wrgif.c

这是JPEG解码、编码的源代码

/*
 * wrgif.c
 *
 * Copyright (C) 1991-1996, Thomas G. Lane.
 * This file is part of the Independent JPEG Group's software.
 * For conditions of distribution and use, see the accompanying README file.
 *
 **************************************************************************
 * WARNING: You will need an LZW patent license from Unisys in order to   *
 * use this file legally in any commercial or shareware application.      *
 **************************************************************************
 *
 * This file contains routines to write output images in GIF format.
 *
 * These routines may need modification for non-Unix environments or
 * specialized applications.  As they stand, they assume output to
 * an ordinary stdio stream.
 */

/*
 * This code is loosely based on ppmtogif from the PBMPLUS distribution
 * of Feb. 1991.  That file contains the following copyright notice:
 *    Based on GIFENCODE by David Rowley <[e-mail]>.
 *    Lempel-Ziv compression based on "compress" by Spencer W. Thomas et al.
 *    Copyright (C) 1989 by Jef Poskanzer.
 *    Permission to use, copy, modify, and distribute this software and its
 *    documentation for any purpose and without fee is hereby granted, provided
 *    that the above copyright notice appear in all copies and that both that
 *    copyright notice and this permission notice appear in supporting
 *    documentation.  This software is provided "as is" without express or
 *    implied warranty.
 *
 * We are also required to state that
 *    "The Graphics Interchange Format(c) is the Copyright property of
 *    CompuServe Incorporated. GIF(sm) is a Service Mark property of
 *    CompuServe Incorporated."
 */

#include "cdjpeg.h"		/* Common decls for cjpeg/djpeg applications */

#ifdef GIF_SUPPORTED


#define	MAX_LZW_BITS	12	/* maximum LZW code size (4096 symbols) */

typedef INT16 code_int;		/* must hold -1 .. 2**MAX_LZW_BITS */

#define LZW_TABLE_SIZE	((code_int) 1 << MAX_LZW_BITS)

#define HSIZE		5003	/* hash table size for 80% occupancy */

typedef int hash_int;		/* must hold -2*HSIZE..2*HSIZE */

#define MAXCODE(n_bits)	(((code_int) 1 << (n_bits)) - 1)


/*
 * The LZW hash table consists of two parallel arrays:
 *   hash_code[i]	code of symbol in slot i, or 0 if empty slot
 *   hash_value[i]	symbol's value; undefined if empty slot
 * where slot values (i) range from 0 to HSIZE-1.  The symbol value is
 * its prefix symbol's code concatenated with its suffix character.
 *
 * Algorithm:  use open addressing double hashing (no chaining) on the
 * prefix code / suffix character combination.  We do a variant of Knuth's
 * algorithm D (vol. 3, sec. 6.4) along with G. Knott's relatively-prime
 * secondary probe.
 *
 * The hash_value[] table is allocated from FAR heap space since it would
 * use up rather a lot of the near data space in a PC.
 */

typedef INT32 hash_entry;	/* must hold (code_int<<8) | byte */

#define HASH_ENTRY(prefix,suffix)  ((((hash_entry) (prefix)) << 8) | (suffix))


/* Private version of data destination object */

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

  j_decompress_ptr cinfo;	/* back link saves passing separate parm */

  /* State for packing variable-width codes into a bitstream */
  int n_bits;			/* current number of bits/code */
  code_int maxcode;		/* maximum code, given n_bits */
  int init_bits;		/* initial n_bits ... restored after clear */
  INT32 cur_accum;		/* holds bits not yet output */
  int cur_bits;			/* # of bits in cur_accum */

  /* LZW string construction */
  code_int waiting_code;	/* symbol not yet output; may be extendable */
  boolean first_byte;		/* if TRUE, waiting_code is not valid */

  /* State for LZW code assignment */
  code_int ClearCode;		/* clear code (doesn't change) */
  code_int EOFCode;		/* EOF code (ditto) */
  code_int free_code;		/* first not-yet-used symbol code */

  /* LZW hash table */
  code_int *hash_code;		/* => hash table of symbol codes */
  hash_entry FAR *hash_value;	/* => hash table of symbol values */

  /* GIF data packet construction buffer */
  int bytesinpkt;		/* # of bytes in current packet */
  char packetbuf[256];		/* workspace for accumulating packet */

} gif_dest_struct;

typedef gif_dest_struct * gif_dest_ptr;


/*
 * Routines to package compressed data bytes into GIF data blocks.
 * A data block consists of a count byte (1..255) and that many data bytes.
 */

LOCAL(void)
flush_packet (gif_dest_ptr dinfo)
/* flush any accumulated data */
{
  if (dinfo->bytesinpkt > 0) {	/* never write zero-length packet */
    dinfo->packetbuf[0] = (char) dinfo->bytesinpkt++;
    if (JFWRITE(dinfo->pub.output_file, dinfo->packetbuf, dinfo->bytesinpkt)
	!= (size_t) dinfo->bytesinpkt)
      ERREXIT(dinfo->cinfo, JERR_FILE_WRITE);
    dinfo->bytesinpkt = 0;
  }
}


/* Add a character to current packet; flush to disk if necessary */
#define CHAR_OUT(dinfo,c)  \
	{ (dinfo)->packetbuf[++(dinfo)->bytesinpkt] = (char) (c);  \
	    if ((dinfo)->bytesinpkt >= 255)  \
	      flush_packet(dinfo);  \
	}


/* Routine to convert variable-width codes into a byte stream */

LOCAL(void)
output (gif_dest_ptr dinfo, code_int code)
/* Emit a code of n_bits bits */
/* Uses cur_accum and cur_bits to reblock into 8-bit bytes */
{
  dinfo->cur_accum |= ((INT32) code) << dinfo->cur_bits;
  dinfo->cur_bits += dinfo->n_bits;

  while (dinfo->cur_bits >= 8) {
    CHAR_OUT(dinfo, dinfo->cur_accum & 0xFF);
    dinfo->cur_accum >>= 8;
    dinfo->cur_bits -= 8;
  }

  /*
   * If the next entry is going to be too big for the code size,
   * then increase it, if possible.  We do this here to ensure
   * that it's done in sync with the decoder's codesize increases.
   */
  if (dinfo->free_code > dinfo->maxcode) {
    dinfo->n_bits++;
    if (dinfo->n_bits == MAX_LZW_BITS)
      dinfo->maxcode = LZW_TABLE_SIZE; /* free_code will never exceed this */
    else
      dinfo->maxcode = MAXCODE(dinfo->n_bits);
  }
}


/* The LZW algorithm proper */


LOCAL(void)
clear_hash (gif_dest_ptr dinfo)
/* Fill the hash table with empty entries */
{
  /* It's sufficient to zero hash_code[] */
  MEMZERO(dinfo->hash_code, HSIZE * SIZEOF(code_int));
}


LOCAL(void)
clear_block (gif_dest_ptr dinfo)
/* Reset compressor and issue a Clear code */
{
  clear_hash(dinfo);			/* delete all the symbols */
  dinfo->free_code = dinfo->ClearCode + 2;
  output(dinfo, dinfo->ClearCode);	/* inform decoder */
  dinfo->n_bits = dinfo->init_bits;	/* reset code size */
  dinfo->maxcode = MAXCODE(dinfo->n_bits);
}


LOCAL(void)
compress_init (gif_dest_ptr dinfo, int i_bits)
/* Initialize LZW compressor */
{
  /* init all the state variables */
  dinfo->n_bits = dinfo->init_bits = i_bits;
  dinfo->maxcode = MAXCODE(dinfo->n_bits);
  dinfo->ClearCode = ((code_int) 1 << (i_bits - 1));
  dinfo->EOFCode = dinfo->ClearCode + 1;
  dinfo->free_code = dinfo->ClearCode + 2;
  dinfo->first_byte = TRUE;	/* no waiting symbol yet */
  /* init output buffering vars */
  dinfo->bytesinpkt = 0;
  dinfo->cur_accum = 0;
  dinfo->cur_bits = 0;
  /* clear hash table */
  clear_hash(dinfo);
  /* GIF specifies an initial Clear code */
  output(dinfo, dinfo->ClearCode);
}


LOCAL(void)
compress_byte (gif_dest_ptr dinfo, int c)
/* Accept and compress one 8-bit byte */
{
  register hash_int i;
  register hash_int disp;
  register hash_entry probe_value;

  if (dinfo->first_byte) {	/* need to initialize waiting_code */
    dinfo->waiting_code = c;
    dinfo->first_byte = FALSE;
    return;
  }

  /* Probe hash table to see if a symbol exists for
   * waiting_code followed by c.
   * If so, replace waiting_code by that symbol and return.
   */
  i = ((hash_int) c << (MAX_LZW_BITS-8)) + dinfo->waiting_code;
  /* i is less than twice 2**MAX_LZW_BITS, therefore less than twice HSIZE */
  if (i >= HSIZE)
    i -= HSIZE;

  probe_value = HASH_ENTRY(dinfo->waiting_code, c);
  
  if (dinfo->hash_code[i] != 0) { /* is first probed slot empty? */
    if (dinfo->hash_value[i] == probe_value) {
      dinfo->waiting_code = dinfo->hash_code[i];
      return;
    }
    if (i == 0)			/* secondary hash (after G. Knott) */
      disp = 1;
    else
      disp = HSIZE - i;
    for (;;) {
      i -= disp;