xvgif.c

/*
 * xvgif.c  -  GIF loading code for 'xv'.  Based strongly on...
 *
 * gif2ras.c - Converts from a Compuserve GIF (tm) image to a Sun Raster image.
 *
 * Copyright (c) 1988, 1989 by Patrick J. Naughton
 *
 * Author: Patrick J. Naughton
 * naughton@wind.sun.com
 *
 * 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 file is provided AS IS with no warranties of any kind.  The author
 * shall have no liability with respect to the infringement of copyrights,
 * trade secrets or any patents by this file or any part thereof.  In no
 * event will the author be liable for any lost revenue or profits or
 * other special, indirect and consequential damages.
 *
 */


#include "xv.h"

typedef int boolean;

#define NEXTBYTE (*dataptr++)
#define SKIPBYTE (dataptr++)	/* quiet some compiler warnings */
#define EXTENSION     0x21
#define IMAGESEP      0x2c	/* a.k.a. Image Descriptor */
#define TRAILER       0x3b
#define INTERLACEMASK 0x40
#define COLORMAPMASK  0x80



static FILE *fp;

static int
    BitOffset = 0,		/* Bit Offset of next code */
    XC = 0, YC = 0,		/* Output X and Y coords of current pixel */
    Pass = 0,			/* Used by output routine if interlaced pic */
    OutCount = 0,		/* Decompressor output 'stack count' */
    RWidth, RHeight,		/* screen dimensions */
    Width, Height,		/* image dimensions */
    LeftOfs, TopOfs,		/* image offset */
    BitsPerPixel,		/* Bits per pixel, read from GIF header */
/*  BytesPerScanline,	*/	/* bytes per scanline in output raster */
    ColorMapSize,		/* number of colors */
    Background,			/* background color */
    Transparent,		/* transparent color (GRR 19980314) */
    CodeSize,			/* Code size, read from GIF header */
    InitCodeSize,		/* Starting code size, used during Clear */
    Code,			/* Value returned by ReadCode */
    MaxCode,			/* limiting value for current code size */
    ClearCode,			/* GIF clear code */
    EOFCode,			/* GIF end-of-information code */
    CurCode, OldCode, InCode,	/* Decompressor variables */
    FirstFree,			/* First free code, generated per GIF spec */
    FreeCode,			/* Decompressor, next free slot in hash table */
    FinChar,			/* Decompressor variable */
    BitMask,			/* AND mask for data size */
    ReadMask,			/* Code AND mask for current code size */
    Misc,                       /* miscellaneous bits (interlace, local cmap)*/
    GlobalBitsPerPixel,		/* may have local colormap of different size */
    GlobalColorMapSize,		/*   (ditto)  */
    GlobalBitMask;		/*   (ditto)  */


static boolean Interlace, HasGlobalColormap;

static byte *RawGIF;		/* The heap array to hold it, raw */
static byte *Raster;		/* The raster data stream, unblocked */
static byte *pic8;
static size_t rasterSize;

    /* The hash table used by the decompressor */
static int Prefix[4096];
static int Suffix[4096];

    /* An output array used by the decompressor */
static int OutCode[4097];

static int gif89 = 0;
static const char *id87 = "GIF87a";
static const char *id89 = "GIF89a";

static int const EGApalette[16][3] = {
  {0,0,0},       {0,0,128},     {0,128,0},     {0,128,128},
  {128,0,0},     {128,0,128},   {128,128,0},   {200,200,200},
  {100,100,100}, {100,100,255}, {100,255,100}, {100,255,255},
  {255,100,100}, {255,100,255}, {255,255,100}, {255,255,255} };


static int   readImage   PARM((PICINFO *));
static int   readCode    PARM((void));
static void  doInterlace PARM((int));
static int   gifError    PARM((PICINFO *, const char *));
static void  gifWarning  PARM((const char *));

static long int   filesize;
static const char *bname;
static byte       *dataptr;


/*****************************/
int LoadGIF(fname, pinfo)
     char *fname;
     PICINFO *pinfo;
/*****************************/
{
  /* returns '1' if successful */

  register byte  ch, *origptr;
  register int   i, block;
  int            aspect;
  char tmpname[256];
  byte r[256], g[256], b[256];

  /* initialize variables */
  BitOffset = XC = YC = OutCount = 0;
  Pass = -1;
  RawGIF = Raster = pic8 = NULL;
  gif89 = 0;
  Transparent = -1;

  pinfo->pic     = (byte *) NULL;
  pinfo->comment = (char *) NULL;
  pinfo->numpages= 0;

  bname = BaseName(fname);
  fp = xv_fopen(fname,"r");
  if (!fp) return ( gifError(pinfo, "can't open file") );


  /* find the size of the file */
  fseek(fp, 0L, 2);
  filesize = ftell(fp);
  fseek(fp, 0L, 0);

  if (filesize > (2147483647L - 256))
    return( gifError(pinfo, "GIF file size is too large") );

  /* the +256's are so we can read truncated GIF files without fear of
     segmentation violation */
  if (!(dataptr = RawGIF = (byte *) calloc((size_t) filesize+256, (size_t) 1)))
    FatalError("LoadGIF: not enough memory to read GIF file");

  rasterSize = filesize+256;
  if (!(Raster = (byte *) calloc(rasterSize, (size_t) 1)))
    FatalError("LoadGIF: not enough memory to read GIF file");

  if (fread(dataptr, (size_t) filesize, (size_t) 1, fp) != 1)
    return( gifError(pinfo, "GIF data read failed") );
  fclose(fp);

  origptr = dataptr;

  if      (strncmp((char *) dataptr, id87, (size_t) 6)==0) gif89 = 0;
  else if (strncmp((char *) dataptr, id89, (size_t) 6)==0) gif89 = 1;
  else    return( gifError(pinfo, "not a GIF file"));

  dataptr += 6;

  /* Get variables from the GIF screen descriptor */

  ch = NEXTBYTE;
  RWidth = ch + 0x100 * NEXTBYTE;	/* screen dimensions... not used. */
  ch = NEXTBYTE;
  RHeight = ch + 0x100 * NEXTBYTE;
  if (DEBUG) fprintf(stderr,"GIF89 logical screen = %d x %d\n",RWidth,RHeight);

  ch = NEXTBYTE;
  HasGlobalColormap = ((ch & COLORMAPMASK) ? True : False);

  /* GRR 20070318:  fix decoding bug when global and local color-table sizes
   *                differ */
  GlobalBitsPerPixel = BitsPerPixel = (ch & 7) + 1;
  GlobalColorMapSize = ColorMapSize = numcols = 1 << BitsPerPixel;
  GlobalBitMask = BitMask = ColorMapSize - 1;

  Background = NEXTBYTE;		/* background color... not used. */

  aspect = NEXTBYTE;
  if (aspect) {
    if (!gif89) return(gifError(pinfo,"corrupt GIF file (screen descriptor)"));
    else normaspect = (float) (aspect + 15) / 64.0;   /* gif89 aspect ratio */
    if (DEBUG) fprintf(stderr,"GIF89 aspect = %f\n", normaspect);
    /* FIXME:  apparently this _should_ apply to all frames in a multi-image
     *         GIF (i.e., PgUp/PgDn), but it doesn't */
  }


  /* Read in global colormap. */

  if (HasGlobalColormap)
    for (i=0; i<ColorMapSize; i++) {
      r[i] = NEXTBYTE;
      g[i] = NEXTBYTE;
      b[i] = NEXTBYTE;
    }
  else {  /* no _global_ colormap in GIF file (but may have local one(s)) */
    /* put std EGA palette (repeated 16 times) into colormap, for lack of
       anything better to do at the moment */

    for (i=0; i<256; i++) {
      r[i] = EGApalette[i&15][0];
      g[i] = EGApalette[i&15][1];
      b[i] = EGApalette[i&15][2];
    }
  }
  memcpy(pinfo->r, r, sizeof r);
  memcpy(pinfo->g, g, sizeof g);
  memcpy(pinfo->b, b, sizeof b);

  if (DEBUG > 1) {
    fprintf(stderr,"  global color table%s:\n",
      HasGlobalColormap? "":" (repeated EGA palette)");
    for (i=0; i<ColorMapSize; i++) {
      fprintf(stderr,"    (%3d  %02x,%02x,%02x)\n", i, pinfo->r[i],
        pinfo->g[i], pinfo->b[i]);
    }
  }

  /* possible things at this point are:
   *   an application extension block
   *   a comment extension block
   *   an (optional) graphic control extension block
   *       followed by either an image
   *	   or a plaintext extension
   */

  while (1) {
    block = NEXTBYTE;

    if (DEBUG) fprintf(stderr,"LoadGIF: ");

    if (block == EXTENSION) {  /* parse extension blocks */
      int i, fn, blocksize, aspnum, aspden;

      /* read extension block */
      fn = NEXTBYTE;

      if (DEBUG) fprintf(stderr,"GIF extension type 0x%02x\n", fn);

      if (fn == 'R') {                  /* GIF87 aspect extension */
	int sbsize;

	blocksize = NEXTBYTE;
	if (blocksize == 2) {
	  aspnum = NEXTBYTE;
	  aspden = NEXTBYTE;
	  if (aspden>0 && aspnum>0)
	    normaspect = (float) aspnum / (float) aspden;
	  else { normaspect = 1.0;  aspnum = aspden = 1; }

	  if (DEBUG) fprintf(stderr,"GIF87 aspect extension: %d:%d = %f\n\n",
			     aspnum, aspden,normaspect);
	}
	else {
	  for (i=0; i<blocksize; i++) SKIPBYTE;
	}

	while ((sbsize=NEXTBYTE)>0) {  /* eat any following data subblocks */
	  for (i=0; i<sbsize; i++) SKIPBYTE;
	}
      }


      else if (fn == 0xFE) {  /* Comment Extension */
	int   ch, j, sbsize, cmtlen;
	byte *ptr1, *cmt, *cmt1, *sp;

	cmtlen = 0;
	ptr1 = dataptr;      /* remember start of comments */

	/* figure out length of comment */
	do {
	  sbsize = NEXTBYTE;
	  cmtlen += sbsize;
	  for (j=0; j<sbsize; j++) ch = NEXTBYTE;
	} while (sbsize);


	if (cmtlen>0) {   /* build into one un-blocked comment */
	  /* this can overflow iff cmtlen == 2G - 1, but then filesize
	   * would have to be > 2GB, which was disallowed above */
	  cmt = (byte *) malloc((size_t) (cmtlen + 1));
	  if (!cmt) FatalError("LoadGIF: couldn't malloc space for comments");
	  /* else */ {
	    sp = cmt;
	    do {
	      sbsize = (*ptr1++);
	      for (j=0; j<sbsize; j++, sp++, ptr1++) *sp = *ptr1;
	    } while (sbsize);
	    *sp = '\0';

	    if (pinfo->comment) {    /* have to strcat onto old comments */
	      cmt1 = (byte *) malloc(strlen(pinfo->comment) + cmtlen + 2);
	      if (!cmt1) {
		free(cmt);
	        FatalError("LoadGIF: couldn't malloc space for comments");
	      }
	      /* else */ {
		strcpy((char *) cmt1, (char *) pinfo->comment);
		strcat((char *) cmt1, (char *) "\n");
		strcat((char *) cmt1, (char *) cmt);
		free(pinfo->comment);
		free(cmt);
		pinfo->comment = (char *) cmt1;
	      }
	    }
	    else pinfo->comment = (char *) cmt;
	  }  /* if (cmt) */
	}  /* if cmtlen>0 */
      }


      else if (fn == 0x01) {  /* PlainText Extension */
	int j,sbsize,ch;
	int tgLeft, tgTop, tgWidth, tgHeight, cWidth, cHeight, fg, bg;

	SetISTR(ISTR_INFO, "%s:  %s", bname,
		"PlainText extension found in GIF file.  Ignored.");

	sbsize   = NEXTBYTE;
	tgLeft   = NEXTBYTE;  tgLeft   += (NEXTBYTE)<<8;
	tgTop    = NEXTBYTE;  tgTop    += (NEXTBYTE)<<8;
	tgWidth  = NEXTBYTE;  tgWidth  += (NEXTBYTE)<<8;
	tgHeight = NEXTBYTE;  tgHeight += (NEXTBYTE)<<8;
	cWidth   = NEXTBYTE;
	cHeight  = NEXTBYTE;
	fg       = NEXTBYTE;
	bg       = NEXTBYTE;
	i=12;
	for ( ; i<sbsize; i++) SKIPBYTE;   /* read rest of first subblock */

	if (DEBUG) fprintf(stderr,
	   "PlainText: tgrid=%d,%d %dx%d  cell=%dx%d  col=%d,%d\n",
	   tgLeft, tgTop, tgWidth, tgHeight, cWidth, cHeight, fg, bg);

	/* read (and ignore) data sub-blocks */
	do {
	  j = 0;
	  sbsize = NEXTBYTE;
	  while (j<sbsize) {
	    ch = NEXTBYTE;  j++;
	    if (DEBUG) fprintf(stderr,"%c", ch);
	  }
	} while (sbsize);
	if (DEBUG) fprintf(stderr,"\n\n");
      }


      else if (fn == 0xF9) {  /* Graphic Control Extension */
	int j, sbsize;

	if (DEBUG) fprintf(stderr,"Graphic Control extension\n\n");

	SetISTR(ISTR_INFO, "%s:  %s", bname,
		"Graphic Control Extension ignored.");

	/* read (and ignore) data sub-blocks, unless compositing with
         * user-defined background */
	do {
	  j = 0;
	  sbsize = NEXTBYTE;
	  /* GRR 19980314:  get transparent index out of block */
	  if (have_imagebg && sbsize == 4 && Transparent < 0) {
	    byte packed_fields = NEXTBYTE;

	    j++;
	    SKIPBYTE;  j++;
	    SKIPBYTE;  j++;
	    if (packed_fields & 1) {
	      Transparent = NEXTBYTE;
	      j++;
	    }
	  }
	  while (j<sbsize) {
	    SKIPBYTE;  j++;
	  }
	} while (sbsize);
      }


      else if (fn == 0xFF) {  /* Application Extension */
	int j, sbsize;

	if (DEBUG) fprintf(stderr,"Application extension\n\n");

	/* read (and ignore) data sub-blocks */
	do {
	  j = 0; sbsize = NEXTBYTE;
	  while (j<sbsize) { SKIPBYTE;  j++; }
	} while (sbsize);
      }


      else { /* unknown extension */
	int j, sbsize;

	if (DEBUG) fprintf(stderr,"unknown GIF extension 0x%02x\n\n", fn);

	SetISTR(ISTR_INFO,
		"%s:  Unknown extension 0x%02x in GIF file.  Ignored.",
		bname, fn);

	/* read (and ignore) data sub-blocks */
	do {
	  j = 0; sbsize = NEXTBYTE;
	  while (j<sbsize) { SKIPBYTE;  j++; }
	} while (sbsize);
      }
    }


    else if (block == IMAGESEP) {
      if (DEBUG) fprintf(stderr, "imagesep (page=%d)\n", pinfo->numpages+1);
      if (DEBUG) fprintf(stderr, "  at start: offset=0x%lx\n",
                         (unsigned long)(dataptr-RawGIF));

      BitOffset = XC = YC = Pass = OutCount = 0;

      if (pinfo->numpages > 0) {   /* do multipage stuff */
	if (pinfo->numpages == 1) {    /* first time only... */
	  xv_mktemp(pinfo->pagebname, "xvpgXXXXXX"); /* a.k.a. close(mkstemp()) */
	  if (pinfo->pagebname[0] == '\0') {
	    ErrPopUp("LoadGIF: Unable to create temporary filename???",
			"\nHow unlikely!");
	    return 0;
	  }
	  /* GRR 20070328:  basename file doesn't go away, at least on Linux
           *  (though all appended-number ones do); ergo, open for reading (see
           *  if it's there), close, and explicitly unlink() if necessary */
          /* GRR 20070506:  could/should call KillPageFiles() (xv.c) instead */
	  fp = xv_fopen(pinfo->pagebname, "r");
	  if (fp) {
	    fclose(fp);
	    unlink(pinfo->pagebname);  /* no errors during testing */
	  }
	}
	sprintf(tmpname, "%s%d", pinfo->pagebname, pinfo->numpages);
	fp = xv_fopen(tmpname, "w");
	if (!fp) {
	  ErrPopUp("LoadGIF: Unable to open temp file", "\nDang!");
	  return 0;
	}
	if (WriteGIF(fp, pinfo->pic, pinfo->type, pinfo->w, pinfo->h, pinfo->r,
		 pinfo->g, pinfo->b, numcols, pinfo->colType, NULL)) {
	  fclose(fp);
	  ErrPopUp("LoadGIF: Error writing temp file", "\nBummer!");
	  return 0;
	}
	fclose(fp);
	free(pinfo->pic);
	pinfo->pic = (byte *) NULL;
	if (HasGlobalColormap) {
	  memcpy(pinfo->r, r, sizeof r);
	  memcpy(pinfo->g, g, sizeof g);
	  memcpy(pinfo->b, b, sizeof b);
	}
        BitsPerPixel = GlobalBitsPerPixel;
        numcols = ColorMapSize = GlobalColorMapSize;
        BitMask = GlobalBitMask;
      }
      if (readImage(pinfo)) ++pinfo->numpages;
      if (DEBUG) fprintf(stderr, "  at end:   offset=0x%lx\n",
                         (unsigned long)(dataptr-RawGIF));
    }


    else if (block == TRAILER) {      /* stop reading blocks */
      if (DEBUG) fprintf(stderr,"trailer");
      break;
    }

    else {      /* unknown block type */
      char str[128];

      if (DEBUG) fprintf(stderr,"block type 0x%02x  ", block);

      /* don't mention bad block if file was trunc'd, as it's all bogus */
      if ((dataptr - origptr) < filesize) {
	sprintf(str, "Unknown block type (0x%02x) at offset 0x%lx",
		block, (unsigned long)(dataptr - origptr) - 1);

	if (!pinfo->numpages) return gifError(pinfo, str);
	else gifWarning(str);
      }

      break;
    }

    if (DEBUG) fprintf(stderr,"\n");
  }

  free(RawGIF);	 RawGIF = NULL;
  free(Raster);  Raster = NULL;

  if (!pinfo->numpages)
     return( gifError(pinfo, "no image data found in GIF file") );
  if (pinfo->numpages > 1) {
    /* write the last page temp file */
    int numpages = pinfo->numpages;
    char *comment = pinfo->comment;
    sprintf(tmpname, "%s%d", pinfo->pagebname, pinfo->numpages);
    fp = xv_fopen(tmpname, "w");
    if (!fp) {
      ErrPopUp("LoadGIF: Unable to open temp file", "\nDang!");
      return 0;
    }
    if (WriteGIF(fp, pinfo->pic, pinfo->type, pinfo->w, pinfo->h, pinfo->r,
		 pinfo->g, pinfo->b, numcols, pinfo->colType, NULL)) {
      fclose(fp);
      ErrPopUp("LoadGIF: Error writing temp file", "\nBummer!");
      return 0;
    }
    fclose(fp);
    free(pinfo->pic);
    pinfo->pic = (byte *) NULL;

    /* load the first page temp file */
    sprintf(tmpname, "%s%d", pinfo->pagebname, 1);
    i = LoadGIF(tmpname, pinfo);
    pinfo->numpages = numpages;
    pinfo->comment = comment;
  }

  return 1;
}


/********************************************/
static int readImage(pinfo)
     PICINFO *pinfo;
{
  register byte ch, ch1, *ptr1, *picptr;
  int           i, npixels, maxpixels;
  boolean       HasLocalColormap;

  npixels = maxpixels = 0;

  /* read in values from the image descriptor */

  ch = NEXTBYTE;
  LeftOfs = ch + 0x100 * NEXTBYTE;
  ch = NEXTBYTE;
  TopOfs  = ch + 0x100 * NEXTBYTE;
  ch = NEXTBYTE;
  Width   = ch + 0x100 * NEXTBYTE;
  ch = NEXTBYTE;
  Height  = ch + 0x100 * NEXTBYTE;

  Misc = NEXTBYTE;
  Interlace = ((Misc & INTERLACEMASK) ? True : False);
  HasLocalColormap = ((Misc & COLORMAPMASK) ? True : False);

  if (HasLocalColormap) {
    BitsPerPixel = (Misc & 7) + 1;
    ColorMapSize = numcols = 1 << BitsPerPixel;  /* GRR 20070318 */
    BitMask = ColorMapSize - 1;
    if (DEBUG) fprintf(stderr,"  local color table, %d bits (%d entries)\n",
      (Misc&7)+1, ColorMapSize);
    for (i=0; i<ColorMapSize; i++) {
      pinfo->r[i] = NEXTBYTE;
      pinfo->g[i] = NEXTBYTE;
      pinfo->b[i] = NEXTBYTE;
    }
    if (DEBUG > 1) {
      for (i=0; i<ColorMapSize; i++) {
        fprintf(stderr,"    (%3d  %02x,%02x,%02x)\n", i, pinfo->r[i],
          pinfo->g[i], pinfo->b[i]);
      }
    }
  }


  if (!HasGlobalColormap && !HasLocalColormap) {
    /* no global or local colormap */
    SetISTR(ISTR_WARNING, "%s:  %s", bname,
	    "No colormap in this GIF file.  Assuming EGA colors.");
  }


  /* GRR 19980314 */
  /* need not worry about size of EGA palette:  full 256 colors */
  if (have_imagebg && Transparent >= 0 &&
      Transparent < ((Misc&0x80)? (1 << ((Misc&7)+1)) : ColorMapSize) )
  {
    pinfo->r[Transparent] = (imagebgR >> 8);
    pinfo->g[Transparent] = (imagebgG >> 8);
    pinfo->b[Transparent] = (imagebgB >> 8);
  }



  /* Start reading the raster data. First we get the intial code size
   * and compute decompressor constant values, based on this code size.
   */

  CodeSize = NEXTBYTE;

  ClearCode = (1 << CodeSize);
  EOFCode = ClearCode + 1;
  FreeCode = FirstFree = ClearCode + 2;

  /* The GIF spec has it that the code size is the code size used to
   * compute the above values is the code size given in the file, but the
   * code size used in compression/decompression is the code size given in
   * the file plus one. (thus the ++).
   */

  CodeSize++;
  InitCodeSize = CodeSize;
  MaxCode = (1 << CodeSize);
  ReadMask = MaxCode - 1;



  /* UNBLOCK:
   * Read the raster data.  Here we just transpose it from the GIF array
   * to the Raster array, turning it from a series of blocks into one long
   * data stream, which makes life much easier for readCode().
   */

  ptr1 = Raster;
  do {
    ch = ch1 = NEXTBYTE;
    while (ch--) { *ptr1 = NEXTBYTE; ptr1++; }
    if ((dataptr - RawGIF) > filesize) {
      SetISTR(ISTR_WARNING,"%s:  %s", bname,
	      "This GIF file seems to be truncated.  Winging it.");
      break;
    }
  } while(ch1);




  if (DEBUG) {
    fprintf(stderr,"LoadGIF: image is %dx%d, %d bits, %sinterlaced\n",
	    Width, Height, BitsPerPixel, Interlace ? "" : "non-");
  }


  /* Allocate the 'pic' */
  maxpixels = Width*Height;  /* 65535*65535 max (but everything is int) */
  if (Width <= 0 || Height <= 0 || maxpixels/Width != Height)
    return( gifError(pinfo, "image dimensions out of range") );
  picptr = pic8 = (byte *) malloc((size_t) maxpixels);
  if (!pic8) FatalError("LoadGIF: couldn't malloc 'pic8'");



  /* Decompress the file, continuing until you see the GIF EOF code.
   * One obvious enhancement is to add checking for corrupt files here.
   */

  Code = readCode();
  while (Code != EOFCode) {
    /* Clear code sets everything back to its initial value, then reads the
     * immediately subsequent code as uncompressed data.
     */

    if (Code == ClearCode) {
      CodeSize = InitCodeSize;
      MaxCode = (1 << CodeSize);
      ReadMask = MaxCode - 1;
      FreeCode = FirstFree;
      Code = readCode();
      CurCode = OldCode = Code;
      FinChar = CurCode & BitMask;
      if (!Interlace) *picptr++ = FinChar;
         else doInterlace(FinChar);
      npixels++;
    }
    else {
      /* If not a clear code, must be data: save same as CurCode and InCode */

      /* if we're at maxcode and didn't get a clear, stop loading */
      if (FreeCode>=4096) { /* printf("freecode blew up\n"); */
			    break; }

      CurCode = InCode = Code;

      /* If greater or equal to FreeCode, not in the hash table yet;
       * repeat the last character decoded
       */

      if (CurCode >= FreeCode) {
	CurCode = OldCode;
	if (OutCount > 4096) {  /* printf("outcount1 blew up\n"); */ break; }
	OutCode[OutCount++] = FinChar;
      }

      /* Unless this code is raw data, pursue the chain pointed to by CurCode
       * through the hash table to its end; each code in the chain puts its
       * associated output code on the output queue.
       */

      while (CurCode >= ClearCode) {  /* Joe Zbiciak fix, 20070621 */
	if (OutCount > 4096) break;   /* corrupt file */
	OutCode[OutCount++] = Suffix[CurCode];
	CurCode = Prefix[CurCode];
      }

      if (OutCount > 4096) { /* printf("outcount blew up\n"); */ break; }

      /* The last code in the chain is treated as raw data. */

      FinChar = CurCode & BitMask;
      OutCode[OutCount++] = FinChar;

      /* Now we put the data out to the Output routine.
       * It's been stacked LIFO, so deal with it that way...
       */

      /* safety thing:  prevent exceeding range of 'pic8' */
      if (npixels + OutCount > maxpixels) OutCount = maxpixels-npixels;

      npixels += OutCount;
      if (!Interlace) for (i=OutCount-1; i>=0; i--) *picptr++ = OutCode[i];
                else  for (i=OutCount-1; i>=0; i--) doInterlace(OutCode[i]);
      OutCount = 0;

      /* Build the hash table on-the-fly. No table is stored in the file. */

      Prefix[FreeCode] = OldCode;
      Suffix[FreeCode] = FinChar;
      OldCode = InCode;

      /* Point to the next slot in the table.  If we exceed the current
       * MaxCode value, increment the code size unless it's already 12.  If it
       * is, do nothing: the next code decompressed better be CLEAR
       */

      FreeCode++;
      if (FreeCode >= MaxCode) {
	if (CodeSize < 12) {
	  CodeSize++;
	  MaxCode *= 2;
	  ReadMask = (1 << CodeSize) - 1;
	}
      }
    }
    Code = readCode();
    if (npixels >= maxpixels) break;
  }

  if (npixels != maxpixels) {
    SetISTR(ISTR_WARNING,"%s:  %s", bname,
	    "This GIF file seems to be truncated.  Winging it.");
    if (!Interlace)  /* clear->EOBuffer */
      xvbzero((char *) pic8+npixels,
	      (size_t) (maxpixels-npixels<0 ? 0 : maxpixels-npixels));
  }

  /* fill in the PICINFO structure */

  pinfo->pic     = pic8;
  pinfo->w       = Width;
  pinfo->h       = Height;
  pinfo->type    = PIC8;
  pinfo->frmType = F_GIF;
  pinfo->colType = F_FULLCOLOR;

  pinfo->normw = pinfo->w;   pinfo->normh = pinfo->h;

  sprintf(pinfo->fullInfo,
	  "GIF%s, %d bit%s per pixel, %sinterlaced.  (%ld bytes)",
 	  (gif89) ? "89" : "87", BitsPerPixel,
	  (BitsPerPixel==1) ? "" : "s",
 	  Interlace ? "" : "non-", filesize);

  sprintf(pinfo->shrtInfo, "%dx%d GIF%s.",Width,Height,(gif89) ? "89" : "87");

  /* pinfo.comment gets handled in main LoadGIF() block-reader */

  return 1;
}



/* Fetch the next code from the raster data stream.  The codes can be
 * any length from 3 to 12 bits, packed into 8-bit bytes, so we have to
 * maintain our location in the Raster array as a BIT Offset.  We compute
 * the byte Offset into the raster array by dividing this by 8, pick up
 * three bytes, compute the bit Offset into our 24-bit chunk, shift to
 * bring the desired code to the bottom, then mask it off and return it.
 */

static int readCode()
{
  int RawCode, ByteOffset;

  ByteOffset = BitOffset / 8;
  if (ByteOffset >= rasterSize-2)
    return 0;
  RawCode = Raster[ByteOffset] + (Raster[ByteOffset + 1] << 8);
  if (CodeSize >= 8)
    RawCode += ( ((int) Raster[ByteOffset + 2]) << 16);
  RawCode >>= (BitOffset % 8);
  BitOffset += CodeSize;

  return(RawCode & ReadMask);
}


/***************************/
static void doInterlace(Index)
     int Index;
{
  static byte *ptr = NULL;
  static int   oldYC = -1;

  if (Pass == -1) {  /* first time through - init stuff */
    oldYC = -1;
    Pass = 0;
  }

  if (oldYC != YC) {  ptr = pic8 + YC * Width;  oldYC = YC; }

  if (YC<Height)
    *ptr++ = Index;

  /* Update the X-coordinate, and if it overflows, update the Y-coordinate */

  if (++XC == Width) {

    /* deal with the interlace as described in the GIF
     * spec.  Put the decoded scan line out to the screen if we haven't gone
     * past the bottom of it
     */

    XC = 0;

    switch (Pass) {
    case 0:
      YC += 8;
      if (YC >= Height) { Pass++; YC = 4; }
      break;

    case 1:
      YC += 8;
      if (YC >= Height) { Pass++; YC = 2; }
      break;

    case 2:
      YC += 4;
      if (YC >= Height) { Pass++; YC = 1; }
      break;

    case 3:
      YC += 2;  break;

    default:
      break;
    }
  }
}



/*****************************/
static int gifError(pinfo, st)
     PICINFO    *pinfo;
     const char *st;
{
  gifWarning(st);

  if (RawGIF != NULL) free(RawGIF);
  if (Raster != NULL) free(Raster);

  if (pinfo->pic) free(pinfo->pic);
  if (pinfo->comment) free(pinfo->comment);

  if (pic8 && pic8 != pinfo->pic) free(pic8);

  pinfo->pic = (byte *) NULL;
  pinfo->comment = (char *) NULL;

  return 0;
}


/*****************************/
static void gifWarning(st)
     const char *st;
{
  SetISTR(ISTR_WARNING,"%s:  %s", bname, st);
}