tor-browser

jquant1.c (32887B)

---

/*
* jquant1.c
*
* This file was part of the Independent JPEG Group's software:
* Copyright (C) 1991-1996, Thomas G. Lane.
* libjpeg-turbo Modifications:
* Copyright (C) 2009, 2015, 2022-2023, D. R. Commander.
* For conditions of distribution and use, see the accompanying README.ijg
* file.
*
* This file contains 1-pass color quantization (color mapping) routines.
* These routines provide mapping to a fixed color map using equally spaced
* color values.  Optional Floyd-Steinberg or ordered dithering is available.
*/

#define JPEG_INTERNALS
#include "jinclude.h"
#include "jpeglib.h"
#include "jsamplecomp.h"

#if defined(QUANT_1PASS_SUPPORTED) && BITS_IN_JSAMPLE != 16


/*
* The main purpose of 1-pass quantization is to provide a fast, if not very
* high quality, colormapped output capability.  A 2-pass quantizer usually
* gives better visual quality; however, for quantized grayscale output this
* quantizer is perfectly adequate.  Dithering is highly recommended with this
* quantizer, though you can turn it off if you really want to.
*
* In 1-pass quantization the colormap must be chosen in advance of seeing the
* image.  We use a map consisting of all combinations of Ncolors[i] color
* values for the i'th component.  The Ncolors[] values are chosen so that
* their product, the total number of colors, is no more than that requested.
* (In most cases, the product will be somewhat less.)
*
* Since the colormap is orthogonal, the representative value for each color
* component can be determined without considering the other components;
* then these indexes can be combined into a colormap index by a standard
* N-dimensional-array-subscript calculation.  Most of the arithmetic involved
* can be precalculated and stored in the lookup table colorindex[].
* colorindex[i][j] maps pixel value j in component i to the nearest
* representative value (grid plane) for that component; this index is
* multiplied by the array stride for component i, so that the
* index of the colormap entry closest to a given pixel value is just
*    sum( colorindex[component-number][pixel-component-value] )
* Aside from being fast, this scheme allows for variable spacing between
* representative values with no additional lookup cost.
*
* If gamma correction has been applied in color conversion, it might be wise
* to adjust the color grid spacing so that the representative colors are
* equidistant in linear space.  At this writing, gamma correction is not
* implemented by jdcolor, so nothing is done here.
*/


/* Declarations for ordered dithering.
*
* We use a standard 16x16 ordered dither array.  The basic concept of ordered
* dithering is described in many references, for instance Dale Schumacher's
* chapter II.2 of Graphics Gems II (James Arvo, ed. Academic Press, 1991).
* In place of Schumacher's comparisons against a "threshold" value, we add a
* "dither" value to the input pixel and then round the result to the nearest
* output value.  The dither value is equivalent to (0.5 - threshold) times
* the distance between output values.  For ordered dithering, we assume that
* the output colors are equally spaced; if not, results will probably be
* worse, since the dither may be too much or too little at a given point.
*
* The normal calculation would be to form pixel value + dither, range-limit
* this to 0.._MAXJSAMPLE, and then index into the colorindex table as usual.
* We can skip the separate range-limiting step by extending the colorindex
* table in both directions.
*/

#define ODITHER_SIZE  16        /* dimension of dither matrix */
/* NB: if ODITHER_SIZE is not a power of 2, ODITHER_MASK uses will break */
#define ODITHER_CELLS  (ODITHER_SIZE * ODITHER_SIZE) /* # cells in matrix */
#define ODITHER_MASK  (ODITHER_SIZE - 1) /* mask for wrapping around
                                           counters */

typedef int ODITHER_MATRIX[ODITHER_SIZE][ODITHER_SIZE];
typedef int (*ODITHER_MATRIX_PTR)[ODITHER_SIZE];

static const UINT8 base_dither_matrix[ODITHER_SIZE][ODITHER_SIZE] = {
 /* Bayer's order-4 dither array.  Generated by the code given in
  * Stephen Hawley's article "Ordered Dithering" in Graphics Gems I.
  * The values in this array must range from 0 to ODITHER_CELLS-1.
  */
 {   0, 192,  48, 240,  12, 204,  60, 252,   3, 195,  51, 243,  15, 207,  63, 255 },
 { 128,  64, 176, 112, 140,  76, 188, 124, 131,  67, 179, 115, 143,  79, 191, 127 },
 {  32, 224,  16, 208,  44, 236,  28, 220,  35, 227,  19, 211,  47, 239,  31, 223 },
 { 160,  96, 144,  80, 172, 108, 156,  92, 163,  99, 147,  83, 175, 111, 159,  95 },
 {   8, 200,  56, 248,   4, 196,  52, 244,  11, 203,  59, 251,   7, 199,  55, 247 },
 { 136,  72, 184, 120, 132,  68, 180, 116, 139,  75, 187, 123, 135,  71, 183, 119 },
 {  40, 232,  24, 216,  36, 228,  20, 212,  43, 235,  27, 219,  39, 231,  23, 215 },
 { 168, 104, 152,  88, 164, 100, 148,  84, 171, 107, 155,  91, 167, 103, 151,  87 },
 {   2, 194,  50, 242,  14, 206,  62, 254,   1, 193,  49, 241,  13, 205,  61, 253 },
 { 130,  66, 178, 114, 142,  78, 190, 126, 129,  65, 177, 113, 141,  77, 189, 125 },
 {  34, 226,  18, 210,  46, 238,  30, 222,  33, 225,  17, 209,  45, 237,  29, 221 },
 { 162,  98, 146,  82, 174, 110, 158,  94, 161,  97, 145,  81, 173, 109, 157,  93 },
 {  10, 202,  58, 250,   6, 198,  54, 246,   9, 201,  57, 249,   5, 197,  53, 245 },
 { 138,  74, 186, 122, 134,  70, 182, 118, 137,  73, 185, 121, 133,  69, 181, 117 },
 {  42, 234,  26, 218,  38, 230,  22, 214,  41, 233,  25, 217,  37, 229,  21, 213 },
 { 170, 106, 154,  90, 166, 102, 150,  86, 169, 105, 153,  89, 165, 101, 149,  85 }
};


/* Declarations for Floyd-Steinberg dithering.
*
* Errors are accumulated into the array fserrors[], at a resolution of
* 1/16th of a pixel count.  The error at a given pixel is propagated
* to its not-yet-processed neighbors using the standard F-S fractions,
*              ...     (here)  7/16
*              3/16    5/16    1/16
* We work left-to-right on even rows, right-to-left on odd rows.
*
* We can get away with a single array (holding one row's worth of errors)
* by using it to store the current row's errors at pixel columns not yet
* processed, but the next row's errors at columns already processed.  We
* need only a few extra variables to hold the errors immediately around the
* current column.  (If we are lucky, those variables are in registers, but
* even if not, they're probably cheaper to access than array elements are.)
*
* The fserrors[] array is indexed [component#][position].
* We provide (#columns + 2) entries per component; the extra entry at each
* end saves us from special-casing the first and last pixels.
*/

#if BITS_IN_JSAMPLE == 8
typedef INT16 FSERROR;          /* 16 bits should be enough */
typedef int LOCFSERROR;         /* use 'int' for calculation temps */
#else
typedef JLONG FSERROR;          /* may need more than 16 bits */
typedef JLONG LOCFSERROR;       /* be sure calculation temps are big enough */
#endif

typedef FSERROR *FSERRPTR;      /* pointer to error array */


/* Private subobject */

#define MAX_Q_COMPS  4          /* max components I can handle */

typedef struct {
 struct jpeg_color_quantizer pub; /* public fields */

 /* Initially allocated colormap is saved here */
 _JSAMPARRAY sv_colormap;      /* The color map as a 2-D pixel array */
 int sv_actual;                /* number of entries in use */

 _JSAMPARRAY colorindex;       /* Precomputed mapping for speed */
 /* colorindex[i][j] = index of color closest to pixel value j in component i,
  * premultiplied as described above.  Since colormap indexes must fit into
  * _JSAMPLEs, the entries of this array will too.
  */
 boolean is_padded;            /* is the colorindex padded for odither? */

 int Ncolors[MAX_Q_COMPS];     /* # of values allocated to each component */

 /* Variables for ordered dithering */
 int row_index;                /* cur row's vertical index in dither matrix */
 ODITHER_MATRIX_PTR odither[MAX_Q_COMPS]; /* one dither array per component */

 /* Variables for Floyd-Steinberg dithering */
 FSERRPTR fserrors[MAX_Q_COMPS]; /* accumulated errors */
 boolean on_odd_row;           /* flag to remember which row we are on */
} my_cquantizer;

typedef my_cquantizer *my_cquantize_ptr;


/*
* Policy-making subroutines for create_colormap and create_colorindex.
* These routines determine the colormap to be used.  The rest of the module
* only assumes that the colormap is orthogonal.
*
*  * select_ncolors decides how to divvy up the available colors
*    among the components.
*  * output_value defines the set of representative values for a component.
*  * largest_input_value defines the mapping from input values to
*    representative values for a component.
* Note that the latter two routines may impose different policies for
* different components, though this is not currently done.
*/


LOCAL(int)
select_ncolors(j_decompress_ptr cinfo, int Ncolors[])
/* Determine allocation of desired colors to components, */
/* and fill in Ncolors[] array to indicate choice. */
/* Return value is total number of colors (product of Ncolors[] values). */
{
 int nc = cinfo->out_color_components; /* number of color components */
 int max_colors = cinfo->desired_number_of_colors;
 int total_colors, iroot, i, j;
 boolean changed;
 long temp;
 int RGB_order[3] = { RGB_GREEN, RGB_RED, RGB_BLUE };
 RGB_order[0] = rgb_green[cinfo->out_color_space];
 RGB_order[1] = rgb_red[cinfo->out_color_space];
 RGB_order[2] = rgb_blue[cinfo->out_color_space];

 /* We can allocate at least the nc'th root of max_colors per component. */
 /* Compute floor(nc'th root of max_colors). */
 iroot = 1;
 do {
   iroot++;
   temp = iroot;               /* set temp = iroot ** nc */
   for (i = 1; i < nc; i++)
     temp *= iroot;
 } while (temp <= (long)max_colors); /* repeat till iroot exceeds root */
 iroot--;                      /* now iroot = floor(root) */

 /* Must have at least 2 color values per component */
 if (iroot < 2)
   ERREXIT1(cinfo, JERR_QUANT_FEW_COLORS, (int)temp);

 /* Initialize to iroot color values for each component */
 total_colors = 1;
 for (i = 0; i < nc; i++) {
   Ncolors[i] = iroot;
   total_colors *= iroot;
 }
 /* We may be able to increment the count for one or more components without
  * exceeding max_colors, though we know not all can be incremented.
  * Sometimes, the first component can be incremented more than once!
  * (Example: for 16 colors, we start at 2*2*2, go to 3*2*2, then 4*2*2.)
  * In RGB colorspace, try to increment G first, then R, then B.
  */
 do {
   changed = FALSE;
   for (i = 0; i < nc; i++) {
     j = (cinfo->out_color_space == JCS_RGB ? RGB_order[i] : i);
     /* calculate new total_colors if Ncolors[j] is incremented */
     temp = total_colors / Ncolors[j];
     temp *= Ncolors[j] + 1;   /* done in long arith to avoid oflo */
     if (temp > (long)max_colors)
       break;                  /* won't fit, done with this pass */
     Ncolors[j]++;             /* OK, apply the increment */
     total_colors = (int)temp;
     changed = TRUE;
   }
 } while (changed);

 return total_colors;
}


LOCAL(int)
output_value(j_decompress_ptr cinfo, int ci, int j, int maxj)
/* Return j'th output value, where j will range from 0 to maxj */
/* The output values must fall in 0.._MAXJSAMPLE in increasing order */
{
 /* We always provide values 0 and _MAXJSAMPLE for each component;
  * any additional values are equally spaced between these limits.
  * (Forcing the upper and lower values to the limits ensures that
  * dithering can't produce a color outside the selected gamut.)
  */
 return (int)(((JLONG)j * _MAXJSAMPLE + maxj / 2) / maxj);
}


LOCAL(int)
largest_input_value(j_decompress_ptr cinfo, int ci, int j, int maxj)
/* Return largest input value that should map to j'th output value */
/* Must have largest(j=0) >= 0, and largest(j=maxj) >= _MAXJSAMPLE */
{
 /* Breakpoints are halfway between values returned by output_value */
 return (int)(((JLONG)(2 * j + 1) * _MAXJSAMPLE + maxj) / (2 * maxj));
}


/*
* Create the colormap.
*/

LOCAL(void)
create_colormap(j_decompress_ptr cinfo)
{
 my_cquantize_ptr cquantize = (my_cquantize_ptr)cinfo->cquantize;
 _JSAMPARRAY colormap;         /* Created colormap */
 int total_colors;             /* Number of distinct output colors */
 int i, j, k, nci, blksize, blkdist, ptr, val;

 /* Select number of colors for each component */
 total_colors = select_ncolors(cinfo, cquantize->Ncolors);

 /* Report selected color counts */
 if (cinfo->out_color_components == 3)
   TRACEMS4(cinfo, 1, JTRC_QUANT_3_NCOLORS, total_colors,
            cquantize->Ncolors[0], cquantize->Ncolors[1],
            cquantize->Ncolors[2]);
 else
   TRACEMS1(cinfo, 1, JTRC_QUANT_NCOLORS, total_colors);

 /* Allocate and fill in the colormap. */
 /* The colors are ordered in the map in standard row-major order, */
 /* i.e. rightmost (highest-indexed) color changes most rapidly. */

 colormap = (_JSAMPARRAY)(*cinfo->mem->alloc_sarray)
   ((j_common_ptr)cinfo, JPOOL_IMAGE,
    (JDIMENSION)total_colors, (JDIMENSION)cinfo->out_color_components);

 /* blksize is number of adjacent repeated entries for a component */
 /* blkdist is distance between groups of identical entries for a component */
 blkdist = total_colors;

 for (i = 0; i < cinfo->out_color_components; i++) {
   /* fill in colormap entries for i'th color component */
   nci = cquantize->Ncolors[i]; /* # of distinct values for this color */
   blksize = blkdist / nci;
   for (j = 0; j < nci; j++) {
     /* Compute j'th output value (out of nci) for component */
     val = output_value(cinfo, i, j, nci - 1);
     /* Fill in all colormap entries that have this value of this component */
     for (ptr = j * blksize; ptr < total_colors; ptr += blkdist) {
       /* fill in blksize entries beginning at ptr */
       for (k = 0; k < blksize; k++)
         colormap[i][ptr + k] = (_JSAMPLE)val;
     }
   }
   blkdist = blksize;          /* blksize of this color is blkdist of next */
 }

 /* Save the colormap in private storage,
  * where it will survive color quantization mode changes.
  */
 cquantize->sv_colormap = colormap;
 cquantize->sv_actual = total_colors;
}


/*
* Create the color index table.
*/

LOCAL(void)
create_colorindex(j_decompress_ptr cinfo)
{
 my_cquantize_ptr cquantize = (my_cquantize_ptr)cinfo->cquantize;
 _JSAMPROW indexptr;
 int i, j, k, nci, blksize, val, pad;

 /* For ordered dither, we pad the color index tables by _MAXJSAMPLE in
  * each direction (input index values can be -_MAXJSAMPLE .. 2*_MAXJSAMPLE).
  * This is not necessary in the other dithering modes.  However, we
  * flag whether it was done in case user changes dithering mode.
  */
 if (cinfo->dither_mode == JDITHER_ORDERED) {
   pad = _MAXJSAMPLE * 2;
   cquantize->is_padded = TRUE;
 } else {
   pad = 0;
   cquantize->is_padded = FALSE;
 }

 cquantize->colorindex = (_JSAMPARRAY)(*cinfo->mem->alloc_sarray)
   ((j_common_ptr)cinfo, JPOOL_IMAGE,
    (JDIMENSION)(_MAXJSAMPLE + 1 + pad),
    (JDIMENSION)cinfo->out_color_components);

 /* blksize is number of adjacent repeated entries for a component */
 blksize = cquantize->sv_actual;

 for (i = 0; i < cinfo->out_color_components; i++) {
   /* fill in colorindex entries for i'th color component */
   nci = cquantize->Ncolors[i]; /* # of distinct values for this color */
   blksize = blksize / nci;

   /* adjust colorindex pointers to provide padding at negative indexes. */
   if (pad)
     cquantize->colorindex[i] += _MAXJSAMPLE;

   /* in loop, val = index of current output value, */
   /* and k = largest j that maps to current val */
   indexptr = cquantize->colorindex[i];
   val = 0;
   k = largest_input_value(cinfo, i, 0, nci - 1);
   for (j = 0; j <= _MAXJSAMPLE; j++) {
     while (j > k)             /* advance val if past boundary */
       k = largest_input_value(cinfo, i, ++val, nci - 1);
     /* premultiply so that no multiplication needed in main processing */
     indexptr[j] = (_JSAMPLE)(val * blksize);
   }
   /* Pad at both ends if necessary */
   if (pad)
     for (j = 1; j <= _MAXJSAMPLE; j++) {
       indexptr[-j] = indexptr[0];
       indexptr[_MAXJSAMPLE + j] = indexptr[_MAXJSAMPLE];
     }
 }
}


/*
* Create an ordered-dither array for a component having ncolors
* distinct output values.
*/

LOCAL(ODITHER_MATRIX_PTR)
make_odither_array(j_decompress_ptr cinfo, int ncolors)
{
 ODITHER_MATRIX_PTR odither;
 int j, k;
 JLONG num, den;

 odither = (ODITHER_MATRIX_PTR)
   (*cinfo->mem->alloc_small) ((j_common_ptr)cinfo, JPOOL_IMAGE,
                               sizeof(ODITHER_MATRIX));
 /* The inter-value distance for this color is _MAXJSAMPLE/(ncolors-1).
  * Hence the dither value for the matrix cell with fill order f
  * (f=0..N-1) should be (N-1-2*f)/(2*N) * _MAXJSAMPLE/(ncolors-1).
  * On 16-bit-int machine, be careful to avoid overflow.
  */
 den = 2 * ODITHER_CELLS * ((JLONG)(ncolors - 1));
 for (j = 0; j < ODITHER_SIZE; j++) {
   for (k = 0; k < ODITHER_SIZE; k++) {
     num = ((JLONG)(ODITHER_CELLS - 1 -
                    2 * ((int)base_dither_matrix[j][k]))) * _MAXJSAMPLE;
     /* Ensure round towards zero despite C's lack of consistency
      * about rounding negative values in integer division...
      */
     odither[j][k] = (int)(num < 0 ? -((-num) / den) : num / den);
   }
 }
 return odither;
}


/*
* Create the ordered-dither tables.
* Components having the same number of representative colors may
* share a dither table.
*/

LOCAL(void)
create_odither_tables(j_decompress_ptr cinfo)
{
 my_cquantize_ptr cquantize = (my_cquantize_ptr)cinfo->cquantize;
 ODITHER_MATRIX_PTR odither;
 int i, j, nci;

 for (i = 0; i < cinfo->out_color_components; i++) {
   nci = cquantize->Ncolors[i]; /* # of distinct values for this color */
   odither = NULL;             /* search for matching prior component */
   for (j = 0; j < i; j++) {
     if (nci == cquantize->Ncolors[j]) {
       odither = cquantize->odither[j];
       break;
     }
   }
   if (odither == NULL)        /* need a new table? */
     odither = make_odither_array(cinfo, nci);
   cquantize->odither[i] = odither;
 }
}


/*
* Map some rows of pixels to the output colormapped representation.
*/

METHODDEF(void)
color_quantize(j_decompress_ptr cinfo, _JSAMPARRAY input_buf,
              _JSAMPARRAY output_buf, int num_rows)
/* General case, no dithering */
{
 my_cquantize_ptr cquantize = (my_cquantize_ptr)cinfo->cquantize;
 _JSAMPARRAY colorindex = cquantize->colorindex;
 register int pixcode, ci;
 register _JSAMPROW ptrin, ptrout;
 int row;
 JDIMENSION col;
 JDIMENSION width = cinfo->output_width;
 register int nc = cinfo->out_color_components;

 for (row = 0; row < num_rows; row++) {
   ptrin = input_buf[row];
   ptrout = output_buf[row];
   for (col = width; col > 0; col--) {
     pixcode = 0;
     for (ci = 0; ci < nc; ci++) {
       pixcode += colorindex[ci][*ptrin++];
     }
     *ptrout++ = (_JSAMPLE)pixcode;
   }
 }
}


METHODDEF(void)
color_quantize3(j_decompress_ptr cinfo, _JSAMPARRAY input_buf,
               _JSAMPARRAY output_buf, int num_rows)
/* Fast path for out_color_components==3, no dithering */
{
 my_cquantize_ptr cquantize = (my_cquantize_ptr)cinfo->cquantize;
 register int pixcode;
 register _JSAMPROW ptrin, ptrout;
 _JSAMPROW colorindex0 = cquantize->colorindex[0];
 _JSAMPROW colorindex1 = cquantize->colorindex[1];
 _JSAMPROW colorindex2 = cquantize->colorindex[2];
 int row;
 JDIMENSION col;
 JDIMENSION width = cinfo->output_width;

 for (row = 0; row < num_rows; row++) {
   ptrin = input_buf[row];
   ptrout = output_buf[row];
   for (col = width; col > 0; col--) {
     pixcode  = colorindex0[*ptrin++];
     pixcode += colorindex1[*ptrin++];
     pixcode += colorindex2[*ptrin++];
     *ptrout++ = (_JSAMPLE)pixcode;
   }
 }
}


METHODDEF(void)
quantize_ord_dither(j_decompress_ptr cinfo, _JSAMPARRAY input_buf,
                   _JSAMPARRAY output_buf, int num_rows)
/* General case, with ordered dithering */
{
 my_cquantize_ptr cquantize = (my_cquantize_ptr)cinfo->cquantize;
 register _JSAMPROW input_ptr;
 register _JSAMPROW output_ptr;
 _JSAMPROW colorindex_ci;
 int *dither;                  /* points to active row of dither matrix */
 int row_index, col_index;     /* current indexes into dither matrix */
 int nc = cinfo->out_color_components;
 int ci;
 int row;
 JDIMENSION col;
 JDIMENSION width = cinfo->output_width;

 for (row = 0; row < num_rows; row++) {
   /* Initialize output values to 0 so can process components separately */
   jzero_far((void *)output_buf[row], (size_t)(width * sizeof(_JSAMPLE)));
   row_index = cquantize->row_index;
   for (ci = 0; ci < nc; ci++) {
     input_ptr = input_buf[row] + ci;
     output_ptr = output_buf[row];
     colorindex_ci = cquantize->colorindex[ci];
     dither = cquantize->odither[ci][row_index];
     col_index = 0;

     for (col = width; col > 0; col--) {
       /* Form pixel value + dither, range-limit to 0.._MAXJSAMPLE,
        * select output value, accumulate into output code for this pixel.
        * Range-limiting need not be done explicitly, as we have extended
        * the colorindex table to produce the right answers for out-of-range
        * inputs.  The maximum dither is +- _MAXJSAMPLE; this sets the
        * required amount of padding.
        */
       *output_ptr +=
         colorindex_ci[*input_ptr + dither[col_index]];
       input_ptr += nc;
       output_ptr++;
       col_index = (col_index + 1) & ODITHER_MASK;
     }
   }
   /* Advance row index for next row */
   row_index = (row_index + 1) & ODITHER_MASK;
   cquantize->row_index = row_index;
 }
}


METHODDEF(void)
quantize3_ord_dither(j_decompress_ptr cinfo, _JSAMPARRAY input_buf,
                    _JSAMPARRAY output_buf, int num_rows)
/* Fast path for out_color_components==3, with ordered dithering */
{
 my_cquantize_ptr cquantize = (my_cquantize_ptr)cinfo->cquantize;
 register int pixcode;
 register _JSAMPROW input_ptr;
 register _JSAMPROW output_ptr;
 _JSAMPROW colorindex0 = cquantize->colorindex[0];
 _JSAMPROW colorindex1 = cquantize->colorindex[1];
 _JSAMPROW colorindex2 = cquantize->colorindex[2];
 int *dither0;                 /* points to active row of dither matrix */
 int *dither1;
 int *dither2;
 int row_index, col_index;     /* current indexes into dither matrix */
 int row;
 JDIMENSION col;
 JDIMENSION width = cinfo->output_width;

 for (row = 0; row < num_rows; row++) {
   row_index = cquantize->row_index;
   input_ptr = input_buf[row];
   output_ptr = output_buf[row];
   dither0 = cquantize->odither[0][row_index];
   dither1 = cquantize->odither[1][row_index];
   dither2 = cquantize->odither[2][row_index];
   col_index = 0;

   for (col = width; col > 0; col--) {
     pixcode  = colorindex0[(*input_ptr++) + dither0[col_index]];
     pixcode += colorindex1[(*input_ptr++) + dither1[col_index]];
     pixcode += colorindex2[(*input_ptr++) + dither2[col_index]];
     *output_ptr++ = (_JSAMPLE)pixcode;
     col_index = (col_index + 1) & ODITHER_MASK;
   }
   row_index = (row_index + 1) & ODITHER_MASK;
   cquantize->row_index = row_index;
 }
}


METHODDEF(void)
quantize_fs_dither(j_decompress_ptr cinfo, _JSAMPARRAY input_buf,
                  _JSAMPARRAY output_buf, int num_rows)
/* General case, with Floyd-Steinberg dithering */
{
 my_cquantize_ptr cquantize = (my_cquantize_ptr)cinfo->cquantize;
 register LOCFSERROR cur;      /* current error or pixel value */
 LOCFSERROR belowerr;          /* error for pixel below cur */
 LOCFSERROR bpreverr;          /* error for below/prev col */
 LOCFSERROR bnexterr;          /* error for below/next col */
 LOCFSERROR delta;
 register FSERRPTR errorptr;   /* => fserrors[] at column before current */
 register _JSAMPROW input_ptr;
 register _JSAMPROW output_ptr;
 _JSAMPROW colorindex_ci;
 _JSAMPROW colormap_ci;
 int pixcode;
 int nc = cinfo->out_color_components;
 int dir;                      /* 1 for left-to-right, -1 for right-to-left */
 int dirnc;                    /* dir * nc */
 int ci;
 int row;
 JDIMENSION col;
 JDIMENSION width = cinfo->output_width;
 _JSAMPLE *range_limit = (_JSAMPLE *)cinfo->sample_range_limit;
 SHIFT_TEMPS

 for (row = 0; row < num_rows; row++) {
   /* Initialize output values to 0 so can process components separately */
   jzero_far((void *)output_buf[row], (size_t)(width * sizeof(_JSAMPLE)));
   for (ci = 0; ci < nc; ci++) {
     input_ptr = input_buf[row] + ci;
     output_ptr = output_buf[row];
     if (cquantize->on_odd_row) {
       /* work right to left in this row */
       input_ptr += (width - 1) * nc; /* so point to rightmost pixel */
       output_ptr += width - 1;
       dir = -1;
       dirnc = -nc;
       errorptr = cquantize->fserrors[ci] + (width + 1); /* => entry after last column */
     } else {
       /* work left to right in this row */
       dir = 1;
       dirnc = nc;
       errorptr = cquantize->fserrors[ci]; /* => entry before first column */
     }
     colorindex_ci = cquantize->colorindex[ci];
     colormap_ci = cquantize->sv_colormap[ci];
     /* Preset error values: no error propagated to first pixel from left */
     cur = 0;
     /* and no error propagated to row below yet */
     belowerr = bpreverr = 0;

     for (col = width; col > 0; col--) {
       /* cur holds the error propagated from the previous pixel on the
        * current line.  Add the error propagated from the previous line
        * to form the complete error correction term for this pixel, and
        * round the error term (which is expressed * 16) to an integer.
        * RIGHT_SHIFT rounds towards minus infinity, so adding 8 is correct
        * for either sign of the error value.
        * Note: errorptr points to *previous* column's array entry.
        */
       cur = RIGHT_SHIFT(cur + errorptr[dir] + 8, 4);
       /* Form pixel value + error, and range-limit to 0.._MAXJSAMPLE.
        * The maximum error is +- _MAXJSAMPLE; this sets the required size
        * of the range_limit array.
        */
       cur += *input_ptr;
       cur = range_limit[cur];
       /* Select output value, accumulate into output code for this pixel */
       pixcode = colorindex_ci[cur];
       *output_ptr += (_JSAMPLE)pixcode;
       /* Compute actual representation error at this pixel */
       /* Note: we can do this even though we don't have the final */
       /* pixel code, because the colormap is orthogonal. */
       cur -= colormap_ci[pixcode];
       /* Compute error fractions to be propagated to adjacent pixels.
        * Add these into the running sums, and simultaneously shift the
        * next-line error sums left by 1 column.
        */
       bnexterr = cur;
       delta = cur * 2;
       cur += delta;           /* form error * 3 */
       errorptr[0] = (FSERROR)(bpreverr + cur);
       cur += delta;           /* form error * 5 */
       bpreverr = belowerr + cur;
       belowerr = bnexterr;
       cur += delta;           /* form error * 7 */
       /* At this point cur contains the 7/16 error value to be propagated
        * to the next pixel on the current line, and all the errors for the
        * next line have been shifted over. We are therefore ready to move on.
        */
       input_ptr += dirnc;     /* advance input ptr to next column */
       output_ptr += dir;      /* advance output ptr to next column */
       errorptr += dir;        /* advance errorptr to current column */
     }
     /* Post-loop cleanup: we must unload the final error value into the
      * final fserrors[] entry.  Note we need not unload belowerr because
      * it is for the dummy column before or after the actual array.
      */
     errorptr[0] = (FSERROR)bpreverr; /* unload prev err into array */
   }
   cquantize->on_odd_row = (cquantize->on_odd_row ? FALSE : TRUE);
 }
}


/*
* Allocate workspace for Floyd-Steinberg errors.
*/

LOCAL(void)
alloc_fs_workspace(j_decompress_ptr cinfo)
{
 my_cquantize_ptr cquantize = (my_cquantize_ptr)cinfo->cquantize;
 size_t arraysize;
 int i;

 arraysize = (size_t)((cinfo->output_width + 2) * sizeof(FSERROR));
 for (i = 0; i < cinfo->out_color_components; i++) {
   cquantize->fserrors[i] = (FSERRPTR)
     (*cinfo->mem->alloc_large) ((j_common_ptr)cinfo, JPOOL_IMAGE, arraysize);
 }
}


/*
* Initialize for one-pass color quantization.
*/

METHODDEF(void)
start_pass_1_quant(j_decompress_ptr cinfo, boolean is_pre_scan)
{
 my_cquantize_ptr cquantize = (my_cquantize_ptr)cinfo->cquantize;
 size_t arraysize;
 int i;

 /* Install my colormap. */
 cinfo->colormap = (JSAMPARRAY)cquantize->sv_colormap;
 cinfo->actual_number_of_colors = cquantize->sv_actual;

 /* Initialize for desired dithering mode. */
 switch (cinfo->dither_mode) {
 case JDITHER_NONE:
   if (cinfo->out_color_components == 3)
     cquantize->pub._color_quantize = color_quantize3;
   else
     cquantize->pub._color_quantize = color_quantize;
   break;
 case JDITHER_ORDERED:
   if (cinfo->out_color_components == 3)
     cquantize->pub._color_quantize = quantize3_ord_dither;
   else
     cquantize->pub._color_quantize = quantize_ord_dither;
   cquantize->row_index = 0;   /* initialize state for ordered dither */
   /* If user changed to ordered dither from another mode,
    * we must recreate the color index table with padding.
    * This will cost extra space, but probably isn't very likely.
    */
   if (!cquantize->is_padded)
     create_colorindex(cinfo);
   /* Create ordered-dither tables if we didn't already. */
   if (cquantize->odither[0] == NULL)
     create_odither_tables(cinfo);
   break;
 case JDITHER_FS:
   cquantize->pub._color_quantize = quantize_fs_dither;
   cquantize->on_odd_row = FALSE; /* initialize state for F-S dither */
   /* Allocate Floyd-Steinberg workspace if didn't already. */
   if (cquantize->fserrors[0] == NULL)
     alloc_fs_workspace(cinfo);
   /* Initialize the propagated errors to zero. */
   arraysize = (size_t)((cinfo->output_width + 2) * sizeof(FSERROR));
   for (i = 0; i < cinfo->out_color_components; i++)
     jzero_far((void *)cquantize->fserrors[i], arraysize);
   break;
 default:
   ERREXIT(cinfo, JERR_NOT_COMPILED);
   break;
 }
}


/*
* Finish up at the end of the pass.
*/

METHODDEF(void)
finish_pass_1_quant(j_decompress_ptr cinfo)
{
 /* no work in 1-pass case */
}


/*
* Switch to a new external colormap between output passes.
* Shouldn't get to this module!
*/

METHODDEF(void)
new_color_map_1_quant(j_decompress_ptr cinfo)
{
 ERREXIT(cinfo, JERR_MODE_CHANGE);
}


/*
* Module initialization routine for 1-pass color quantization.
*/

GLOBAL(void)
_jinit_1pass_quantizer(j_decompress_ptr cinfo)
{
 my_cquantize_ptr cquantize;

 if (cinfo->data_precision != BITS_IN_JSAMPLE)
   ERREXIT1(cinfo, JERR_BAD_PRECISION, cinfo->data_precision);

 /* Color quantization is not supported with lossless JPEG images */
 if (cinfo->master->lossless)
   ERREXIT(cinfo, JERR_NOTIMPL);

 cquantize = (my_cquantize_ptr)
   (*cinfo->mem->alloc_small) ((j_common_ptr)cinfo, JPOOL_IMAGE,
                               sizeof(my_cquantizer));
 cinfo->cquantize = (struct jpeg_color_quantizer *)cquantize;
 cquantize->pub.start_pass = start_pass_1_quant;
 cquantize->pub.finish_pass = finish_pass_1_quant;
 cquantize->pub.new_color_map = new_color_map_1_quant;
 cquantize->fserrors[0] = NULL; /* Flag FS workspace not allocated */
 cquantize->odither[0] = NULL; /* Also flag odither arrays not allocated */

 /* Make sure my internal arrays won't overflow */
 if (cinfo->out_color_components > MAX_Q_COMPS)
   ERREXIT1(cinfo, JERR_QUANT_COMPONENTS, MAX_Q_COMPS);
 /* Make sure colormap indexes can be represented by _JSAMPLEs */
 if (cinfo->desired_number_of_colors > (_MAXJSAMPLE + 1))
   ERREXIT1(cinfo, JERR_QUANT_MANY_COLORS, _MAXJSAMPLE + 1);

 /* Create the colormap and color index table. */
 create_colormap(cinfo);
 create_colorindex(cinfo);

 /* Allocate Floyd-Steinberg workspace now if requested.
  * We do this now since it may affect the memory manager's space
  * calculations.  If the user changes to FS dither mode in a later pass, we
  * will allocate the space then, and will possibly overrun the
  * max_memory_to_use setting.
  */
 if (cinfo->dither_mode == JDITHER_FS)
   alloc_fs_workspace(cinfo);
}

#endif /* defined(QUANT_1PASS_SUPPORTED) && BITS_IN_JSAMPLE != 16 */