jchuff.h (1766B)
1 /* 2 * jchuff.h 3 * 4 * This file was part of the Independent JPEG Group's software: 5 * Copyright (C) 1991-1997, Thomas G. Lane. 6 * libjpeg-turbo Modifications: 7 * Copyright (C) 2022, D. R. Commander. 8 * For conditions of distribution and use, see the accompanying README.ijg 9 * file. 10 * 11 * This file contains declarations for Huffman entropy encoding routines 12 * that are shared between the sequential encoder (jchuff.c) and the 13 * progressive encoder (jcphuff.c). No other modules need to see these. 14 */ 15 16 /* The legal range of a DCT coefficient is 17 * -1024 .. +1023 for 8-bit data; 18 * -16384 .. +16383 for 12-bit data. 19 * Hence the magnitude should always fit in 10 or 14 bits respectively. 20 */ 21 22 /* The progressive Huffman encoder uses an unsigned 16-bit data type to store 23 * absolute values of coefficients, because it is possible to inject a 24 * coefficient value of -32768 into the encoder by attempting to transform a 25 * malformed 12-bit JPEG image, and the absolute value of -32768 would overflow 26 * a signed 16-bit integer. 27 */ 28 typedef unsigned short UJCOEF; 29 30 /* Derived data constructed for each Huffman table */ 31 32 typedef struct { 33 unsigned int ehufco[256]; /* code for each symbol */ 34 char ehufsi[256]; /* length of code for each symbol */ 35 /* If no code has been allocated for a symbol S, ehufsi[S] contains 0 */ 36 } c_derived_tbl; 37 38 /* Expand a Huffman table definition into the derived format */ 39 EXTERN(void) jpeg_make_c_derived_tbl(j_compress_ptr cinfo, boolean isDC, 40 int tblno, c_derived_tbl **pdtbl); 41 42 /* Generate an optimal table definition given the specified counts */ 43 EXTERN(void) jpeg_gen_optimal_table(j_compress_ptr cinfo, JHUFF_TBL *htbl, 44 long freq[]);