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src/Source/LibJPEG/jcarith.c  view on Meta::CPAN

 *
 * This file contains portable arithmetic entropy encoding routines for JPEG
 * (implementing the ISO/IEC IS 10918-1 and CCITT Recommendation ITU-T T.81).
 *
 * Both sequential and progressive modes are supported in this single module.
 *
 * Suspension is not currently supported in this module.
 */

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


/* Expanded entropy encoder object for arithmetic encoding. */

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

  INT32 c; /* C register, base of coding interval, layout as in sec. D.1.3 */
  INT32 a;               /* A register, normalized size of coding interval */
  INT32 sc;        /* counter for stacked 0xFF values which might overflow */
  INT32 zc;          /* counter for pending 0x00 output values which might *
                          * be discarded at the end ("Pacman" termination) */
  int ct;  /* bit shift counter, determines when next byte will be written */
  int buffer;                /* buffer for most recent output byte != 0xFF */

  int last_dc_val[MAX_COMPS_IN_SCAN]; /* last DC coef for each component */
  int dc_context[MAX_COMPS_IN_SCAN]; /* context index for DC conditioning */

  unsigned int restarts_to_go;	/* MCUs left in this restart interval */
  int next_restart_num;		/* next restart number to write (0-7) */

  /* Pointers to statistics areas (these workspaces have image lifespan) */
  unsigned char * dc_stats[NUM_ARITH_TBLS];
  unsigned char * ac_stats[NUM_ARITH_TBLS];

  /* Statistics bin for coding with fixed probability 0.5 */
  unsigned char fixed_bin[4];
} arith_entropy_encoder;

typedef arith_entropy_encoder * arith_entropy_ptr;

/* The following two definitions specify the allocation chunk size
 * for the statistics area.
 * According to sections F.1.4.4.1.3 and F.1.4.4.2, we need at least
 * 49 statistics bins for DC, and 245 statistics bins for AC coding.
 *
 * We use a compact representation with 1 byte per statistics bin,
 * thus the numbers directly represent byte sizes.
 * This 1 byte per statistics bin contains the meaning of the MPS
 * (more probable symbol) in the highest bit (mask 0x80), and the
 * index into the probability estimation state machine table
 * in the lower bits (mask 0x7F).
 */

#define DC_STAT_BINS 64
#define AC_STAT_BINS 256

/* NOTE: Uncomment the following #define if you want to use the
 * given formula for calculating the AC conditioning parameter Kx
 * for spectral selection progressive coding in section G.1.3.2
 * of the spec (Kx = Kmin + SRL (8 + Se - Kmin) 4).
 * Although the spec and P&M authors claim that this "has proven
 * to give good results for 8 bit precision samples", I'm not
 * convinced yet that this is really beneficial.
 * Early tests gave only very marginal compression enhancements
 * (a few - around 5 or so - bytes even for very large files),
 * which would turn out rather negative if we'd suppress the
 * DAC (Define Arithmetic Conditioning) marker segments for
 * the default parameters in the future.
 * Note that currently the marker writing module emits 12-byte
 * DAC segments for a full-component scan in a color image.
 * This is not worth worrying about IMHO. However, since the
 * spec defines the default values to be used if the tables
 * are omitted (unlike Huffman tables, which are required
 * anyway), one might optimize this behaviour in the future,
 * and then it would be disadvantageous to use custom tables if
 * they don't provide sufficient gain to exceed the DAC size.
 *
 * On the other hand, I'd consider it as a reasonable result
 * that the conditioning has no significant influence on the
 * compression performance. This means that the basic
 * statistical model is already rather stable.
 *
 * Thus, at the moment, we use the default conditioning values
 * anyway, and do not use the custom formula.
 *
#define CALCULATE_SPECTRAL_CONDITIONING
 */

/* IRIGHT_SHIFT is like RIGHT_SHIFT, but works on int rather than INT32.
 * We assume that int right shift is unsigned if INT32 right shift is,
 * which should be safe.
 */

#ifdef RIGHT_SHIFT_IS_UNSIGNED
#define ISHIFT_TEMPS	int ishift_temp;
#define IRIGHT_SHIFT(x,shft)  \
	((ishift_temp = (x)) < 0 ? \
	 (ishift_temp >> (shft)) | ((~0) << (16-(shft))) : \
	 (ishift_temp >> (shft)))
#else
#define ISHIFT_TEMPS
#define IRIGHT_SHIFT(x,shft)	((x) >> (shft))
#endif


LOCAL(void)
emit_byte (int val, j_compress_ptr cinfo)
/* Write next output byte; we do not support suspension in this module. */
{
  struct jpeg_destination_mgr * dest = cinfo->dest;

  *dest->next_output_byte++ = (JOCTET) val;
  if (--dest->free_in_buffer == 0)
    if (! (*dest->empty_output_buffer) (cinfo))
      ERREXIT(cinfo, JERR_CANT_SUSPEND);
}


/*
 * Finish up at the end of an arithmetic-compressed scan.
 */

METHODDEF(void)
finish_pass (j_compress_ptr cinfo)
{
  arith_entropy_ptr e = (arith_entropy_ptr) cinfo->entropy;
  INT32 temp;

  /* Section D.1.8: Termination of encoding */

  /* Find the e->c in the coding interval with the largest
   * number of trailing zero bits */
  if ((temp = (e->a - 1 + e->c) & 0xFFFF0000L) < e->c)
    e->c = temp + 0x8000L;
  else
    e->c = temp;
  /* Send remaining bytes to output */
  e->c <<= e->ct;
  if (e->c & 0xF8000000L) {
    /* One final overflow has to be handled */
    if (e->buffer >= 0) {
      if (e->zc)
	do emit_byte(0x00, cinfo);
	while (--e->zc);