jwrgif.c

/*
 * jwrgif.c
 *
 * Copyright (C) 1991, 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.
 *
 * 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.
 *
 * These routines are invoked via the methods put_pixel_rows, put_color_map,
 * and output_init/term.
 */

/*
 * 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.
 *
 * 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 "jinclude.h"

#ifdef GIF_SUPPORTED


static decompress_info_ptr dcinfo; /* to avoid passing to all functions */

#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 */

static int n_bits;		/* current number of bits/code */
static code_int maxcode;	/* maximum code, given n_bits */
#define MAXCODE(n_bits)	(((code_int) 1 << (n_bits)) - 1)

static int init_bits;		/* initial n_bits ... restored after clear */

static code_int ClearCode;	/* clear code (doesn't change) */
static code_int EOFCode;	/* EOF code (ditto) */

static code_int free_code;	/* first not-yet-used symbol code */

/*
 * The LZW hash table consists of three parallel arrays:
 *   hash_code[i]	code of symbol in slot i, or 0 if empty slot
 *   hash_prefix[i]	symbol's prefix code; undefined if empty slot
 *   hash_suffix[i]	symbol's suffix character; undefined if empty slot
 * where slot values (i) range from 0 to HSIZE-1.
 *
 * 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 tables are allocated from FAR heap space since they would use up
 * rather a lot of the near data space in a PC.
 */

static code_int FAR *hash_code;	/* => hash table of symbol codes */
static code_int FAR *hash_prefix; /* => hash table of prefix symbols */
static UINT8 FAR *hash_suffix;	/* => hash table of suffix bytes */


/*
 * 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.
 */

static int bytesinpkt;		/* # of bytes in current packet */
static char packetbuf[256];	/* workspace for accumulating packet */


LOCAL void
flush_packet (void)
/* flush any accumulated data */
{
  if (bytesinpkt > 0) {		/* never write zero-length packet */
    packetbuf[0] = bytesinpkt++;
    if (fwrite(packetbuf, 1, bytesinpkt, dcinfo->output_file) != bytesinpkt)
      ERREXIT(dcinfo->emethods, "Output file write error");
    bytesinpkt = 0;
  }
}


LOCAL void
char_out (int c)
/* Add a character to current packet; flush to disk if necessary */
{
  packetbuf[++bytesinpkt] = c;
  if (bytesinpkt >= 255)
    flush_packet();
}


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

static INT32 cur_accum;		/* holds bits not yet output */
static int cur_bits;		/* # of bits in cur_accum */


LOCAL void
output (code_int code)
/* Emit a code of n_bits bits */
/* Uses cur_accum and cur_bits to reblock into 8-bit bytes */
{
  if (cur_bits > 0)
    cur_accum |= ((INT32) code << cur_bits);
  else
    cur_accum = code;
  cur_bits += n_bits;

  while (cur_bits >= 8) {
    char_out((int) (cur_accum & 0xFF));
    cur_accum >>= 8;
    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 (free_code > maxcode) {
    n_bits++;
    if (n_bits == MAX_LZW_BITS)
      maxcode = LZW_TABLE_SIZE;	/* free_code will never exceed this */
    else
      maxcode = MAXCODE(n_bits);
  }
}


/* The LZW algorithm proper */

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


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


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


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


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

  if (first_byte) {		/* need to initialize waiting_code */
    waiting_code = c;
    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)) + waiting_code;
  /* i is less than twice 2**MAX_LZW_BITS, therefore less than twice HSIZE */
  if (i >= HSIZE)
    i -= HSIZE;
  
  if (hash_code[i] != 0) {	/* is first probed slot empty? */
    if (hash_prefix[i] == waiting_code && hash_suffix[i] == c) {
      waiting_code = hash_code[i];
      return;
    }
    if (i == 0)			/* secondary hash (after G. Knott) */
      disp = 1;
    else
      disp = HSIZE - i;
    while (1) {
      i -= disp;
      if (i < 0)
	i += HSIZE;
      if (hash_code[i] == 0)
	break;			/* hit empty slot */
      if (hash_prefix[i] == waiting_code && hash_suffix[i] == c) {
	waiting_code = hash_code[i];
	return;
      }
    }
  }

  /* here when hashtable[i] is an empty slot; desired symbol not in table */
  output(waiting_code);
  if (free_code < LZW_TABLE_SIZE) {
    hash_code[i] = free_code++;	/* add symbol to hashtable */
    hash_prefix[i] = waiting_code;
    hash_suffix[i] = c;
  } else
    clear_block();
  waiting_code = c;
}


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


/* GIF header construction */


LOCAL void
put_word (UINT16 w)
/* Emit a 16-bit word, LSB first */
{
  putc(w & 0xFF, dcinfo->output_file);
  putc((w >> 8) & 0xFF, dcinfo->output_file);
}


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


LOCAL void
emit_header (int num_colors, JSAMPARRAY colormap)
/* Output the GIF file header, including color map */
/* If colormap==NULL, synthesize a gray-scale colormap */
{
  int BitsPerPixel, ColorMapSize, InitCodeSize, FlagByte;
  int i;

  if (num_colors > 256)
    ERREXIT(dcinfo->emethods, "GIF can only handle 256 colors");
  /* Compute bits/pixel and related values */
  if (num_colors <= 2)
    BitsPerPixel = 1;
  else if (num_colors <= 4)
    BitsPerPixel = 2;
  else if (num_colors <= 8)
    BitsPerPixel = 3;
  else if (num_colors <= 16)
    BitsPerPixel = 4;
  else if (num_colors <= 32)
    BitsPerPixel = 5;
  else if (num_colors <= 64)
    BitsPerPixel = 6;
  else if (num_colors <= 128)
    BitsPerPixel = 7;
  else
    BitsPerPixel = 8;
  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.
   */
  fwrite("GIF87a", 1, 6, dcinfo->output_file);
  /* Write the Logical Screen Descriptor */
  put_word((UINT16) dcinfo->image_width);
  put_word((UINT16) dcinfo->image_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, dcinfo->output_file);
  putc(0, dcinfo->output_file);	/* Background color index */
  putc(0, dcinfo->output_file);	/* Reserved in GIF87 (aspect ratio in GIF89) */
  /* Write the Global Color Map */
  for (i=0; i < ColorMapSize; i++) {
    if (i < num_colors) {
      if (colormap != NULL) {
	if (dcinfo->out_color_space == CS_RGB) {
	  /* Normal case: RGB color map */
	  putc(GETJSAMPLE(colormap[0][i]), dcinfo->output_file);
	  putc(GETJSAMPLE(colormap[1][i]), dcinfo->output_file);
	  putc(GETJSAMPLE(colormap[2][i]), dcinfo->output_file);
	} else {
	  /* Grayscale "color map": possible if quantizing grayscale image */
	  put_3bytes(GETJSAMPLE(colormap[0][i]));
	}
      } else {
	/* Create a gray-scale map of num_colors values, range 0..255 */
	put_3bytes((i * 255 + (num_colors-1)/2) / (num_colors-1));
      }
    } else {
      /* fill out the map to a power of 2 */
      put_3bytes(0);
    }
  }
  /* Write image separator and Image Descriptor */
  putc(',', dcinfo->output_file); /* separator */
  put_word((UINT16) 0);		/* left/top offset */
  put_word((UINT16) 0);
  put_word((UINT16) dcinfo->image_width); /* image size */
  put_word((UINT16) dcinfo->image_height);
  /* flag byte: not interlaced, no local color map */
  putc(0x00, dcinfo->output_file);
  /* Write Initial Code Size byte */
  putc(InitCodeSize, dcinfo->output_file);

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



/*
 * Initialize for GIF output.
 */

METHODDEF void
output_init (decompress_info_ptr cinfo)
{
  dcinfo = cinfo;		/* save for use by local routines */
  if (cinfo->final_out_comps != 1) /* safety check */
    ERREXIT(cinfo->emethods, "GIF output got confused");
  /* Allocate space for hash table */
  hash_code = (code_int FAR *) (*cinfo->emethods->alloc_medium)
				(HSIZE * SIZEOF(code_int));
  hash_prefix = (code_int FAR *) (*cinfo->emethods->alloc_medium)
				(HSIZE * SIZEOF(code_int));
  hash_suffix = (UINT8 FAR *) (*cinfo->emethods->alloc_medium)
				(HSIZE * SIZEOF(UINT8));
  /*
   * If we aren't quantizing, put_color_map won't be called,
   * so emit the header now.  This only happens with gray scale output.
   * (If we are quantizing, wait for the color map to be provided.)
   */
  if (! cinfo->quantize_colors)
    emit_header(256, (JSAMPARRAY) NULL);
}


/*
 * Write the color map.
 */

METHODDEF void
put_color_map (decompress_info_ptr cinfo, int num_colors, JSAMPARRAY colormap)
{
  emit_header(num_colors, colormap);
}


/*
 * Write some pixel data.
 */

METHODDEF void
put_pixel_rows (decompress_info_ptr cinfo, int num_rows,
		JSAMPIMAGE pixel_data)
{
  register JSAMPROW ptr;
  register long col;
  register long width = cinfo->image_width;
  register int row;
  
  for (row = 0; row < num_rows; row++) {
    ptr = pixel_data[0][row];
    for (col = width; col > 0; col--) {
      compress_byte(GETJSAMPLE(*ptr));
      ptr++;
    }
  }
}


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

METHODDEF void
output_term (decompress_info_ptr cinfo)
{
  /* Flush LZW mechanism */
  compress_term();
  /* Write a zero-length data block to end the series */
  putc(0, cinfo->output_file);
  /* Write the GIF terminator mark */
  putc(';', cinfo->output_file);
  /* Make sure we wrote the output file OK */
  fflush(cinfo->output_file);
  if (ferror(cinfo->output_file))
    ERREXIT(cinfo->emethods, "Output file write error");
  /* Free space */
  (*cinfo->emethods->free_medium) ((void FAR *) hash_code);
  (*cinfo->emethods->free_medium) ((void FAR *) hash_prefix);
  (*cinfo->emethods->free_medium) ((void FAR *) hash_suffix);
}


/*
 * The method selection routine for GIF format output.
 * This should be called from d_ui_method_selection if GIF output is wanted.
 */

GLOBAL void
jselwgif (decompress_info_ptr cinfo)
{
  cinfo->methods->output_init = output_init;
  cinfo->methods->put_color_map = put_color_map;
  cinfo->methods->put_pixel_rows = put_pixel_rows;
  cinfo->methods->output_term = output_term;

  if (cinfo->out_color_space != CS_GRAYSCALE &&
      cinfo->out_color_space != CS_RGB)
    ERREXIT(cinfo->emethods, "GIF output must be grayscale or RGB");

  /* Force quantization if color or if > 8 bits input */
  if (cinfo->out_color_space == CS_RGB || 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;
  }
}

#endif /* GIF_SUPPORTED */