wrgif.c

/*
 * wrgif.c
 *
 * Copyright (C) 1991-1996, Thomas G. Lane.
 * Modified 2015-2019 by Guido Vollbeding.
 * This file is part of the Independent JPEG Group's software.
 * For conditions of distribution and use, see the accompanying README file.
 *
 * 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 <mgardi@watdscu.waterloo.edu>.
 *    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.
 */

#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 GIF code assignment */
  code_int ClearCode;		/* clear code (doesn't change) */
  code_int EOFCode;		/* EOF code (ditto) */
  code_int free_code;		/* LZW: first not-yet-used symbol code */
  code_int code_counter;	/* not LZW: counts output symbols */

  /* 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 finished 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);
  }
}


/* Compression initialization & termination */


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 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->code_counter = 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 */
  if (dinfo->hash_code != NULL)
    clear_hash(dinfo);
  /* GIF specifies an initial Clear code */
  output(dinfo, dinfo->ClearCode);
}


LOCAL(void)
compress_term (gif_dest_ptr dinfo)
/* Clean up at end */
{
  /* Flush out the buffered LZW code */
  if (! dinfo->first_byte)
    output(dinfo, dinfo->waiting_code);
  /* Send an EOF code */
  output(dinfo, dinfo->EOFCode);
  /* Flush the bit-packing buffer */
  if (dinfo->cur_bits > 0) {
    CHAR_OUT(dinfo, dinfo->cur_accum & 0xFF);
  }
  /* Flush the packet buffer */
  flush_packet(dinfo);
}


/* GIF header construction */


LOCAL(void)
put_word (gif_dest_ptr dinfo, unsigned int w)
/* Emit a 16-bit word, LSB first */
{
  putc(w & 0xFF, dinfo->pub.output_file);
  putc((w >> 8) & 0xFF, dinfo->pub.output_file);
}


LOCAL(void)
put_3bytes (gif_dest_ptr dinfo, int val)
/* Emit 3 copies of same byte value --- handy subr for colormap construction */
{
  putc(val, dinfo->pub.output_file);
  putc(val, dinfo->pub.output_file);
  putc(val, dinfo->pub.output_file);
}


LOCAL(void)
emit_header (gif_dest_ptr dinfo, int num_colors, JSAMPARRAY colormap)
/* Output the GIF file header, including color map */
/* If colormap == NULL, synthesize a grayscale colormap */
{
  int BitsPerPixel, ColorMapSize, InitCodeSize, FlagByte;
  int cshift = dinfo->cinfo->data_precision - 8;
  int i;

  if (num_colors > 256)
    ERREXIT1(dinfo->cinfo, JERR_TOO_MANY_COLORS, num_colors);
  /* Compute bits/pixel and related values */
  BitsPerPixel = 1;
  while (num_colors > (1 << BitsPerPixel))
    BitsPerPixel++;
  ColorMapSize = 1 << BitsPerPixel;
  if (BitsPerPixel <= 1)
    InitCodeSize = 2;
  else
    InitCodeSize = BitsPerPixel;
  /*
   * Write the GIF header.
   * Note that we generate a plain GIF87 header for maximum compatibility.
   */
  putc('G', dinfo->pub.output_file);
  putc('I', dinfo->pub.output_file);
  putc('F', dinfo->pub.output_file);
  putc('8', dinfo->pub.output_file);
  putc('7', dinfo->pub.output_file);
  putc('a', dinfo->pub.output_file);
  /* Write the Logical Screen Descriptor */
  put_word(dinfo, (unsigned int) dinfo->cinfo->output_width);
  put_word(dinfo, (unsigned int) dinfo->cinfo->output_height);
  FlagByte = 0x80;		/* Yes, there is a global color table */
  FlagByte |= (BitsPerPixel - 1) << 4;	/* color resolution */
  FlagByte |= (BitsPerPixel - 1);	/* size of global color table */
  putc(FlagByte, dinfo->pub.output_file);
  putc(0, dinfo->pub.output_file); /* Background color index */
  putc(0, dinfo->pub.output_file); /* Reserved (aspect ratio in GIF89) */
  /* Write the Global Color Map */
  /* If the color map is more than 8 bits precision, */
  /* we reduce it to 8 bits by shifting */
  for (i = 0; i < ColorMapSize; i++) {
    if (i < num_colors) {
      if (colormap != NULL) {
	if (dinfo->cinfo->out_color_space == JCS_RGB) {
	  /* Normal case: RGB color map */
	  putc(GETJSAMPLE(colormap[0][i]) >> cshift, dinfo->pub.output_file);
	  putc(GETJSAMPLE(colormap[1][i]) >> cshift, dinfo->pub.output_file);
	  putc(GETJSAMPLE(colormap[2][i]) >> cshift, dinfo->pub.output_file);
	} else {
	  /* Grayscale "color map": possible if quantizing grayscale image */
	  put_3bytes(dinfo, GETJSAMPLE(colormap[0][i]) >> cshift);
	}
      } else {
	/* Create a grayscale map of num_colors values, range 0..255 */
	put_3bytes(dinfo, (i * 255 + (num_colors - 1) / 2) / (num_colors - 1));
      }
    } else {
      /* fill out the map to a power of 2 */
      put_3bytes(dinfo, CENTERJSAMPLE >> cshift);
    }
  }
  /* Write image separator and Image Descriptor */
  putc(',', dinfo->pub.output_file); /* separator */
  put_word(dinfo, 0);		/* left/top offset */
  put_word(dinfo, 0);
  put_word(dinfo, (unsigned int) dinfo->cinfo->output_width); /* image size */
  put_word(dinfo, (unsigned int) dinfo->cinfo->output_height);
  /* flag byte: not interlaced, no local color map */
  putc(0x00, dinfo->pub.output_file);
  /* Write Initial Code Size byte */
  putc(InitCodeSize, dinfo->pub.output_file);

  /* Initialize for compression of image data */
  compress_init(dinfo, InitCodeSize + 1);
}


/*
 * Startup: write the file header.
 */

METHODDEF(void)
start_output_gif (j_decompress_ptr cinfo, djpeg_dest_ptr dinfo)
{
  gif_dest_ptr dest = (gif_dest_ptr) dinfo;

  if (cinfo->quantize_colors)
    emit_header(dest, cinfo->actual_number_of_colors, cinfo->colormap);
  else
    emit_header(dest, 256, (JSAMPARRAY) NULL);
}


/*
 * Write some pixel data.
 * In this module rows_supplied will always be 1.
 */


/*
 * The LZW algorithm proper
 */

METHODDEF(void)
put_LZW_pixel_rows (j_decompress_ptr cinfo, djpeg_dest_ptr dinfo,
		    JDIMENSION rows_supplied)
{
  gif_dest_ptr dest = (gif_dest_ptr) dinfo;
  register JSAMPROW ptr;
  register JDIMENSION col;
  code_int c;
  register hash_int i;
  register hash_int disp;
  register hash_entry probe_value;

  ptr = dest->pub.buffer[0];
  for (col = cinfo->output_width; col > 0; col--) {
    /* Accept and compress one 8-bit byte */
    c = (code_int) GETJSAMPLE(*ptr++);

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

    /* Probe hash table to see if a symbol exists for
     * waiting_code followed by c.
     * If so, replace waiting_code by that symbol and continue.
     */
    i = ((hash_int) c << (MAX_LZW_BITS-8)) + dest->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(dest->waiting_code, c);

    if (dest->hash_code[i] == 0) {
      /* hit empty slot; desired symbol not in table */
      output(dest, dest->waiting_code);
      if (dest->free_code < LZW_TABLE_SIZE) {
	dest->hash_code[i] = dest->free_code++; /* add symbol to hashtable */
	dest->hash_value[i] = probe_value;
      } else
	clear_block(dest);
      dest->waiting_code = c;
      continue;
    }
    if (dest->hash_value[i] == probe_value) {
      dest->waiting_code = dest->hash_code[i];
      continue;
    }

    if (i == 0)			/* secondary hash (after G. Knott) */
      disp = 1;
    else
      disp = HSIZE - i;
    for (;;) {
      i -= disp;
      if (i < 0)
	i += HSIZE;
      if (dest->hash_code[i] == 0) {
	/* hit empty slot; desired symbol not in table */
	output(dest, dest->waiting_code);
	if (dest->free_code < LZW_TABLE_SIZE) {
	  dest->hash_code[i] = dest->free_code++; /* add symbol to hashtable */
	  dest->hash_value[i] = probe_value;
	} else
	  clear_block(dest);
	dest->waiting_code = c;
	break;
      }
      if (dest->hash_value[i] == probe_value) {
	dest->waiting_code = dest->hash_code[i];
	break;
      }
    }
  }
}


/*
 * The pseudo-compression algorithm.
 *
 * In this version we simply output each pixel value as a separate symbol;
 * thus, no compression occurs.  In fact, there is expansion of one bit per
 * pixel, because we use a symbol width one bit wider than the pixel width.
 *
 * GIF ordinarily uses variable-width symbols, and the decoder will expect
 * to ratchet up the symbol width after a fixed number of symbols.
 * To simplify the logic and keep the expansion penalty down, we emit a
 * GIF Clear code to reset the decoder just before the width would ratchet up.
 * Thus, all the symbols in the output file will have the same bit width.
 * Note that emitting the Clear codes at the right times is a mere matter of
 * counting output symbols and is in no way dependent on the LZW algorithm.
 *
 * With a small basic pixel width (low color count), Clear codes will be
 * needed very frequently, causing the file to expand even more.  So this
 * simplistic approach wouldn't work too well on bilevel images, for example.
 * But for output of JPEG conversions the pixel width will usually be 8 bits
 * (129 to 256 colors), so the overhead added by Clear symbols is only about
 * one symbol in every 256.
 */

METHODDEF(void)
put_raw_pixel_rows (j_decompress_ptr cinfo, djpeg_dest_ptr dinfo,
		    JDIMENSION rows_supplied)
{
  gif_dest_ptr dest = (gif_dest_ptr) dinfo;
  register JSAMPROW ptr;
  register JDIMENSION col;
  code_int c;

  ptr = dest->pub.buffer[0];
  for (col = cinfo->output_width; col > 0; col--) {
    c = (code_int) GETJSAMPLE(*ptr++);
    /* Accept and output one pixel value.
     * The given value must be less than n_bits wide.
     */

    /* Output the given pixel value as a symbol. */
    output(dest, c);
    /* Issue Clear codes often enough to keep the reader from ratcheting up
     * its symbol size.
     */
    if (dest->code_counter < dest->maxcode) {
      dest->code_counter++;
    } else {
      output(dest, dest->ClearCode);
      dest->code_counter = dest->ClearCode + 2;	/* reset the counter */
    }
  }
}


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

METHODDEF(void)
finish_output_gif (j_decompress_ptr cinfo, djpeg_dest_ptr dinfo)
{
  gif_dest_ptr dest = (gif_dest_ptr) dinfo;

  /* Flush compression mechanism */
  compress_term(dest);
  /* Write a zero-length data block to end the series */
  putc(0, dest->pub.output_file);
  /* Write the GIF terminator mark */
  putc(';', dest->pub.output_file);
  /* Make sure we wrote the output file OK */
  JFFLUSH(dest->pub.output_file);
  if (JFERROR(dest->pub.output_file))
    ERREXIT(cinfo, JERR_FILE_WRITE);
}


/*
 * The module selection routine for GIF format output.
 */

GLOBAL(djpeg_dest_ptr)
jinit_write_gif (j_decompress_ptr cinfo, boolean is_lzw)
{
  gif_dest_ptr dest;

  /* Create module interface object, fill in method pointers */
  dest = (gif_dest_ptr) (*cinfo->mem->alloc_small)
    ((j_common_ptr) cinfo, JPOOL_IMAGE, SIZEOF(gif_dest_struct));
  dest->cinfo = cinfo;		/* make back link for subroutines */
  dest->pub.start_output = start_output_gif;
  dest->pub.finish_output = finish_output_gif;

  if (cinfo->out_color_space != JCS_GRAYSCALE &&
      cinfo->out_color_space != JCS_RGB)
    ERREXIT(cinfo, JERR_GIF_COLORSPACE);

  /* Force quantization if color or if > 8 bits input */
  if (cinfo->out_color_space != JCS_GRAYSCALE || cinfo->data_precision > 8) {
    /* Force quantization to at most 256 colors */
    cinfo->quantize_colors = TRUE;
    if (cinfo->desired_number_of_colors > 256)
      cinfo->desired_number_of_colors = 256;
  }

  /* Calculate output image dimensions so we can allocate space */
  jpeg_calc_output_dimensions(cinfo);

  if (cinfo->output_components != 1) /* safety check: just one component? */
    ERREXIT(cinfo, JERR_GIF_BUG);

  /* Create decompressor output buffer. */
  dest->pub.buffer = (*cinfo->mem->alloc_sarray)
    ((j_common_ptr) cinfo, JPOOL_IMAGE, cinfo->output_width, (JDIMENSION) 1);
  dest->pub.buffer_height = 1;

  if (is_lzw) {
    dest->pub.put_pixel_rows = put_LZW_pixel_rows;
    /* Allocate space for hash table */
    dest->hash_code = (code_int *) (*cinfo->mem->alloc_small)
      ((j_common_ptr) cinfo, JPOOL_IMAGE, HSIZE * SIZEOF(code_int));
    dest->hash_value = (hash_entry FAR *) (*cinfo->mem->alloc_large)
      ((j_common_ptr) cinfo, JPOOL_IMAGE, HSIZE * SIZEOF(hash_entry));
  } else {
    dest->pub.put_pixel_rows = put_raw_pixel_rows;
    /* Mark tables unused */
    dest->hash_code = NULL;
    dest->hash_value = NULL;
  }

  return &dest->pub;
}

#endif /* GIF_SUPPORTED */