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pngwutil.c (85703B)

---

/* pngwutil.c - utilities to write a PNG file
*
* Copyright (c) 2018-2025 Cosmin Truta
* Copyright (c) 1998-2002,2004,2006-2018 Glenn Randers-Pehrson
* Copyright (c) 1996-1997 Andreas Dilger
* Copyright (c) 1995-1996 Guy Eric Schalnat, Group 42, Inc.
*
* This code is released under the libpng license.
* For conditions of distribution and use, see the disclaimer
* and license in png.h
*
* This file contains routines that are only called from within
* libpng itself during the course of writing an image.
*/

#include "pngpriv.h"

#ifdef PNG_WRITE_SUPPORTED

#ifdef PNG_WRITE_INTERLACING_SUPPORTED
/* Arrays to facilitate interlacing - use pass (0 - 6) as index. */

/* Start of interlace block */
static const png_byte png_pass_start[7] = {0, 4, 0, 2, 0, 1, 0};
/* Offset to next interlace block */
static const png_byte png_pass_inc[7] = {8, 8, 4, 4, 2, 2, 1};
/* Start of interlace block in the y direction */
static const png_byte png_pass_ystart[7] = {0, 0, 4, 0, 2, 0, 1};
/* Offset to next interlace block in the y direction */
static const png_byte png_pass_yinc[7] = {8, 8, 8, 4, 4, 2, 2};

/* TODO: Move these arrays to a common utility module to avoid duplication. */
#endif

#ifdef PNG_WRITE_INT_FUNCTIONS_SUPPORTED
/* Place a 32-bit number into a buffer in PNG byte order.  We work
* with unsigned numbers for convenience, although one supported
* ancillary chunk uses signed (two's complement) numbers.
*/
void PNGAPI
png_save_uint_32(png_bytep buf, png_uint_32 i)
{
  buf[0] = (png_byte)((i >> 24) & 0xffU);
  buf[1] = (png_byte)((i >> 16) & 0xffU);
  buf[2] = (png_byte)((i >>  8) & 0xffU);
  buf[3] = (png_byte)( i        & 0xffU);
}

/* Place a 16-bit number into a buffer in PNG byte order.
* The parameter is declared unsigned int, not png_uint_16,
* just to avoid potential problems on pre-ANSI C compilers.
*/
void PNGAPI
png_save_uint_16(png_bytep buf, unsigned int i)
{
  buf[0] = (png_byte)((i >> 8) & 0xffU);
  buf[1] = (png_byte)( i       & 0xffU);
}
#endif

/* Simple function to write the signature.  If we have already written
* the magic bytes of the signature, or more likely, the PNG stream is
* being embedded into another stream and doesn't need its own signature,
* we should call png_set_sig_bytes() to tell libpng how many of the
* bytes have already been written.
*/
void PNGAPI
png_write_sig(png_structrp png_ptr)
{
  png_byte png_signature[8] = {137, 80, 78, 71, 13, 10, 26, 10};

#ifdef PNG_IO_STATE_SUPPORTED
  /* Inform the I/O callback that the signature is being written */
  png_ptr->io_state = PNG_IO_WRITING | PNG_IO_SIGNATURE;
#endif

  /* Write the rest of the 8 byte signature */
  png_write_data(png_ptr, &png_signature[png_ptr->sig_bytes],
      (size_t)(8 - png_ptr->sig_bytes));

  if (png_ptr->sig_bytes < 3)
     png_ptr->mode |= PNG_HAVE_PNG_SIGNATURE;
}

/* Write the start of a PNG chunk.  The type is the chunk type.
* The total_length is the sum of the lengths of all the data you will be
* passing in png_write_chunk_data().
*/
static void
png_write_chunk_header(png_structrp png_ptr, png_uint_32 chunk_name,
   png_uint_32 length)
{
  png_byte buf[8];

#if defined(PNG_DEBUG) && (PNG_DEBUG > 0)
  PNG_CSTRING_FROM_CHUNK(buf, chunk_name);
  png_debug2(0, "Writing %s chunk, length = %lu", buf, (unsigned long)length);
#endif

  if (png_ptr == NULL)
     return;

#ifdef PNG_IO_STATE_SUPPORTED
  /* Inform the I/O callback that the chunk header is being written.
   * PNG_IO_CHUNK_HDR requires a single I/O call.
   */
  png_ptr->io_state = PNG_IO_WRITING | PNG_IO_CHUNK_HDR;
#endif

  /* Write the length and the chunk name */
  png_save_uint_32(buf, length);
  png_save_uint_32(buf + 4, chunk_name);
  png_write_data(png_ptr, buf, 8);

  /* Put the chunk name into png_ptr->chunk_name */
  png_ptr->chunk_name = chunk_name;

  /* Reset the crc and run it over the chunk name */
  png_reset_crc(png_ptr);

  png_calculate_crc(png_ptr, buf + 4, 4);

#ifdef PNG_IO_STATE_SUPPORTED
  /* Inform the I/O callback that chunk data will (possibly) be written.
   * PNG_IO_CHUNK_DATA does NOT require a specific number of I/O calls.
   */
  png_ptr->io_state = PNG_IO_WRITING | PNG_IO_CHUNK_DATA;
#endif
}

void PNGAPI
png_write_chunk_start(png_structrp png_ptr, png_const_bytep chunk_string,
   png_uint_32 length)
{
  png_write_chunk_header(png_ptr, PNG_CHUNK_FROM_STRING(chunk_string), length);
}

/* Write the data of a PNG chunk started with png_write_chunk_header().
* Note that multiple calls to this function are allowed, and that the
* sum of the lengths from these calls *must* add up to the total_length
* given to png_write_chunk_header().
*/
void PNGAPI
png_write_chunk_data(png_structrp png_ptr, png_const_bytep data, size_t length)
{
  /* Write the data, and run the CRC over it */
  if (png_ptr == NULL)
     return;

  if (data != NULL && length > 0)
  {
     png_write_data(png_ptr, data, length);

     /* Update the CRC after writing the data,
      * in case the user I/O routine alters it.
      */
     png_calculate_crc(png_ptr, data, length);
  }
}

/* Finish a chunk started with png_write_chunk_header(). */
void PNGAPI
png_write_chunk_end(png_structrp png_ptr)
{
  png_byte buf[4];

  if (png_ptr == NULL) return;

#ifdef PNG_IO_STATE_SUPPORTED
  /* Inform the I/O callback that the chunk CRC is being written.
   * PNG_IO_CHUNK_CRC requires a single I/O function call.
   */
  png_ptr->io_state = PNG_IO_WRITING | PNG_IO_CHUNK_CRC;
#endif

  /* Write the crc in a single operation */
  png_save_uint_32(buf, png_ptr->crc);

  png_write_data(png_ptr, buf, 4);
}

/* Write a PNG chunk all at once.  The type is an array of ASCII characters
* representing the chunk name.  The array must be at least 4 bytes in
* length, and does not need to be null terminated.  To be safe, pass the
* pre-defined chunk names here, and if you need a new one, define it
* where the others are defined.  The length is the length of the data.
* All the data must be present.  If that is not possible, use the
* png_write_chunk_start(), png_write_chunk_data(), and png_write_chunk_end()
* functions instead.
*/
static void
png_write_complete_chunk(png_structrp png_ptr, png_uint_32 chunk_name,
   png_const_bytep data, size_t length)
{
  if (png_ptr == NULL)
     return;

  /* On 64-bit architectures 'length' may not fit in a png_uint_32. */
  if (length > PNG_UINT_31_MAX)
     png_error(png_ptr, "length exceeds PNG maximum");

  png_write_chunk_header(png_ptr, chunk_name, (png_uint_32)length);
  png_write_chunk_data(png_ptr, data, length);
  png_write_chunk_end(png_ptr);
}

/* This is the API that calls the internal function above. */
void PNGAPI
png_write_chunk(png_structrp png_ptr, png_const_bytep chunk_string,
   png_const_bytep data, size_t length)
{
  png_write_complete_chunk(png_ptr, PNG_CHUNK_FROM_STRING(chunk_string), data,
      length);
}

/* This is used below to find the size of an image to pass to png_deflate_claim,
* so it only needs to be accurate if the size is less than 16384 bytes (the
* point at which a lower LZ window size can be used.)
*/
static png_alloc_size_t
png_image_size(png_structrp png_ptr)
{
  /* Only return sizes up to the maximum of a png_uint_32; do this by limiting
   * the width and height used to 15 bits.
   */
  png_uint_32 h = png_ptr->height;

  if (png_ptr->rowbytes < 32768 && h < 32768)
  {
     if (png_ptr->interlaced != 0)
     {
        /* Interlacing makes the image larger because of the replication of
         * both the filter byte and the padding to a byte boundary.
         */
        png_uint_32 w = png_ptr->width;
        unsigned int pd = png_ptr->pixel_depth;
        png_alloc_size_t cb_base;
        int pass;

        for (cb_base=0, pass=0; pass<=6; ++pass)
        {
           png_uint_32 pw = PNG_PASS_COLS(w, pass);

           if (pw > 0)
              cb_base += (PNG_ROWBYTES(pd, pw)+1) * PNG_PASS_ROWS(h, pass);
        }

        return cb_base;
     }

     else
        return (png_ptr->rowbytes+1) * h;
  }

  else
     return 0xffffffffU;
}

#ifdef PNG_WRITE_OPTIMIZE_CMF_SUPPORTED
  /* This is the code to hack the first two bytes of the deflate stream (the
   * deflate header) to correct the windowBits value to match the actual data
   * size.  Note that the second argument is the *uncompressed* size but the
   * first argument is the *compressed* data (and it must be deflate
   * compressed.)
   */
static void
optimize_cmf(png_bytep data, png_alloc_size_t data_size)
{
  /* Optimize the CMF field in the zlib stream.  The resultant zlib stream is
   * still compliant to the stream specification.
   */
  if (data_size <= 16384) /* else windowBits must be 15 */
  {
     unsigned int z_cmf = data[0];  /* zlib compression method and flags */

     if ((z_cmf & 0x0f) == 8 && (z_cmf & 0xf0) <= 0x70)
     {
        unsigned int z_cinfo;
        unsigned int half_z_window_size;

        z_cinfo = z_cmf >> 4;
        half_z_window_size = 1U << (z_cinfo + 7);

        if (data_size <= half_z_window_size) /* else no change */
        {
           unsigned int tmp;

           do
           {
              half_z_window_size >>= 1;
              --z_cinfo;
           }
           while (z_cinfo > 0 && data_size <= half_z_window_size);

           z_cmf = (z_cmf & 0x0f) | (z_cinfo << 4);

           data[0] = (png_byte)z_cmf;
           tmp = data[1] & 0xe0;
           tmp += 0x1f - ((z_cmf << 8) + tmp) % 0x1f;
           data[1] = (png_byte)tmp;
        }
     }
  }
}
#endif /* WRITE_OPTIMIZE_CMF */

/* Initialize the compressor for the appropriate type of compression. */
static int
png_deflate_claim(png_structrp png_ptr, png_uint_32 owner,
   png_alloc_size_t data_size)
{
  if (png_ptr->zowner != 0)
  {
#if defined(PNG_WARNINGS_SUPPORTED) || defined(PNG_ERROR_TEXT_SUPPORTED)
     char msg[64];

     PNG_STRING_FROM_CHUNK(msg, owner);
     msg[4] = ':';
     msg[5] = ' ';
     PNG_STRING_FROM_CHUNK(msg+6, png_ptr->zowner);
     /* So the message that results is "<chunk> using zstream"; this is an
      * internal error, but is very useful for debugging.  i18n requirements
      * are minimal.
      */
     (void)png_safecat(msg, (sizeof msg), 10, " using zstream");
#endif
#if PNG_RELEASE_BUILD
        png_warning(png_ptr, msg);

        /* Attempt sane error recovery */
        if (png_ptr->zowner == png_IDAT) /* don't steal from IDAT */
        {
           png_ptr->zstream.msg = PNGZ_MSG_CAST("in use by IDAT");
           return Z_STREAM_ERROR;
        }

        png_ptr->zowner = 0;
#else
        png_error(png_ptr, msg);
#endif
  }

  {
     int level = png_ptr->zlib_level;
     int method = png_ptr->zlib_method;
     int windowBits = png_ptr->zlib_window_bits;
     int memLevel = png_ptr->zlib_mem_level;
     int strategy; /* set below */
     int ret; /* zlib return code */

     if (owner == png_IDAT)
     {
        if ((png_ptr->flags & PNG_FLAG_ZLIB_CUSTOM_STRATEGY) != 0)
           strategy = png_ptr->zlib_strategy;

#ifdef PNG_WRITE_FILTER_SUPPORTED
        else if (png_ptr->do_filter != PNG_FILTER_NONE)
           strategy = PNG_Z_DEFAULT_STRATEGY;
#endif

        else
           strategy = PNG_Z_DEFAULT_NOFILTER_STRATEGY;
     }

     else
     {
#ifdef PNG_WRITE_CUSTOMIZE_ZTXT_COMPRESSION_SUPPORTED
           level = png_ptr->zlib_text_level;
           method = png_ptr->zlib_text_method;
           windowBits = png_ptr->zlib_text_window_bits;
           memLevel = png_ptr->zlib_text_mem_level;
           strategy = png_ptr->zlib_text_strategy;
#else
           /* If customization is not supported the values all come from the
            * IDAT values except for the strategy, which is fixed to the
            * default.  (This is the pre-1.6.0 behavior too, although it was
            * implemented in a very different way.)
            */
           strategy = Z_DEFAULT_STRATEGY;
#endif
     }

     /* Adjust 'windowBits' down if larger than 'data_size'; to stop this
      * happening just pass 32768 as the data_size parameter.  Notice that zlib
      * requires an extra 262 bytes in the window in addition to the data to be
      * able to see the whole of the data, so if data_size+262 takes us to the
      * next windowBits size we need to fix up the value later.  (Because even
      * though deflate needs the extra window, inflate does not!)
      */
     if (data_size <= 16384)
     {
        /* IMPLEMENTATION NOTE: this 'half_window_size' stuff is only here to
         * work round a Microsoft Visual C misbehavior which, contrary to C-90,
         * widens the result of the following shift to 64-bits if (and,
         * apparently, only if) it is used in a test.
         */
        unsigned int half_window_size = 1U << (windowBits-1);

        while (data_size + 262 <= half_window_size)
        {
           half_window_size >>= 1;
           --windowBits;
        }
     }

     /* Check against the previous initialized values, if any. */
     if ((png_ptr->flags & PNG_FLAG_ZSTREAM_INITIALIZED) != 0 &&
        (png_ptr->zlib_set_level != level ||
        png_ptr->zlib_set_method != method ||
        png_ptr->zlib_set_window_bits != windowBits ||
        png_ptr->zlib_set_mem_level != memLevel ||
        png_ptr->zlib_set_strategy != strategy))
     {
        if (deflateEnd(&png_ptr->zstream) != Z_OK)
           png_warning(png_ptr, "deflateEnd failed (ignored)");

        png_ptr->flags &= ~PNG_FLAG_ZSTREAM_INITIALIZED;
     }

     /* For safety clear out the input and output pointers (currently zlib
      * doesn't use them on Init, but it might in the future).
      */
     png_ptr->zstream.next_in = NULL;
     png_ptr->zstream.avail_in = 0;
     png_ptr->zstream.next_out = NULL;
     png_ptr->zstream.avail_out = 0;

     /* Now initialize if required, setting the new parameters, otherwise just
      * do a simple reset to the previous parameters.
      */
     if ((png_ptr->flags & PNG_FLAG_ZSTREAM_INITIALIZED) != 0)
        ret = deflateReset(&png_ptr->zstream);

     else
     {
        ret = deflateInit2(&png_ptr->zstream, level, method, windowBits,
            memLevel, strategy);

        if (ret == Z_OK)
           png_ptr->flags |= PNG_FLAG_ZSTREAM_INITIALIZED;
     }

     /* The return code is from either deflateReset or deflateInit2; they have
      * pretty much the same set of error codes.
      */
     if (ret == Z_OK)
        png_ptr->zowner = owner;

     else
        png_zstream_error(png_ptr, ret);

     return ret;
  }
}

/* Clean up (or trim) a linked list of compression buffers. */
void /* PRIVATE */
png_free_buffer_list(png_structrp png_ptr, png_compression_bufferp *listp)
{
  png_compression_bufferp list = *listp;

  if (list != NULL)
  {
     *listp = NULL;

     do
     {
        png_compression_bufferp next = list->next;

        png_free(png_ptr, list);
        list = next;
     }
     while (list != NULL);
  }
}

#ifdef PNG_WRITE_COMPRESSED_TEXT_SUPPORTED
/* This pair of functions encapsulates the operation of (a) compressing a
* text string, and (b) issuing it later as a series of chunk data writes.
* The compression_state structure is shared context for these functions
* set up by the caller to allow access to the relevant local variables.
*
* compression_buffer (new in 1.6.0) is just a linked list of zbuffer_size
* temporary buffers.  From 1.6.0 it is retained in png_struct so that it will
* be correctly freed in the event of a write error (previous implementations
* just leaked memory.)
*/
typedef struct
{
  png_const_bytep      input;        /* The uncompressed input data */
  png_alloc_size_t     input_len;    /* Its length */
  png_uint_32          output_len;   /* Final compressed length */
  png_byte             output[1024]; /* First block of output */
} compression_state;

static void
png_text_compress_init(compression_state *comp, png_const_bytep input,
   png_alloc_size_t input_len)
{
  comp->input = input;
  comp->input_len = input_len;
  comp->output_len = 0;
}

/* Compress the data in the compression state input */
static int
png_text_compress(png_structrp png_ptr, png_uint_32 chunk_name,
   compression_state *comp, png_uint_32 prefix_len)
{
  int ret;

  /* To find the length of the output it is necessary to first compress the
   * input. The result is buffered rather than using the two-pass algorithm
   * that is used on the inflate side; deflate is assumed to be slower and a
   * PNG writer is assumed to have more memory available than a PNG reader.
   *
   * IMPLEMENTATION NOTE: the zlib API deflateBound() can be used to find an
   * upper limit on the output size, but it is always bigger than the input
   * size so it is likely to be more efficient to use this linked-list
   * approach.
   */
  ret = png_deflate_claim(png_ptr, chunk_name, comp->input_len);

  if (ret != Z_OK)
     return ret;

  /* Set up the compression buffers, we need a loop here to avoid overflowing a
   * uInt.  Use ZLIB_IO_MAX to limit the input.  The output is always limited
   * by the output buffer size, so there is no need to check that.  Since this
   * is ANSI-C we know that an 'int', hence a uInt, is always at least 16 bits
   * in size.
   */
  {
     png_compression_bufferp *end = &png_ptr->zbuffer_list;
     png_alloc_size_t input_len = comp->input_len; /* may be zero! */
     png_uint_32 output_len;

     /* zlib updates these for us: */
     png_ptr->zstream.next_in = PNGZ_INPUT_CAST(comp->input);
     png_ptr->zstream.avail_in = 0; /* Set below */
     png_ptr->zstream.next_out = comp->output;
     png_ptr->zstream.avail_out = (sizeof comp->output);

     output_len = png_ptr->zstream.avail_out;

     do
     {
        uInt avail_in = ZLIB_IO_MAX;

        if (avail_in > input_len)
           avail_in = (uInt)input_len;

        input_len -= avail_in;

        png_ptr->zstream.avail_in = avail_in;

        if (png_ptr->zstream.avail_out == 0)
        {
           png_compression_buffer *next;

           /* Chunk data is limited to 2^31 bytes in length, so the prefix
            * length must be counted here.
            */
           if (output_len + prefix_len > PNG_UINT_31_MAX)
           {
              ret = Z_MEM_ERROR;
              break;
           }

           /* Need a new (malloc'ed) buffer, but there may be one present
            * already.
            */
           next = *end;
           if (next == NULL)
           {
              next = png_voidcast(png_compression_bufferp, png_malloc_base
                 (png_ptr, PNG_COMPRESSION_BUFFER_SIZE(png_ptr)));

              if (next == NULL)
              {
                 ret = Z_MEM_ERROR;
                 break;
              }

              /* Link in this buffer (so that it will be freed later) */
              next->next = NULL;
              *end = next;
           }

           png_ptr->zstream.next_out = next->output;
           png_ptr->zstream.avail_out = png_ptr->zbuffer_size;
           output_len += png_ptr->zstream.avail_out;

           /* Move 'end' to the next buffer pointer. */
           end = &next->next;
        }

        /* Compress the data */
        ret = deflate(&png_ptr->zstream,
            input_len > 0 ? Z_NO_FLUSH : Z_FINISH);

        /* Claw back input data that was not consumed (because avail_in is
         * reset above every time round the loop).
         */
        input_len += png_ptr->zstream.avail_in;
        png_ptr->zstream.avail_in = 0; /* safety */
     }
     while (ret == Z_OK);

     /* There may be some space left in the last output buffer. This needs to
      * be subtracted from output_len.
      */
     output_len -= png_ptr->zstream.avail_out;
     png_ptr->zstream.avail_out = 0; /* safety */
     comp->output_len = output_len;

     /* Now double check the output length, put in a custom message if it is
      * too long.  Otherwise ensure the z_stream::msg pointer is set to
      * something.
      */
     if (output_len + prefix_len >= PNG_UINT_31_MAX)
     {
        png_ptr->zstream.msg = PNGZ_MSG_CAST("compressed data too long");
        ret = Z_MEM_ERROR;
     }

     else
        png_zstream_error(png_ptr, ret);

     /* Reset zlib for another zTXt/iTXt or image data */
     png_ptr->zowner = 0;

     /* The only success case is Z_STREAM_END, input_len must be 0; if not this
      * is an internal error.
      */
     if (ret == Z_STREAM_END && input_len == 0)
     {
#ifdef PNG_WRITE_OPTIMIZE_CMF_SUPPORTED
        /* Fix up the deflate header, if required */
        optimize_cmf(comp->output, comp->input_len);
#endif
        /* But Z_OK is returned, not Z_STREAM_END; this allows the claim
         * function above to return Z_STREAM_END on an error (though it never
         * does in the current versions of zlib.)
         */
        return Z_OK;
     }

     else
        return ret;
  }
}

/* Ship the compressed text out via chunk writes */
static void
png_write_compressed_data_out(png_structrp png_ptr, compression_state *comp)
{
  png_uint_32 output_len = comp->output_len;
  png_const_bytep output = comp->output;
  png_uint_32 avail = (sizeof comp->output);
  png_compression_buffer *next = png_ptr->zbuffer_list;

  for (;;)
  {
     if (avail > output_len)
        avail = output_len;

     png_write_chunk_data(png_ptr, output, avail);

     output_len -= avail;

     if (output_len == 0 || next == NULL)
        break;

     avail = png_ptr->zbuffer_size;
     output = next->output;
     next = next->next;
  }

  /* This is an internal error; 'next' must have been NULL! */
  if (output_len > 0)
     png_error(png_ptr, "error writing ancillary chunked compressed data");
}
#endif /* WRITE_COMPRESSED_TEXT */

/* Write the IHDR chunk, and update the png_struct with the necessary
* information.  Note that the rest of this code depends upon this
* information being correct.
*/
void /* PRIVATE */
png_write_IHDR(png_structrp png_ptr, png_uint_32 width, png_uint_32 height,
   int bit_depth, int color_type, int compression_type, int filter_type,
   int interlace_type)
{
  png_byte buf[13]; /* Buffer to store the IHDR info */
  int is_invalid_depth;

  png_debug(1, "in png_write_IHDR");

  /* Check that we have valid input data from the application info */
  switch (color_type)
  {
     case PNG_COLOR_TYPE_GRAY:
        switch (bit_depth)
        {
           case 1:
           case 2:
           case 4:
           case 8:
#ifdef PNG_WRITE_16BIT_SUPPORTED
           case 16:
#endif
              png_ptr->channels = 1; break;

           default:
              png_error(png_ptr,
                  "Invalid bit depth for grayscale image");
        }
        break;

     case PNG_COLOR_TYPE_RGB:
        is_invalid_depth = (bit_depth != 8);
#ifdef PNG_WRITE_16BIT_SUPPORTED
        is_invalid_depth = (is_invalid_depth && bit_depth != 16);
#endif
        if (is_invalid_depth)
           png_error(png_ptr, "Invalid bit depth for RGB image");

        png_ptr->channels = 3;
        break;

     case PNG_COLOR_TYPE_PALETTE:
        switch (bit_depth)
        {
           case 1:
           case 2:
           case 4:
           case 8:
              png_ptr->channels = 1;
              break;

           default:
              png_error(png_ptr, "Invalid bit depth for paletted image");
        }
        break;

     case PNG_COLOR_TYPE_GRAY_ALPHA:
        is_invalid_depth = (bit_depth != 8);
#ifdef PNG_WRITE_16BIT_SUPPORTED
        is_invalid_depth = (is_invalid_depth && bit_depth != 16);
#endif
        if (is_invalid_depth)
           png_error(png_ptr, "Invalid bit depth for grayscale+alpha image");

        png_ptr->channels = 2;
        break;

     case PNG_COLOR_TYPE_RGB_ALPHA:
        is_invalid_depth = (bit_depth != 8);
#ifdef PNG_WRITE_16BIT_SUPPORTED
        is_invalid_depth = (is_invalid_depth && bit_depth != 16);
#endif
        if (is_invalid_depth)
           png_error(png_ptr, "Invalid bit depth for RGBA image");

        png_ptr->channels = 4;
        break;

     default:
        png_error(png_ptr, "Invalid image color type specified");
  }

  if (compression_type != PNG_COMPRESSION_TYPE_BASE)
  {
     png_warning(png_ptr, "Invalid compression type specified");
     compression_type = PNG_COMPRESSION_TYPE_BASE;
  }

  /* Write filter_method 64 (intrapixel differencing) only if
   * 1. Libpng was compiled with PNG_MNG_FEATURES_SUPPORTED and
   * 2. Libpng did not write a PNG signature (this filter_method is only
   *    used in PNG datastreams that are embedded in MNG datastreams) and
   * 3. The application called png_permit_mng_features with a mask that
   *    included PNG_FLAG_MNG_FILTER_64 and
   * 4. The filter_method is 64 and
   * 5. The color_type is RGB or RGBA
   */
  if (
#ifdef PNG_MNG_FEATURES_SUPPORTED
      !((png_ptr->mng_features_permitted & PNG_FLAG_MNG_FILTER_64) != 0 &&
      ((png_ptr->mode & PNG_HAVE_PNG_SIGNATURE) == 0) &&
      (color_type == PNG_COLOR_TYPE_RGB ||
       color_type == PNG_COLOR_TYPE_RGB_ALPHA) &&
      (filter_type == PNG_INTRAPIXEL_DIFFERENCING)) &&
#endif
      filter_type != PNG_FILTER_TYPE_BASE)
  {
     png_warning(png_ptr, "Invalid filter type specified");
     filter_type = PNG_FILTER_TYPE_BASE;
  }

#ifdef PNG_WRITE_INTERLACING_SUPPORTED
  if (interlace_type != PNG_INTERLACE_NONE &&
      interlace_type != PNG_INTERLACE_ADAM7)
  {
     png_warning(png_ptr, "Invalid interlace type specified");
     interlace_type = PNG_INTERLACE_ADAM7;
  }
#else
  interlace_type=PNG_INTERLACE_NONE;
#endif

  /* Save the relevant information */
  png_ptr->bit_depth = (png_byte)bit_depth;
  png_ptr->color_type = (png_byte)color_type;
  png_ptr->interlaced = (png_byte)interlace_type;
#ifdef PNG_MNG_FEATURES_SUPPORTED
  png_ptr->filter_type = (png_byte)filter_type;
#endif
  png_ptr->compression_type = (png_byte)compression_type;
  png_ptr->width = width;
  png_ptr->height = height;

  png_ptr->pixel_depth = (png_byte)(bit_depth * png_ptr->channels);
  png_ptr->rowbytes = PNG_ROWBYTES(png_ptr->pixel_depth, width);
  /* Set the usr info, so any transformations can modify it */
  png_ptr->usr_width = png_ptr->width;
  png_ptr->usr_bit_depth = png_ptr->bit_depth;
  png_ptr->usr_channels = png_ptr->channels;

  /* Pack the header information into the buffer */
  png_save_uint_32(buf, width);
  png_save_uint_32(buf + 4, height);
  buf[8] = (png_byte)bit_depth;
  buf[9] = (png_byte)color_type;
  buf[10] = (png_byte)compression_type;
  buf[11] = (png_byte)filter_type;
  buf[12] = (png_byte)interlace_type;

  /* Write the chunk */
  png_write_complete_chunk(png_ptr, png_IHDR, buf, 13);

#ifdef PNG_WRITE_APNG_SUPPORTED
  png_ptr->first_frame_width = width;
  png_ptr->first_frame_height = height;
#endif

  if ((png_ptr->do_filter) == PNG_NO_FILTERS)
  {
#ifdef PNG_WRITE_FILTER_SUPPORTED
     if (png_ptr->color_type == PNG_COLOR_TYPE_PALETTE ||
         png_ptr->bit_depth < 8)
        png_ptr->do_filter = PNG_FILTER_NONE;

     else
        png_ptr->do_filter = PNG_ALL_FILTERS;
#else
     png_ptr->do_filter = PNG_FILTER_NONE;
#endif
  }

  png_ptr->mode = PNG_HAVE_IHDR; /* not READY_FOR_ZTXT */
}

/* Write the palette.  We are careful not to trust png_color to be in the
* correct order for PNG, so people can redefine it to any convenient
* structure.
*/
void /* PRIVATE */
png_write_PLTE(png_structrp png_ptr, png_const_colorp palette,
   png_uint_32 num_pal)
{
  png_uint_32 max_palette_length, i;
  png_const_colorp pal_ptr;
  png_byte buf[3];

  png_debug(1, "in png_write_PLTE");

  max_palette_length = (png_ptr->color_type == PNG_COLOR_TYPE_PALETTE) ?
     (1 << png_ptr->bit_depth) : PNG_MAX_PALETTE_LENGTH;

  if ((
#ifdef PNG_MNG_FEATURES_SUPPORTED
      (png_ptr->mng_features_permitted & PNG_FLAG_MNG_EMPTY_PLTE) == 0 &&
#endif
      num_pal == 0) || num_pal > max_palette_length)
  {
     if (png_ptr->color_type == PNG_COLOR_TYPE_PALETTE)
     {
        png_error(png_ptr, "Invalid number of colors in palette");
     }

     else
     {
        png_warning(png_ptr, "Invalid number of colors in palette");
        return;
     }
  }

  if ((png_ptr->color_type & PNG_COLOR_MASK_COLOR) == 0)
  {
     png_warning(png_ptr,
         "Ignoring request to write a PLTE chunk in grayscale PNG");

     return;
  }

  png_ptr->num_palette = (png_uint_16)num_pal;
  png_debug1(3, "num_palette = %d", png_ptr->num_palette);

  png_write_chunk_header(png_ptr, png_PLTE, (png_uint_32)(num_pal * 3));
#ifdef PNG_POINTER_INDEXING_SUPPORTED

  for (i = 0, pal_ptr = palette; i < num_pal; i++, pal_ptr++)
  {
     buf[0] = pal_ptr->red;
     buf[1] = pal_ptr->green;
     buf[2] = pal_ptr->blue;
     png_write_chunk_data(png_ptr, buf, 3);
  }

#else
  /* This is a little slower but some buggy compilers need to do this
   * instead
   */
  pal_ptr=palette;

  for (i = 0; i < num_pal; i++)
  {
     buf[0] = pal_ptr[i].red;
     buf[1] = pal_ptr[i].green;
     buf[2] = pal_ptr[i].blue;
     png_write_chunk_data(png_ptr, buf, 3);
  }

#endif
  png_write_chunk_end(png_ptr);
  png_ptr->mode |= PNG_HAVE_PLTE;
}

/* This is similar to png_text_compress, above, except that it does not require
* all of the data at once and, instead of buffering the compressed result,
* writes it as IDAT chunks.  Unlike png_text_compress it *can* png_error out
* because it calls the write interface.  As a result it does its own error
* reporting and does not return an error code.  In the event of error it will
* just call png_error.  The input data length may exceed 32-bits.  The 'flush'
* parameter is exactly the same as that to deflate, with the following
* meanings:
*
* Z_NO_FLUSH: normal incremental output of compressed data
* Z_SYNC_FLUSH: do a SYNC_FLUSH, used by png_write_flush
* Z_FINISH: this is the end of the input, do a Z_FINISH and clean up
*
* The routine manages the acquire and release of the png_ptr->zstream by
* checking and (at the end) clearing png_ptr->zowner; it does some sanity
* checks on the 'mode' flags while doing this.
*/
void /* PRIVATE */
png_compress_IDAT(png_structrp png_ptr, png_const_bytep input,
   png_alloc_size_t input_len, int flush)
{
  if (png_ptr->zowner != png_IDAT)
  {
     /* First time.   Ensure we have a temporary buffer for compression and
      * trim the buffer list if it has more than one entry to free memory.
      * If 'WRITE_COMPRESSED_TEXT' is not set the list will never have been
      * created at this point, but the check here is quick and safe.
      */
     if (png_ptr->zbuffer_list == NULL)
     {
        png_ptr->zbuffer_list = png_voidcast(png_compression_bufferp,
            png_malloc(png_ptr, PNG_COMPRESSION_BUFFER_SIZE(png_ptr)));
        png_ptr->zbuffer_list->next = NULL;
     }

     else
        png_free_buffer_list(png_ptr, &png_ptr->zbuffer_list->next);

     /* It is a terminal error if we can't claim the zstream. */
     if (png_deflate_claim(png_ptr, png_IDAT, png_image_size(png_ptr)) != Z_OK)
        png_error(png_ptr, png_ptr->zstream.msg);

     /* The output state is maintained in png_ptr->zstream, so it must be
      * initialized here after the claim.
      */
     png_ptr->zstream.next_out = png_ptr->zbuffer_list->output;
     png_ptr->zstream.avail_out = png_ptr->zbuffer_size;
  }

  /* Now loop reading and writing until all the input is consumed or an error
   * terminates the operation.  The _out values are maintained across calls to
   * this function, but the input must be reset each time.
   */
  png_ptr->zstream.next_in = PNGZ_INPUT_CAST(input);
  png_ptr->zstream.avail_in = 0; /* set below */
  for (;;)
  {
     int ret;

     /* INPUT: from the row data */
     uInt avail = ZLIB_IO_MAX;

     if (avail > input_len)
        avail = (uInt)input_len; /* safe because of the check */

     png_ptr->zstream.avail_in = avail;
     input_len -= avail;

     ret = deflate(&png_ptr->zstream, input_len > 0 ? Z_NO_FLUSH : flush);

     /* Include as-yet unconsumed input */
     input_len += png_ptr->zstream.avail_in;
     png_ptr->zstream.avail_in = 0;

     /* OUTPUT: write complete IDAT chunks when avail_out drops to zero. Note
      * that these two zstream fields are preserved across the calls, therefore
      * there is no need to set these up on entry to the loop.
      */
     if (png_ptr->zstream.avail_out == 0)
     {
        png_bytep data = png_ptr->zbuffer_list->output;
        uInt size = png_ptr->zbuffer_size;

        /* Write an IDAT containing the data then reset the buffer.  The
         * first IDAT may need deflate header optimization.
         */
#ifdef PNG_WRITE_OPTIMIZE_CMF_SUPPORTED
           if ((png_ptr->mode & PNG_HAVE_IDAT) == 0 &&
               png_ptr->compression_type == PNG_COMPRESSION_TYPE_BASE)
              optimize_cmf(data, png_image_size(png_ptr));
#endif

        if (size > 0)
#ifdef PNG_WRITE_APNG_SUPPORTED
        {
           if (png_ptr->num_frames_written == 0)
#endif
           png_write_complete_chunk(png_ptr, png_IDAT, data, size);
#ifdef PNG_WRITE_APNG_SUPPORTED
           else
              png_write_fdAT(png_ptr, data, size);
        }
#endif /* WRITE_APNG */

        png_ptr->mode |= PNG_HAVE_IDAT;

        png_ptr->zstream.next_out = data;
        png_ptr->zstream.avail_out = size;

        /* For SYNC_FLUSH or FINISH it is essential to keep calling zlib with
         * the same flush parameter until it has finished output, for NO_FLUSH
         * it doesn't matter.
         */
        if (ret == Z_OK && flush != Z_NO_FLUSH)
           continue;
     }

     /* The order of these checks doesn't matter much; it just affects which
      * possible error might be detected if multiple things go wrong at once.
      */
     if (ret == Z_OK) /* most likely return code! */
     {
        /* If all the input has been consumed then just return.  If Z_FINISH
         * was used as the flush parameter something has gone wrong if we get
         * here.
         */
        if (input_len == 0)
        {
           if (flush == Z_FINISH)
              png_error(png_ptr, "Z_OK on Z_FINISH with output space");

           return;
        }
     }

     else if (ret == Z_STREAM_END && flush == Z_FINISH)
     {
        /* This is the end of the IDAT data; any pending output must be
         * flushed.  For small PNG files we may still be at the beginning.
         */
        png_bytep data = png_ptr->zbuffer_list->output;
        uInt size = png_ptr->zbuffer_size - png_ptr->zstream.avail_out;

#ifdef PNG_WRITE_OPTIMIZE_CMF_SUPPORTED
        if ((png_ptr->mode & PNG_HAVE_IDAT) == 0 &&
            png_ptr->compression_type == PNG_COMPRESSION_TYPE_BASE)
           optimize_cmf(data, png_image_size(png_ptr));
#endif

        if (size > 0)
#ifdef PNG_WRITE_APNG_SUPPORTED
        {
           if (png_ptr->num_frames_written == 0)
#endif
           png_write_complete_chunk(png_ptr, png_IDAT, data, size);
#ifdef PNG_WRITE_APNG_SUPPORTED
           else
              png_write_fdAT(png_ptr, data, size);
        }
#endif /* WRITE_APNG */

        png_ptr->zstream.avail_out = 0;
        png_ptr->zstream.next_out = NULL;
        png_ptr->mode |= PNG_HAVE_IDAT | PNG_AFTER_IDAT;

        png_ptr->zowner = 0; /* Release the stream */
        return;
     }

     else
     {
        /* This is an error condition. */
        png_zstream_error(png_ptr, ret);
        png_error(png_ptr, png_ptr->zstream.msg);
     }
  }
}

/* Write an IEND chunk */
void /* PRIVATE */
png_write_IEND(png_structrp png_ptr)
{
  png_debug(1, "in png_write_IEND");

  png_write_complete_chunk(png_ptr, png_IEND, NULL, 0);
  png_ptr->mode |= PNG_HAVE_IEND;
}

#ifdef PNG_WRITE_gAMA_SUPPORTED
/* Write a gAMA chunk */
void /* PRIVATE */
png_write_gAMA_fixed(png_structrp png_ptr, png_fixed_point file_gamma)
{
  png_byte buf[4];

  png_debug(1, "in png_write_gAMA");

  /* file_gamma is saved in 1/100,000ths */
  png_save_uint_32(buf, (png_uint_32)file_gamma);
  png_write_complete_chunk(png_ptr, png_gAMA, buf, 4);
}
#endif

#ifdef PNG_WRITE_sRGB_SUPPORTED
/* Write a sRGB chunk */
void /* PRIVATE */
png_write_sRGB(png_structrp png_ptr, int srgb_intent)
{
  png_byte buf[1];

  png_debug(1, "in png_write_sRGB");

  if (srgb_intent >= PNG_sRGB_INTENT_LAST)
     png_warning(png_ptr,
         "Invalid sRGB rendering intent specified");

  buf[0]=(png_byte)srgb_intent;
  png_write_complete_chunk(png_ptr, png_sRGB, buf, 1);
}
#endif

#ifdef PNG_WRITE_iCCP_SUPPORTED
/* Write an iCCP chunk */
void /* PRIVATE */
png_write_iCCP(png_structrp png_ptr, png_const_charp name,
   png_const_bytep profile, png_uint_32 profile_len)
{
  png_uint_32 name_len;
  png_byte new_name[81]; /* 1 byte for the compression byte */
  compression_state comp;
  png_uint_32 temp;

  png_debug(1, "in png_write_iCCP");

  /* These are all internal problems: the profile should have been checked
   * before when it was stored.
   */
  if (profile == NULL)
     png_error(png_ptr, "No profile for iCCP chunk"); /* internal error */

  if (profile_len < 132)
     png_error(png_ptr, "ICC profile too short");

  if (png_get_uint_32(profile) != profile_len)
     png_error(png_ptr, "Incorrect data in iCCP");

  temp = (png_uint_32) (*(profile+8));
  if (temp > 3 && (profile_len & 0x03))
     png_error(png_ptr, "ICC profile length invalid (not a multiple of 4)");

  {
     png_uint_32 embedded_profile_len = png_get_uint_32(profile);

     if (profile_len != embedded_profile_len)
        png_error(png_ptr, "Profile length does not match profile");
  }

  name_len = png_check_keyword(png_ptr, name, new_name);

  if (name_len == 0)
     png_error(png_ptr, "iCCP: invalid keyword");

  new_name[++name_len] = PNG_COMPRESSION_TYPE_BASE;

  /* Make sure we include the NULL after the name and the compression type */
  ++name_len;

  png_text_compress_init(&comp, profile, profile_len);

  /* Allow for keyword terminator and compression byte */
  if (png_text_compress(png_ptr, png_iCCP, &comp, name_len) != Z_OK)
     png_error(png_ptr, png_ptr->zstream.msg);

  png_write_chunk_header(png_ptr, png_iCCP, name_len + comp.output_len);

  png_write_chunk_data(png_ptr, new_name, name_len);

  png_write_compressed_data_out(png_ptr, &comp);

  png_write_chunk_end(png_ptr);
}
#endif

#ifdef PNG_WRITE_sPLT_SUPPORTED
/* Write a sPLT chunk */
void /* PRIVATE */
png_write_sPLT(png_structrp png_ptr, png_const_sPLT_tp spalette)
{
  png_uint_32 name_len;
  png_byte new_name[80];
  png_byte entrybuf[10];
  size_t entry_size = (spalette->depth == 8 ? 6 : 10);
  size_t palette_size = entry_size * (size_t)spalette->nentries;
  png_sPLT_entryp ep;
#ifndef PNG_POINTER_INDEXING_SUPPORTED
  int i;
#endif

  png_debug(1, "in png_write_sPLT");

  name_len = png_check_keyword(png_ptr, spalette->name, new_name);

  if (name_len == 0)
     png_error(png_ptr, "sPLT: invalid keyword");

  /* Make sure we include the NULL after the name */
  png_write_chunk_header(png_ptr, png_sPLT,
      (png_uint_32)(name_len + 2 + palette_size));

  png_write_chunk_data(png_ptr, (png_bytep)new_name, (size_t)(name_len + 1));

  png_write_chunk_data(png_ptr, &spalette->depth, 1);

  /* Loop through each palette entry, writing appropriately */
#ifdef PNG_POINTER_INDEXING_SUPPORTED
  for (ep = spalette->entries; ep<spalette->entries + spalette->nentries; ep++)
  {
     if (spalette->depth == 8)
     {
        entrybuf[0] = (png_byte)ep->red;
        entrybuf[1] = (png_byte)ep->green;
        entrybuf[2] = (png_byte)ep->blue;
        entrybuf[3] = (png_byte)ep->alpha;
        png_save_uint_16(entrybuf + 4, ep->frequency);
     }

     else
     {
        png_save_uint_16(entrybuf + 0, ep->red);
        png_save_uint_16(entrybuf + 2, ep->green);
        png_save_uint_16(entrybuf + 4, ep->blue);
        png_save_uint_16(entrybuf + 6, ep->alpha);
        png_save_uint_16(entrybuf + 8, ep->frequency);
     }

     png_write_chunk_data(png_ptr, entrybuf, entry_size);
  }
#else
  ep=spalette->entries;
  for (i = 0; i>spalette->nentries; i++)
  {
     if (spalette->depth == 8)
     {
        entrybuf[0] = (png_byte)ep[i].red;
        entrybuf[1] = (png_byte)ep[i].green;
        entrybuf[2] = (png_byte)ep[i].blue;
        entrybuf[3] = (png_byte)ep[i].alpha;
        png_save_uint_16(entrybuf + 4, ep[i].frequency);
     }

     else
     {
        png_save_uint_16(entrybuf + 0, ep[i].red);
        png_save_uint_16(entrybuf + 2, ep[i].green);
        png_save_uint_16(entrybuf + 4, ep[i].blue);
        png_save_uint_16(entrybuf + 6, ep[i].alpha);
        png_save_uint_16(entrybuf + 8, ep[i].frequency);
     }

     png_write_chunk_data(png_ptr, entrybuf, entry_size);
  }
#endif

  png_write_chunk_end(png_ptr);
}
#endif

#ifdef PNG_WRITE_sBIT_SUPPORTED
/* Write the sBIT chunk */
void /* PRIVATE */
png_write_sBIT(png_structrp png_ptr, png_const_color_8p sbit, int color_type)
{
  png_byte buf[4];
  size_t size;

  png_debug(1, "in png_write_sBIT");

  /* Make sure we don't depend upon the order of PNG_COLOR_8 */
  if ((color_type & PNG_COLOR_MASK_COLOR) != 0)
  {
     png_byte maxbits;

     maxbits = (png_byte)(color_type==PNG_COLOR_TYPE_PALETTE ? 8 :
         png_ptr->usr_bit_depth);

     if (sbit->red == 0 || sbit->red > maxbits ||
         sbit->green == 0 || sbit->green > maxbits ||
         sbit->blue == 0 || sbit->blue > maxbits)
     {
        png_warning(png_ptr, "Invalid sBIT depth specified");
        return;
     }

     buf[0] = sbit->red;
     buf[1] = sbit->green;
     buf[2] = sbit->blue;
     size = 3;
  }

  else
  {
     if (sbit->gray == 0 || sbit->gray > png_ptr->usr_bit_depth)
     {
        png_warning(png_ptr, "Invalid sBIT depth specified");
        return;
     }

     buf[0] = sbit->gray;
     size = 1;
  }

  if ((color_type & PNG_COLOR_MASK_ALPHA) != 0)
  {
     if (sbit->alpha == 0 || sbit->alpha > png_ptr->usr_bit_depth)
     {
        png_warning(png_ptr, "Invalid sBIT depth specified");
        return;
     }

     buf[size++] = sbit->alpha;
  }

  png_write_complete_chunk(png_ptr, png_sBIT, buf, size);
}
#endif

#ifdef PNG_WRITE_cHRM_SUPPORTED
/* Write the cHRM chunk */
void /* PRIVATE */
png_write_cHRM_fixed(png_structrp png_ptr, const png_xy *xy)
{
  png_byte buf[32];

  png_debug(1, "in png_write_cHRM");

  /* Each value is saved in 1/100,000ths */
  png_save_int_32(buf,      xy->whitex);
  png_save_int_32(buf +  4, xy->whitey);

  png_save_int_32(buf +  8, xy->redx);
  png_save_int_32(buf + 12, xy->redy);

  png_save_int_32(buf + 16, xy->greenx);
  png_save_int_32(buf + 20, xy->greeny);

  png_save_int_32(buf + 24, xy->bluex);
  png_save_int_32(buf + 28, xy->bluey);

  png_write_complete_chunk(png_ptr, png_cHRM, buf, 32);
}
#endif

#ifdef PNG_WRITE_tRNS_SUPPORTED
/* Write the tRNS chunk */
void /* PRIVATE */
png_write_tRNS(png_structrp png_ptr, png_const_bytep trans_alpha,
   png_const_color_16p tran, int num_trans, int color_type)
{
  png_byte buf[6];

  png_debug(1, "in png_write_tRNS");

  if (color_type == PNG_COLOR_TYPE_PALETTE)
  {
     if (num_trans <= 0 || num_trans > (int)png_ptr->num_palette)
     {
        png_app_warning(png_ptr,
            "Invalid number of transparent colors specified");
        return;
     }

     /* Write the chunk out as it is */
     png_write_complete_chunk(png_ptr, png_tRNS, trans_alpha,
         (size_t)num_trans);
  }

  else if (color_type == PNG_COLOR_TYPE_GRAY)
  {
     /* One 16-bit value */
     if (tran->gray >= (1 << png_ptr->bit_depth))
     {
        png_app_warning(png_ptr,
            "Ignoring attempt to write tRNS chunk out-of-range for bit_depth");

        return;
     }

     png_save_uint_16(buf, tran->gray);
     png_write_complete_chunk(png_ptr, png_tRNS, buf, 2);
  }

  else if (color_type == PNG_COLOR_TYPE_RGB)
  {
     /* Three 16-bit values */
     png_save_uint_16(buf, tran->red);
     png_save_uint_16(buf + 2, tran->green);
     png_save_uint_16(buf + 4, tran->blue);
#ifdef PNG_WRITE_16BIT_SUPPORTED
     if (png_ptr->bit_depth == 8 && (buf[0] | buf[2] | buf[4]) != 0)
#else
     if ((buf[0] | buf[2] | buf[4]) != 0)
#endif
     {
        png_app_warning(png_ptr,
            "Ignoring attempt to write 16-bit tRNS chunk when bit_depth is 8");
        return;
     }

     png_write_complete_chunk(png_ptr, png_tRNS, buf, 6);
  }

  else
  {
     png_app_warning(png_ptr, "Can't write tRNS with an alpha channel");
  }
}
#endif

#ifdef PNG_WRITE_bKGD_SUPPORTED
/* Write the background chunk */
void /* PRIVATE */
png_write_bKGD(png_structrp png_ptr, png_const_color_16p back, int color_type)
{
  png_byte buf[6];

  png_debug(1, "in png_write_bKGD");

  if (color_type == PNG_COLOR_TYPE_PALETTE)
  {
     if (
#ifdef PNG_MNG_FEATURES_SUPPORTED
         (png_ptr->num_palette != 0 ||
         (png_ptr->mng_features_permitted & PNG_FLAG_MNG_EMPTY_PLTE) == 0) &&
#endif
        back->index >= png_ptr->num_palette)
     {
        png_warning(png_ptr, "Invalid background palette index");
        return;
     }

     buf[0] = back->index;
     png_write_complete_chunk(png_ptr, png_bKGD, buf, 1);
  }

  else if ((color_type & PNG_COLOR_MASK_COLOR) != 0)
  {
     png_save_uint_16(buf, back->red);
     png_save_uint_16(buf + 2, back->green);
     png_save_uint_16(buf + 4, back->blue);
#ifdef PNG_WRITE_16BIT_SUPPORTED
     if (png_ptr->bit_depth == 8 && (buf[0] | buf[2] | buf[4]) != 0)
#else
     if ((buf[0] | buf[2] | buf[4]) != 0)
#endif
     {
        png_warning(png_ptr,
            "Ignoring attempt to write 16-bit bKGD chunk "
            "when bit_depth is 8");

        return;
     }

     png_write_complete_chunk(png_ptr, png_bKGD, buf, 6);
  }

  else
  {
     if (back->gray >= (1 << png_ptr->bit_depth))
     {
        png_warning(png_ptr,
            "Ignoring attempt to write bKGD chunk out-of-range for bit_depth");

        return;
     }

     png_save_uint_16(buf, back->gray);
     png_write_complete_chunk(png_ptr, png_bKGD, buf, 2);
  }
}
#endif

#ifdef PNG_WRITE_cICP_SUPPORTED
/* Write the cICP data */
void /* PRIVATE */
png_write_cICP(png_structrp png_ptr,
              png_byte colour_primaries, png_byte transfer_function,
              png_byte matrix_coefficients, png_byte video_full_range_flag)
{
  png_byte buf[4];

  png_debug(1, "in png_write_cICP");

  png_write_chunk_header(png_ptr, png_cICP, 4);

  buf[0] = colour_primaries;
  buf[1] = transfer_function;
  buf[2] = matrix_coefficients;
  buf[3] = video_full_range_flag;
  png_write_chunk_data(png_ptr, buf, 4);

  png_write_chunk_end(png_ptr);
}
#endif

#ifdef PNG_WRITE_cLLI_SUPPORTED
void /* PRIVATE */
png_write_cLLI_fixed(png_structrp png_ptr, png_uint_32 maxCLL,
  png_uint_32 maxFALL)
{
  png_byte buf[8];

  png_debug(1, "in png_write_cLLI_fixed");

  png_save_uint_32(buf, maxCLL);
  png_save_uint_32(buf + 4, maxFALL);

  png_write_complete_chunk(png_ptr, png_cLLI, buf, 8);
}
#endif

#ifdef PNG_WRITE_mDCV_SUPPORTED
void /* PRIVATE */
png_write_mDCV_fixed(png_structrp png_ptr,
  png_uint_16 red_x, png_uint_16 red_y,
  png_uint_16 green_x, png_uint_16 green_y,
  png_uint_16 blue_x, png_uint_16 blue_y,
  png_uint_16 white_x, png_uint_16 white_y,
  png_uint_32 maxDL, png_uint_32 minDL)
{
  png_byte buf[24];

  png_debug(1, "in png_write_mDCV_fixed");

  png_save_uint_16(buf +  0, red_x);
  png_save_uint_16(buf +  2, red_y);
  png_save_uint_16(buf +  4, green_x);
  png_save_uint_16(buf +  6, green_y);
  png_save_uint_16(buf +  8, blue_x);
  png_save_uint_16(buf + 10, blue_y);
  png_save_uint_16(buf + 12, white_x);
  png_save_uint_16(buf + 14, white_y);
  png_save_uint_32(buf + 16, maxDL);
  png_save_uint_32(buf + 20, minDL);

  png_write_complete_chunk(png_ptr, png_mDCV, buf, 24);
}
#endif

#ifdef PNG_WRITE_eXIf_SUPPORTED
/* Write the Exif data */
void /* PRIVATE */
png_write_eXIf(png_structrp png_ptr, png_bytep exif, int num_exif)
{
  int i;
  png_byte buf[1];

  png_debug(1, "in png_write_eXIf");

  png_write_chunk_header(png_ptr, png_eXIf, (png_uint_32)(num_exif));

  for (i = 0; i < num_exif; i++)
  {
     buf[0] = exif[i];
     png_write_chunk_data(png_ptr, buf, 1);
  }

  png_write_chunk_end(png_ptr);
}
#endif

#ifdef PNG_WRITE_hIST_SUPPORTED
/* Write the histogram */
void /* PRIVATE */
png_write_hIST(png_structrp png_ptr, png_const_uint_16p hist, int num_hist)
{
  int i;
  png_byte buf[3];

  png_debug(1, "in png_write_hIST");

  if (num_hist > (int)png_ptr->num_palette)
  {
     png_debug2(3, "num_hist = %d, num_palette = %d", num_hist,
         png_ptr->num_palette);

     png_warning(png_ptr, "Invalid number of histogram entries specified");
     return;
  }

  png_write_chunk_header(png_ptr, png_hIST, (png_uint_32)(num_hist * 2));

  for (i = 0; i < num_hist; i++)
  {
     png_save_uint_16(buf, hist[i]);
     png_write_chunk_data(png_ptr, buf, 2);
  }

  png_write_chunk_end(png_ptr);
}
#endif

#ifdef PNG_WRITE_tEXt_SUPPORTED
/* Write a tEXt chunk */
void /* PRIVATE */
png_write_tEXt(png_structrp png_ptr, png_const_charp key, png_const_charp text,
   size_t text_len)
{
  png_uint_32 key_len;
  png_byte new_key[80];

  png_debug(1, "in png_write_tEXt");

  key_len = png_check_keyword(png_ptr, key, new_key);

  if (key_len == 0)
     png_error(png_ptr, "tEXt: invalid keyword");

  if (text == NULL || *text == '\0')
     text_len = 0;

  else
     text_len = strlen(text);

  if (text_len > PNG_UINT_31_MAX - (key_len+1))
     png_error(png_ptr, "tEXt: text too long");

  /* Make sure we include the 0 after the key */
  png_write_chunk_header(png_ptr, png_tEXt,
      (png_uint_32)/*checked above*/(key_len + text_len + 1));
  /*
   * We leave it to the application to meet PNG-1.0 requirements on the
   * contents of the text.  PNG-1.0 through PNG-1.2 discourage the use of
   * any non-Latin-1 characters except for NEWLINE.  ISO PNG will forbid them.
   * The NUL character is forbidden by PNG-1.0 through PNG-1.2 and ISO PNG.
   */
  png_write_chunk_data(png_ptr, new_key, key_len + 1);

  if (text_len != 0)
     png_write_chunk_data(png_ptr, (png_const_bytep)text, text_len);

  png_write_chunk_end(png_ptr);
}
#endif

#ifdef PNG_WRITE_zTXt_SUPPORTED
/* Write a compressed text chunk */
void /* PRIVATE */
png_write_zTXt(png_structrp png_ptr, png_const_charp key, png_const_charp text,
   int compression)
{
  png_uint_32 key_len;
  png_byte new_key[81];
  compression_state comp;

  png_debug(1, "in png_write_zTXt");

  if (compression == PNG_TEXT_COMPRESSION_NONE)
  {
     png_write_tEXt(png_ptr, key, text, 0);
     return;
  }

  if (compression != PNG_TEXT_COMPRESSION_zTXt)
     png_error(png_ptr, "zTXt: invalid compression type");

  key_len = png_check_keyword(png_ptr, key, new_key);

  if (key_len == 0)
     png_error(png_ptr, "zTXt: invalid keyword");

  /* Add the compression method and 1 for the keyword separator. */
  new_key[++key_len] = PNG_COMPRESSION_TYPE_BASE;
  ++key_len;

  /* Compute the compressed data; do it now for the length */
  png_text_compress_init(&comp, (png_const_bytep)text,
      text == NULL ? 0 : strlen(text));

  if (png_text_compress(png_ptr, png_zTXt, &comp, key_len) != Z_OK)
     png_error(png_ptr, png_ptr->zstream.msg);

  /* Write start of chunk */
  png_write_chunk_header(png_ptr, png_zTXt, key_len + comp.output_len);

  /* Write key */
  png_write_chunk_data(png_ptr, new_key, key_len);

  /* Write the compressed data */
  png_write_compressed_data_out(png_ptr, &comp);

  /* Close the chunk */
  png_write_chunk_end(png_ptr);
}
#endif

#ifdef PNG_WRITE_iTXt_SUPPORTED
/* Write an iTXt chunk */
void /* PRIVATE */
png_write_iTXt(png_structrp png_ptr, int compression, png_const_charp key,
   png_const_charp lang, png_const_charp lang_key, png_const_charp text)
{
  png_uint_32 key_len, prefix_len;
  size_t lang_len, lang_key_len;
  png_byte new_key[82];
  compression_state comp;

  png_debug(1, "in png_write_iTXt");

  key_len = png_check_keyword(png_ptr, key, new_key);

  if (key_len == 0)
     png_error(png_ptr, "iTXt: invalid keyword");

  /* Set the compression flag */
  switch (compression)
  {
     case PNG_ITXT_COMPRESSION_NONE:
     case PNG_TEXT_COMPRESSION_NONE:
        compression = new_key[++key_len] = 0; /* no compression */
        break;

     case PNG_TEXT_COMPRESSION_zTXt:
     case PNG_ITXT_COMPRESSION_zTXt:
        compression = new_key[++key_len] = 1; /* compressed */
        break;

     default:
        png_error(png_ptr, "iTXt: invalid compression");
  }

  new_key[++key_len] = PNG_COMPRESSION_TYPE_BASE;
  ++key_len; /* for the keywod separator */

  /* We leave it to the application to meet PNG-1.0 requirements on the
   * contents of the text.  PNG-1.0 through PNG-1.2 discourage the use of
   * any non-Latin-1 characters except for NEWLINE.  ISO PNG, however,
   * specifies that the text is UTF-8 and this really doesn't require any
   * checking.
   *
   * The NUL character is forbidden by PNG-1.0 through PNG-1.2 and ISO PNG.
   *
   * TODO: validate the language tag correctly (see the spec.)
   */
  if (lang == NULL) lang = ""; /* empty language is valid */
  lang_len = strlen(lang)+1;
  if (lang_key == NULL) lang_key = ""; /* may be empty */
  lang_key_len = strlen(lang_key)+1;
  if (text == NULL) text = ""; /* may be empty */

  prefix_len = key_len;
  if (lang_len > PNG_UINT_31_MAX-prefix_len)
     prefix_len = PNG_UINT_31_MAX;
  else
     prefix_len = (png_uint_32)(prefix_len + lang_len);

  if (lang_key_len > PNG_UINT_31_MAX-prefix_len)
     prefix_len = PNG_UINT_31_MAX;
  else
     prefix_len = (png_uint_32)(prefix_len + lang_key_len);

  png_text_compress_init(&comp, (png_const_bytep)text, strlen(text));

  if (compression != 0)
  {
     if (png_text_compress(png_ptr, png_iTXt, &comp, prefix_len) != Z_OK)
        png_error(png_ptr, png_ptr->zstream.msg);
  }

  else
  {
     if (comp.input_len > PNG_UINT_31_MAX-prefix_len)
        png_error(png_ptr, "iTXt: uncompressed text too long");

     /* So the string will fit in a chunk: */
     comp.output_len = (png_uint_32)/*SAFE*/comp.input_len;
  }

  png_write_chunk_header(png_ptr, png_iTXt, comp.output_len + prefix_len);

  png_write_chunk_data(png_ptr, new_key, key_len);

  png_write_chunk_data(png_ptr, (png_const_bytep)lang, lang_len);

  png_write_chunk_data(png_ptr, (png_const_bytep)lang_key, lang_key_len);

  if (compression != 0)
     png_write_compressed_data_out(png_ptr, &comp);

  else
     png_write_chunk_data(png_ptr, (png_const_bytep)text, comp.output_len);

  png_write_chunk_end(png_ptr);
}
#endif

#ifdef PNG_WRITE_oFFs_SUPPORTED
/* Write the oFFs chunk */
void /* PRIVATE */
png_write_oFFs(png_structrp png_ptr, png_int_32 x_offset, png_int_32 y_offset,
   int unit_type)
{
  png_byte buf[9];

  png_debug(1, "in png_write_oFFs");

  if (unit_type >= PNG_OFFSET_LAST)
     png_warning(png_ptr, "Unrecognized unit type for oFFs chunk");

  png_save_int_32(buf, x_offset);
  png_save_int_32(buf + 4, y_offset);
  buf[8] = (png_byte)unit_type;

  png_write_complete_chunk(png_ptr, png_oFFs, buf, 9);
}
#endif
#ifdef PNG_WRITE_pCAL_SUPPORTED
/* Write the pCAL chunk (described in the PNG extensions document) */
void /* PRIVATE */
png_write_pCAL(png_structrp png_ptr, png_charp purpose, png_int_32 X0,
   png_int_32 X1, int type, int nparams, png_const_charp units,
   png_charpp params)
{
  png_uint_32 purpose_len;
  size_t units_len, total_len;
  size_t *params_len;
  png_byte buf[10];
  png_byte new_purpose[80];
  int i;

  png_debug1(1, "in png_write_pCAL (%d parameters)", nparams);

  if (type >= PNG_EQUATION_LAST)
     png_error(png_ptr, "Unrecognized equation type for pCAL chunk");

  purpose_len = png_check_keyword(png_ptr, purpose, new_purpose);

  if (purpose_len == 0)
     png_error(png_ptr, "pCAL: invalid keyword");

  ++purpose_len; /* terminator */

  png_debug1(3, "pCAL purpose length = %d", (int)purpose_len);
  units_len = strlen(units) + (nparams == 0 ? 0 : 1);
  png_debug1(3, "pCAL units length = %d", (int)units_len);
  total_len = purpose_len + units_len + 10;

  params_len = (size_t *)png_malloc(png_ptr,
      (png_alloc_size_t)((png_alloc_size_t)nparams * (sizeof (size_t))));

  /* Find the length of each parameter, making sure we don't count the
   * null terminator for the last parameter.
   */
  for (i = 0; i < nparams; i++)
  {
     params_len[i] = strlen(params[i]) + (i == nparams - 1 ? 0 : 1);
     png_debug2(3, "pCAL parameter %d length = %lu", i,
         (unsigned long)params_len[i]);
     total_len += params_len[i];
  }

  png_debug1(3, "pCAL total length = %d", (int)total_len);
  png_write_chunk_header(png_ptr, png_pCAL, (png_uint_32)total_len);
  png_write_chunk_data(png_ptr, new_purpose, purpose_len);
  png_save_int_32(buf, X0);
  png_save_int_32(buf + 4, X1);
  buf[8] = (png_byte)type;
  buf[9] = (png_byte)nparams;
  png_write_chunk_data(png_ptr, buf, 10);
  png_write_chunk_data(png_ptr, (png_const_bytep)units, (size_t)units_len);

  for (i = 0; i < nparams; i++)
  {
     png_write_chunk_data(png_ptr, (png_const_bytep)params[i], params_len[i]);
  }

  png_free(png_ptr, params_len);
  png_write_chunk_end(png_ptr);
}
#endif

#ifdef PNG_WRITE_sCAL_SUPPORTED
/* Write the sCAL chunk */
void /* PRIVATE */
png_write_sCAL_s(png_structrp png_ptr, int unit, png_const_charp width,
   png_const_charp height)
{
  png_byte buf[64];
  size_t wlen, hlen, total_len;

  png_debug(1, "in png_write_sCAL_s");

  wlen = strlen(width);
  hlen = strlen(height);
  total_len = wlen + hlen + 2;

  if (total_len > 64)
  {
     png_warning(png_ptr, "Can't write sCAL (buffer too small)");
     return;
  }

  buf[0] = (png_byte)unit;
  memcpy(buf + 1, width, wlen + 1);      /* Append the '\0' here */
  memcpy(buf + wlen + 2, height, hlen);  /* Do NOT append the '\0' here */

  png_debug1(3, "sCAL total length = %u", (unsigned int)total_len);
  png_write_complete_chunk(png_ptr, png_sCAL, buf, total_len);
}
#endif

#ifdef PNG_WRITE_pHYs_SUPPORTED
/* Write the pHYs chunk */
void /* PRIVATE */
png_write_pHYs(png_structrp png_ptr, png_uint_32 x_pixels_per_unit,
   png_uint_32 y_pixels_per_unit,
   int unit_type)
{
  png_byte buf[9];

  png_debug(1, "in png_write_pHYs");

  if (unit_type >= PNG_RESOLUTION_LAST)
     png_warning(png_ptr, "Unrecognized unit type for pHYs chunk");

  png_save_uint_32(buf, x_pixels_per_unit);
  png_save_uint_32(buf + 4, y_pixels_per_unit);
  buf[8] = (png_byte)unit_type;

  png_write_complete_chunk(png_ptr, png_pHYs, buf, 9);
}
#endif

#ifdef PNG_WRITE_tIME_SUPPORTED
/* Write the tIME chunk.  Use either png_convert_from_struct_tm()
* or png_convert_from_time_t(), or fill in the structure yourself.
*/
void /* PRIVATE */
png_write_tIME(png_structrp png_ptr, png_const_timep mod_time)
{
  png_byte buf[7];

  png_debug(1, "in png_write_tIME");

  if (mod_time->month  > 12 || mod_time->month  < 1 ||
      mod_time->day    > 31 || mod_time->day    < 1 ||
      mod_time->hour   > 23 || mod_time->second > 60)
  {
     png_warning(png_ptr, "Invalid time specified for tIME chunk");
     return;
  }

  png_save_uint_16(buf, mod_time->year);
  buf[2] = mod_time->month;
  buf[3] = mod_time->day;
  buf[4] = mod_time->hour;
  buf[5] = mod_time->minute;
  buf[6] = mod_time->second;

  png_write_complete_chunk(png_ptr, png_tIME, buf, 7);
}
#endif

#ifdef PNG_WRITE_APNG_SUPPORTED
void /* PRIVATE */
png_write_acTL(png_structp png_ptr,
   png_uint_32 num_frames, png_uint_32 num_plays)
{
   png_byte buf[8];

   png_debug(1, "in png_write_acTL");

   png_ptr->num_frames_to_write = num_frames;

   if ((png_ptr->apng_flags & PNG_FIRST_FRAME_HIDDEN) != 0)
       num_frames--;

   png_save_uint_32(buf, num_frames);
   png_save_uint_32(buf + 4, num_plays);

   png_write_complete_chunk(png_ptr, png_acTL, buf, (png_size_t)8);
}

void /* PRIVATE */
png_write_fcTL(png_structp png_ptr, png_uint_32 width, png_uint_32 height,
   png_uint_32 x_offset, png_uint_32 y_offset,
   png_uint_16 delay_num, png_uint_16 delay_den, png_byte dispose_op,
   png_byte blend_op)
{
   png_byte buf[26];

   png_debug(1, "in png_write_fcTL");

   if (png_ptr->num_frames_written == 0 && (x_offset != 0 || y_offset != 0))
       png_error(png_ptr, "x and/or y offset for the first frame aren't 0");
   if (png_ptr->num_frames_written == 0 &&
       (width != png_ptr->first_frame_width ||
        height != png_ptr->first_frame_height))
       png_error(png_ptr, "width and/or height in the first frame's fcTL "
                          "don't match the ones in IHDR");

   /* more error checking */
   png_ensure_fcTL_is_valid(png_ptr, width, height, x_offset, y_offset,
                            delay_num, delay_den, dispose_op, blend_op);

   png_save_uint_32(buf, png_ptr->next_seq_num);
   png_save_uint_32(buf + 4, width);
   png_save_uint_32(buf + 8, height);
   png_save_uint_32(buf + 12, x_offset);
   png_save_uint_32(buf + 16, y_offset);
   png_save_uint_16(buf + 20, delay_num);
   png_save_uint_16(buf + 22, delay_den);
   buf[24] = dispose_op;
   buf[25] = blend_op;

   png_write_complete_chunk(png_ptr, png_fcTL, buf, (png_size_t)26);

   png_ptr->next_seq_num++;
}

void /* PRIVATE */
png_write_fdAT(png_structp png_ptr,
   png_const_bytep data, png_size_t length)
{
   png_byte buf[4];

   png_write_chunk_header(png_ptr, png_fdAT, (png_uint_32)(4 + length));

   png_save_uint_32(buf, png_ptr->next_seq_num);
   png_write_chunk_data(png_ptr, buf, 4);

   png_write_chunk_data(png_ptr, data, length);

   png_write_chunk_end(png_ptr);

   png_ptr->next_seq_num++;
}
#endif /* WRITE_APNG */

/* Initializes the row writing capability of libpng */
void /* PRIVATE */
png_write_start_row(png_structrp png_ptr)
{
  png_alloc_size_t buf_size;
  int usr_pixel_depth;

#ifdef PNG_WRITE_FILTER_SUPPORTED
  png_byte filters;
#endif

  png_debug(1, "in png_write_start_row");

  usr_pixel_depth = png_ptr->usr_channels * png_ptr->usr_bit_depth;
  buf_size = PNG_ROWBYTES(usr_pixel_depth, png_ptr->width) + 1;

  /* 1.5.6: added to allow checking in the row write code. */
  png_ptr->transformed_pixel_depth = png_ptr->pixel_depth;
  png_ptr->maximum_pixel_depth = (png_byte)usr_pixel_depth;

  /* Set up row buffer */
  png_ptr->row_buf = png_voidcast(png_bytep, png_malloc(png_ptr, buf_size));

  png_ptr->row_buf[0] = PNG_FILTER_VALUE_NONE;

#ifdef PNG_WRITE_FILTER_SUPPORTED
  filters = png_ptr->do_filter;

  if (png_ptr->height == 1)
     filters &= 0xff & ~(PNG_FILTER_UP|PNG_FILTER_AVG|PNG_FILTER_PAETH);

  if (png_ptr->width == 1)
     filters &= 0xff & ~(PNG_FILTER_SUB|PNG_FILTER_AVG|PNG_FILTER_PAETH);

  if (filters == 0)
     filters = PNG_FILTER_NONE;

  png_ptr->do_filter = filters;

  if (((filters & (PNG_FILTER_SUB | PNG_FILTER_UP | PNG_FILTER_AVG |
      PNG_FILTER_PAETH)) != 0) && png_ptr->try_row == NULL)
  {
     int num_filters = 0;

     png_ptr->try_row = png_voidcast(png_bytep, png_malloc(png_ptr, buf_size));

     if (filters & PNG_FILTER_SUB)
        num_filters++;

     if (filters & PNG_FILTER_UP)
        num_filters++;

     if (filters & PNG_FILTER_AVG)
        num_filters++;

     if (filters & PNG_FILTER_PAETH)
        num_filters++;

     if (num_filters > 1)
        png_ptr->tst_row = png_voidcast(png_bytep, png_malloc(png_ptr,
            buf_size));
  }

  /* We only need to keep the previous row if we are using one of the following
   * filters.
   */
  if ((filters & (PNG_FILTER_AVG | PNG_FILTER_UP | PNG_FILTER_PAETH)) != 0)
     png_ptr->prev_row = png_voidcast(png_bytep,
         png_calloc(png_ptr, buf_size));
#endif /* WRITE_FILTER */

#ifdef PNG_WRITE_INTERLACING_SUPPORTED
  /* If interlaced, we need to set up width and height of pass */
  if (png_ptr->interlaced != 0)
  {
     if ((png_ptr->transformations & PNG_INTERLACE) == 0)
     {
        png_ptr->num_rows = (png_ptr->height + png_pass_yinc[0] - 1 -
            png_pass_ystart[0]) / png_pass_yinc[0];

        png_ptr->usr_width = (png_ptr->width + png_pass_inc[0] - 1 -
            png_pass_start[0]) / png_pass_inc[0];
     }

     else
     {
        png_ptr->num_rows = png_ptr->height;
        png_ptr->usr_width = png_ptr->width;
     }
  }

  else
#endif
  {
     png_ptr->num_rows = png_ptr->height;
     png_ptr->usr_width = png_ptr->width;
  }
}

/* Internal use only.  Called when finished processing a row of data. */
void /* PRIVATE */
png_write_finish_row(png_structrp png_ptr)
{
  png_debug(1, "in png_write_finish_row");

  /* Next row */
  png_ptr->row_number++;

  /* See if we are done */
  if (png_ptr->row_number < png_ptr->num_rows)
     return;

#ifdef PNG_WRITE_INTERLACING_SUPPORTED
  /* If interlaced, go to next pass */
  if (png_ptr->interlaced != 0)
  {
     png_ptr->row_number = 0;
     if ((png_ptr->transformations & PNG_INTERLACE) != 0)
     {
        png_ptr->pass++;
     }

     else
     {
        /* Loop until we find a non-zero width or height pass */
        do
        {
           png_ptr->pass++;

           if (png_ptr->pass >= 7)
              break;

           png_ptr->usr_width = (png_ptr->width +
               png_pass_inc[png_ptr->pass] - 1 -
               png_pass_start[png_ptr->pass]) /
               png_pass_inc[png_ptr->pass];

           png_ptr->num_rows = (png_ptr->height +
               png_pass_yinc[png_ptr->pass] - 1 -
               png_pass_ystart[png_ptr->pass]) /
               png_pass_yinc[png_ptr->pass];

           if ((png_ptr->transformations & PNG_INTERLACE) != 0)
              break;

        } while (png_ptr->usr_width == 0 || png_ptr->num_rows == 0);

     }

     /* Reset the row above the image for the next pass */
     if (png_ptr->pass < 7)
     {
        if (png_ptr->prev_row != NULL)
           memset(png_ptr->prev_row, 0,
               PNG_ROWBYTES(png_ptr->usr_channels *
               png_ptr->usr_bit_depth, png_ptr->width) + 1);

        return;
     }
  }
#endif

  /* If we get here, we've just written the last row, so we need
     to flush the compressor */
  png_compress_IDAT(png_ptr, NULL, 0, Z_FINISH);
}

#ifdef PNG_WRITE_INTERLACING_SUPPORTED
/* Pick out the correct pixels for the interlace pass.
* The basic idea here is to go through the row with a source
* pointer and a destination pointer (sp and dp), and copy the
* correct pixels for the pass.  As the row gets compacted,
* sp will always be >= dp, so we should never overwrite anything.
* See the default: case for the easiest code to understand.
*/
void /* PRIVATE */
png_do_write_interlace(png_row_infop row_info, png_bytep row, int pass)
{
  png_debug(1, "in png_do_write_interlace");

  /* We don't have to do anything on the last pass (6) */
  if (pass < 6)
  {
     /* Each pixel depth is handled separately */
     switch (row_info->pixel_depth)
     {
        case 1:
        {
           png_bytep sp;
           png_bytep dp;
           unsigned int shift;
           int d;
           int value;
           png_uint_32 i;
           png_uint_32 row_width = row_info->width;

           dp = row;
           d = 0;
           shift = 7;

           for (i = png_pass_start[pass]; i < row_width;
              i += png_pass_inc[pass])
           {
              sp = row + (size_t)(i >> 3);
              value = (int)(*sp >> (7 - (int)(i & 0x07))) & 0x01;
              d |= (value << shift);

              if (shift == 0)
              {
                 shift = 7;
                 *dp++ = (png_byte)d;
                 d = 0;
              }

              else
                 shift--;

           }
           if (shift != 7)
              *dp = (png_byte)d;

           break;
        }

        case 2:
        {
           png_bytep sp;
           png_bytep dp;
           unsigned int shift;
           int d;
           int value;
           png_uint_32 i;
           png_uint_32 row_width = row_info->width;

           dp = row;
           shift = 6;
           d = 0;

           for (i = png_pass_start[pass]; i < row_width;
              i += png_pass_inc[pass])
           {
              sp = row + (size_t)(i >> 2);
              value = (*sp >> ((3 - (int)(i & 0x03)) << 1)) & 0x03;
              d |= (value << shift);

              if (shift == 0)
              {
                 shift = 6;
                 *dp++ = (png_byte)d;
                 d = 0;
              }

              else
                 shift -= 2;
           }
           if (shift != 6)
              *dp = (png_byte)d;

           break;
        }

        case 4:
        {
           png_bytep sp;
           png_bytep dp;
           unsigned int shift;
           int d;
           int value;
           png_uint_32 i;
           png_uint_32 row_width = row_info->width;

           dp = row;
           shift = 4;
           d = 0;
           for (i = png_pass_start[pass]; i < row_width;
               i += png_pass_inc[pass])
           {
              sp = row + (size_t)(i >> 1);
              value = (*sp >> ((1 - (int)(i & 0x01)) << 2)) & 0x0f;
              d |= (value << shift);

              if (shift == 0)
              {
                 shift = 4;
                 *dp++ = (png_byte)d;
                 d = 0;
              }

              else
                 shift -= 4;
           }
           if (shift != 4)
              *dp = (png_byte)d;

           break;
        }

        default:
        {
           png_bytep sp;
           png_bytep dp;
           png_uint_32 i;
           png_uint_32 row_width = row_info->width;
           size_t pixel_bytes;

           /* Start at the beginning */
           dp = row;

           /* Find out how many bytes each pixel takes up */
           pixel_bytes = (row_info->pixel_depth >> 3);

           /* Loop through the row, only looking at the pixels that matter */
           for (i = png_pass_start[pass]; i < row_width;
              i += png_pass_inc[pass])
           {
              /* Find out where the original pixel is */
              sp = row + (size_t)i * pixel_bytes;

              /* Move the pixel */
              if (dp != sp)
                 memcpy(dp, sp, pixel_bytes);

              /* Next pixel */
              dp += pixel_bytes;
           }
           break;
        }
     }
     /* Set new row width */
     row_info->width = (row_info->width +
         png_pass_inc[pass] - 1 -
         png_pass_start[pass]) /
         png_pass_inc[pass];

     row_info->rowbytes = PNG_ROWBYTES(row_info->pixel_depth,
         row_info->width);
  }
}
#endif


/* This filters the row, chooses which filter to use, if it has not already
* been specified by the application, and then writes the row out with the
* chosen filter.
*/
static void /* PRIVATE */
png_write_filtered_row(png_structrp png_ptr, png_bytep filtered_row,
   size_t row_bytes);

#ifdef PNG_WRITE_FILTER_SUPPORTED
static size_t /* PRIVATE */
png_setup_sub_row(png_structrp png_ptr, png_uint_32 bpp,
   size_t row_bytes, size_t lmins)
{
  png_bytep rp, dp, lp;
  size_t i;
  size_t sum = 0;
  unsigned int v;

  png_ptr->try_row[0] = PNG_FILTER_VALUE_SUB;

  for (i = 0, rp = png_ptr->row_buf + 1, dp = png_ptr->try_row + 1; i < bpp;
       i++, rp++, dp++)
  {
     v = *dp = *rp;
#ifdef PNG_USE_ABS
     sum += 128 - abs((int)v - 128);
#else
     sum += (v < 128) ? v : 256 - v;
#endif
  }

  for (lp = png_ptr->row_buf + 1; i < row_bytes;
     i++, rp++, lp++, dp++)
  {
     v = *dp = (png_byte)(((int)*rp - (int)*lp) & 0xff);
#ifdef PNG_USE_ABS
     sum += 128 - abs((int)v - 128);
#else
     sum += (v < 128) ? v : 256 - v;
#endif

     if (sum > lmins)  /* We are already worse, don't continue. */
       break;
  }

  return sum;
}

static void /* PRIVATE */
png_setup_sub_row_only(png_structrp png_ptr, png_uint_32 bpp,
   size_t row_bytes)
{
  png_bytep rp, dp, lp;
  size_t i;

  png_ptr->try_row[0] = PNG_FILTER_VALUE_SUB;

  for (i = 0, rp = png_ptr->row_buf + 1, dp = png_ptr->try_row + 1; i < bpp;
       i++, rp++, dp++)
  {
     *dp = *rp;
  }

  for (lp = png_ptr->row_buf + 1; i < row_bytes;
     i++, rp++, lp++, dp++)
  {
     *dp = (png_byte)(((int)*rp - (int)*lp) & 0xff);
  }
}

static size_t /* PRIVATE */
png_setup_up_row(png_structrp png_ptr, size_t row_bytes, size_t lmins)
{
  png_bytep rp, dp, pp;
  size_t i;
  size_t sum = 0;
  unsigned int v;

  png_ptr->try_row[0] = PNG_FILTER_VALUE_UP;

  for (i = 0, rp = png_ptr->row_buf + 1, dp = png_ptr->try_row + 1,
      pp = png_ptr->prev_row + 1; i < row_bytes;
      i++, rp++, pp++, dp++)
  {
     v = *dp = (png_byte)(((int)*rp - (int)*pp) & 0xff);
#ifdef PNG_USE_ABS
     sum += 128 - abs((int)v - 128);
#else
     sum += (v < 128) ? v : 256 - v;
#endif

     if (sum > lmins)  /* We are already worse, don't continue. */
       break;
  }

  return sum;
}
static void /* PRIVATE */
png_setup_up_row_only(png_structrp png_ptr, size_t row_bytes)
{
  png_bytep rp, dp, pp;
  size_t i;

  png_ptr->try_row[0] = PNG_FILTER_VALUE_UP;

  for (i = 0, rp = png_ptr->row_buf + 1, dp = png_ptr->try_row + 1,
      pp = png_ptr->prev_row + 1; i < row_bytes;
      i++, rp++, pp++, dp++)
  {
     *dp = (png_byte)(((int)*rp - (int)*pp) & 0xff);
  }
}

static size_t /* PRIVATE */
png_setup_avg_row(png_structrp png_ptr, png_uint_32 bpp,
   size_t row_bytes, size_t lmins)
{
  png_bytep rp, dp, pp, lp;
  png_uint_32 i;
  size_t sum = 0;
  unsigned int v;

  png_ptr->try_row[0] = PNG_FILTER_VALUE_AVG;

  for (i = 0, rp = png_ptr->row_buf + 1, dp = png_ptr->try_row + 1,
      pp = png_ptr->prev_row + 1; i < bpp; i++)
  {
     v = *dp++ = (png_byte)(((int)*rp++ - ((int)*pp++ / 2)) & 0xff);

#ifdef PNG_USE_ABS
     sum += 128 - abs((int)v - 128);
#else
     sum += (v < 128) ? v : 256 - v;
#endif
  }

  for (lp = png_ptr->row_buf + 1; i < row_bytes; i++)
  {
     v = *dp++ = (png_byte)(((int)*rp++ - (((int)*pp++ + (int)*lp++) / 2))
         & 0xff);

#ifdef PNG_USE_ABS
     sum += 128 - abs((int)v - 128);
#else
     sum += (v < 128) ? v : 256 - v;
#endif

     if (sum > lmins)  /* We are already worse, don't continue. */
       break;
  }

  return sum;
}
static void /* PRIVATE */
png_setup_avg_row_only(png_structrp png_ptr, png_uint_32 bpp,
   size_t row_bytes)
{
  png_bytep rp, dp, pp, lp;
  png_uint_32 i;

  png_ptr->try_row[0] = PNG_FILTER_VALUE_AVG;

  for (i = 0, rp = png_ptr->row_buf + 1, dp = png_ptr->try_row + 1,
      pp = png_ptr->prev_row + 1; i < bpp; i++)
  {
     *dp++ = (png_byte)(((int)*rp++ - ((int)*pp++ / 2)) & 0xff);
  }

  for (lp = png_ptr->row_buf + 1; i < row_bytes; i++)
  {
     *dp++ = (png_byte)(((int)*rp++ - (((int)*pp++ + (int)*lp++) / 2))
         & 0xff);
  }
}

static size_t /* PRIVATE */
png_setup_paeth_row(png_structrp png_ptr, png_uint_32 bpp,
   size_t row_bytes, size_t lmins)
{
  png_bytep rp, dp, pp, cp, lp;
  size_t i;
  size_t sum = 0;
  unsigned int v;

  png_ptr->try_row[0] = PNG_FILTER_VALUE_PAETH;

  for (i = 0, rp = png_ptr->row_buf + 1, dp = png_ptr->try_row + 1,
      pp = png_ptr->prev_row + 1; i < bpp; i++)
  {
     v = *dp++ = (png_byte)(((int)*rp++ - (int)*pp++) & 0xff);

#ifdef PNG_USE_ABS
     sum += 128 - abs((int)v - 128);
#else
     sum += (v < 128) ? v : 256 - v;
#endif
  }

  for (lp = png_ptr->row_buf + 1, cp = png_ptr->prev_row + 1; i < row_bytes;
       i++)
  {
     int a, b, c, pa, pb, pc, p;

     b = *pp++;
     c = *cp++;
     a = *lp++;

     p = b - c;
     pc = a - c;

#ifdef PNG_USE_ABS
     pa = abs(p);
     pb = abs(pc);
     pc = abs(p + pc);
#else
     pa = p < 0 ? -p : p;
     pb = pc < 0 ? -pc : pc;
     pc = (p + pc) < 0 ? -(p + pc) : p + pc;
#endif

     p = (pa <= pb && pa <=pc) ? a : (pb <= pc) ? b : c;

     v = *dp++ = (png_byte)(((int)*rp++ - p) & 0xff);

#ifdef PNG_USE_ABS
     sum += 128 - abs((int)v - 128);
#else
     sum += (v < 128) ? v : 256 - v;
#endif

     if (sum > lmins)  /* We are already worse, don't continue. */
       break;
  }

  return sum;
}
static void /* PRIVATE */
png_setup_paeth_row_only(png_structrp png_ptr, png_uint_32 bpp,
   size_t row_bytes)
{
  png_bytep rp, dp, pp, cp, lp;
  size_t i;

  png_ptr->try_row[0] = PNG_FILTER_VALUE_PAETH;

  for (i = 0, rp = png_ptr->row_buf + 1, dp = png_ptr->try_row + 1,
      pp = png_ptr->prev_row + 1; i < bpp; i++)
  {
     *dp++ = (png_byte)(((int)*rp++ - (int)*pp++) & 0xff);
  }

  for (lp = png_ptr->row_buf + 1, cp = png_ptr->prev_row + 1; i < row_bytes;
       i++)
  {
     int a, b, c, pa, pb, pc, p;

     b = *pp++;
     c = *cp++;
     a = *lp++;

     p = b - c;
     pc = a - c;

#ifdef PNG_USE_ABS
     pa = abs(p);
     pb = abs(pc);
     pc = abs(p + pc);
#else
     pa = p < 0 ? -p : p;
     pb = pc < 0 ? -pc : pc;
     pc = (p + pc) < 0 ? -(p + pc) : p + pc;
#endif

     p = (pa <= pb && pa <=pc) ? a : (pb <= pc) ? b : c;

     *dp++ = (png_byte)(((int)*rp++ - p) & 0xff);
  }
}
#endif /* WRITE_FILTER */

void /* PRIVATE */
png_write_find_filter(png_structrp png_ptr, png_row_infop row_info)
{
#ifndef PNG_WRITE_FILTER_SUPPORTED
  png_write_filtered_row(png_ptr, png_ptr->row_buf, row_info->rowbytes+1);
#else
  unsigned int filter_to_do = png_ptr->do_filter;
  png_bytep row_buf;
  png_bytep best_row;
  png_uint_32 bpp;
  size_t mins;
  size_t row_bytes = row_info->rowbytes;

  png_debug(1, "in png_write_find_filter");

  /* Find out how many bytes offset each pixel is */
  bpp = (row_info->pixel_depth + 7) >> 3;

  row_buf = png_ptr->row_buf;
  mins = PNG_SIZE_MAX - 256/* so we can detect potential overflow of the
                              running sum */;

  /* The prediction method we use is to find which method provides the
   * smallest value when summing the absolute values of the distances
   * from zero, using anything >= 128 as negative numbers.  This is known
   * as the "minimum sum of absolute differences" heuristic.  Other
   * heuristics are the "weighted minimum sum of absolute differences"
   * (experimental and can in theory improve compression), and the "zlib
   * predictive" method (not implemented yet), which does test compressions
   * of lines using different filter methods, and then chooses the
   * (series of) filter(s) that give minimum compressed data size (VERY
   * computationally expensive).
   *
   * GRR 980525:  consider also
   *
   *   (1) minimum sum of absolute differences from running average (i.e.,
   *       keep running sum of non-absolute differences & count of bytes)
   *       [track dispersion, too?  restart average if dispersion too large?]
   *
   *  (1b) minimum sum of absolute differences from sliding average, probably
   *       with window size <= deflate window (usually 32K)
   *
   *   (2) minimum sum of squared differences from zero or running average
   *       (i.e., ~ root-mean-square approach)
   */


  /* We don't need to test the 'no filter' case if this is the only filter
   * that has been chosen, as it doesn't actually do anything to the data.
   */
  best_row = png_ptr->row_buf;

  if (PNG_SIZE_MAX/128 <= row_bytes)
  {
     /* Overflow can occur in the calculation, just select the lowest set
      * filter.
      */
     filter_to_do &= 0U-filter_to_do;
  }
  else if ((filter_to_do & PNG_FILTER_NONE) != 0 &&
        filter_to_do != PNG_FILTER_NONE)
  {
     /* Overflow not possible and multiple filters in the list, including the
      * 'none' filter.
      */
     png_bytep rp;
     size_t sum = 0;
     size_t i;
     unsigned int v;

     {
        for (i = 0, rp = row_buf + 1; i < row_bytes; i++, rp++)
        {
           v = *rp;
#ifdef PNG_USE_ABS
           sum += 128 - abs((int)v - 128);
#else
           sum += (v < 128) ? v : 256 - v;
#endif
        }
     }

     mins = sum;
  }

  /* Sub filter */
  if (filter_to_do == PNG_FILTER_SUB)
  /* It's the only filter so no testing is needed */
  {
     png_setup_sub_row_only(png_ptr, bpp, row_bytes);
     best_row = png_ptr->try_row;
  }

  else if ((filter_to_do & PNG_FILTER_SUB) != 0)
  {
     size_t sum;
     size_t lmins = mins;

     sum = png_setup_sub_row(png_ptr, bpp, row_bytes, lmins);

     if (sum < mins)
     {
        mins = sum;
        best_row = png_ptr->try_row;
        if (png_ptr->tst_row != NULL)
        {
           png_ptr->try_row = png_ptr->tst_row;
           png_ptr->tst_row = best_row;
        }
     }
  }

  /* Up filter */
  if (filter_to_do == PNG_FILTER_UP)
  {
     png_setup_up_row_only(png_ptr, row_bytes);
     best_row = png_ptr->try_row;
  }

  else if ((filter_to_do & PNG_FILTER_UP) != 0)
  {
     size_t sum;
     size_t lmins = mins;

     sum = png_setup_up_row(png_ptr, row_bytes, lmins);

     if (sum < mins)
     {
        mins = sum;
        best_row = png_ptr->try_row;
        if (png_ptr->tst_row != NULL)
        {
           png_ptr->try_row = png_ptr->tst_row;
           png_ptr->tst_row = best_row;
        }
     }
  }

  /* Avg filter */
  if (filter_to_do == PNG_FILTER_AVG)
  {
     png_setup_avg_row_only(png_ptr, bpp, row_bytes);
     best_row = png_ptr->try_row;
  }

  else if ((filter_to_do & PNG_FILTER_AVG) != 0)
  {
     size_t sum;
     size_t lmins = mins;

     sum= png_setup_avg_row(png_ptr, bpp, row_bytes, lmins);

     if (sum < mins)
     {
        mins = sum;
        best_row = png_ptr->try_row;
        if (png_ptr->tst_row != NULL)
        {
           png_ptr->try_row = png_ptr->tst_row;
           png_ptr->tst_row = best_row;
        }
     }
  }

  /* Paeth filter */
  if (filter_to_do == PNG_FILTER_PAETH)
  {
     png_setup_paeth_row_only(png_ptr, bpp, row_bytes);
     best_row = png_ptr->try_row;
  }

  else if ((filter_to_do & PNG_FILTER_PAETH) != 0)
  {
     size_t sum;
     size_t lmins = mins;

     sum = png_setup_paeth_row(png_ptr, bpp, row_bytes, lmins);

     if (sum < mins)
     {
        best_row = png_ptr->try_row;
        if (png_ptr->tst_row != NULL)
        {
           png_ptr->try_row = png_ptr->tst_row;
           png_ptr->tst_row = best_row;
        }
     }
  }

  /* Do the actual writing of the filtered row data from the chosen filter. */
  png_write_filtered_row(png_ptr, best_row, row_info->rowbytes+1);

#endif /* WRITE_FILTER */
}


/* Do the actual writing of a previously filtered row. */
static void
png_write_filtered_row(png_structrp png_ptr, png_bytep filtered_row,
   size_t full_row_length/*includes filter byte*/)
{
  png_debug(1, "in png_write_filtered_row");

  png_debug1(2, "filter = %d", filtered_row[0]);

  png_compress_IDAT(png_ptr, filtered_row, full_row_length, Z_NO_FLUSH);

#ifdef PNG_WRITE_FILTER_SUPPORTED
  /* Swap the current and previous rows */
  if (png_ptr->prev_row != NULL)
  {
     png_bytep tptr;

     tptr = png_ptr->prev_row;
     png_ptr->prev_row = png_ptr->row_buf;
     png_ptr->row_buf = tptr;
  }
#endif /* WRITE_FILTER */

  /* Finish row - updates counters and flushes zlib if last row */
  png_write_finish_row(png_ptr);

#ifdef PNG_WRITE_FLUSH_SUPPORTED
  png_ptr->flush_rows++;

  if (png_ptr->flush_dist > 0 &&
      png_ptr->flush_rows >= png_ptr->flush_dist)
  {
     png_write_flush(png_ptr);
  }
#endif /* WRITE_FLUSH */
}

#ifdef PNG_WRITE_APNG_SUPPORTED
void /* PRIVATE */
png_write_reset(png_structp png_ptr)
{
   png_ptr->row_number = 0;
   png_ptr->pass = 0;
   png_ptr->mode &= ~PNG_HAVE_IDAT;
}

void /* PRIVATE */
png_write_reinit(png_structp png_ptr, png_infop info_ptr,
                png_uint_32 width, png_uint_32 height)
{
   if (png_ptr->num_frames_written == 0 &&
       (width != png_ptr->first_frame_width ||
        height != png_ptr->first_frame_height))
       png_error(png_ptr, "width and/or height in the first frame's fcTL "
                          "don't match the ones in IHDR");
   if (width > png_ptr->first_frame_width ||
       height > png_ptr->first_frame_height)
       png_error(png_ptr, "width and/or height for a frame greater than "
                          "the ones in IHDR");

   png_set_IHDR(png_ptr, info_ptr, width, height,
                info_ptr->bit_depth, info_ptr->color_type,
                info_ptr->interlace_type, info_ptr->compression_type,
                info_ptr->filter_type);

   png_ptr->width = width;
   png_ptr->height = height;
   png_ptr->rowbytes = PNG_ROWBYTES(png_ptr->pixel_depth, width);
   png_ptr->usr_width = png_ptr->width;
}
#endif /* WRITE_APNG */
#endif /* WRITE */