media/libjpeg/jchuff.h

   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 #if BITS_IN_JSAMPLE == 8
  23 #define MAX_COEF_BITS  10
  24 #else
  25 #define MAX_COEF_BITS  14
  26 #endif
  27
  28 /* The progressive Huffman encoder uses an unsigned 16-bit data type to store
  29  * absolute values of coefficients, because it is possible to inject a
  30  * coefficient value of -32768 into the encoder by attempting to transform a
  31  * malformed 12-bit JPEG image, and the absolute value of -32768 would overflow
  32  * a signed 16-bit integer.
  33  */
  34 typedef unsigned short UJCOEF;
  35
  36 /* Derived data constructed for each Huffman table */
  37
  38 typedef struct {
  39   unsigned int ehufco[256];     /* code for each symbol */
  40   char ehufsi[256];             /* length of code for each symbol */
  41   /* If no code has been allocated for a symbol S, ehufsi[S] contains 0 */
  42 } c_derived_tbl;
  43
  44 /* Expand a Huffman table definition into the derived format */
  45 EXTERN(void) jpeg_make_c_derived_tbl(j_compress_ptr cinfo, boolean isDC,
  46                                      int tblno, c_derived_tbl **pdtbl);
  47
  48 /* Generate an optimal table definition given the specified counts */
  49 EXTERN(void) jpeg_gen_optimal_table(j_compress_ptr cinfo, JHUFF_TBL *htbl,
  50                                     long freq[]);