path: root/generic/tkImgGIF.c

/*
 * tkImgGIF.c --
 *
 *	A photo image file handler for GIF files. Reads 87a and 89a GIF files.
 *	At present, there only is a file write function. GIF images may be
 *	read using the -data option of the photo image. The data may be given
 *	as a binary string in a Tcl_Obj or by representing the data as BASE64
 *	encoded ascii.  Derived from the giftoppm code found in the pbmplus
 *	package and tkImgFmtPPM.c in the tk4.0b2 distribution.
 *
 * Copyright (c) Reed Wade (wade@cs.utk.edu), University of Tennessee
 * Copyright (c) 1995-1997 Sun Microsystems, Inc.
 * Copyright (c) 1997 Australian National University
 * Copyright (c) 2005 Donal K. Fellows
 *
 * See the file "license.terms" for information on usage and redistribution
 * of this file, and for a DISCLAIMER OF ALL WARRANTIES.
 *
 * This file also contains code from the giftoppm program, which is
 * copyrighted as follows:
 *
 * +--------------------------------------------------------------------+
 * | Copyright 1990, David Koblas.					|
 * |   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.		|
 * +--------------------------------------------------------------------+
 *
 * This file also contains code from miGIF. See lower down in file for the
 * applicable copyright notice for that portion.
 *
 * RCS: @(#) $Id: tkImgGIF.c,v 1.27 2005/06/19 21:49:08 dkf Exp $
 */

/*
 * GIF's are represented as data in either binary or base64 format.  base64
 * strings consist of 4 6-bit characters -> 3 8 bit bytes.  A-Z, a-z, 0-9, +
 * and / represent the 64 values (in order).  '=' is a trailing padding char
 * when the un-encoded data is not a multiple of 3 bytes.  We'll ignore white
 * space when encountered.  Any other invalid character is treated as an EOF
 */

#define GIF_SPECIAL	(256)
#define GIF_PAD		(GIF_SPECIAL+1)
#define GIF_SPACE	(GIF_SPECIAL+2)
#define GIF_BAD		(GIF_SPECIAL+3)
#define GIF_DONE	(GIF_SPECIAL+4)

/*
 * structure to "mimic" FILE for Mread, so we can look like fread.  The
 * decoder state keeps track of which byte we are about to read, or EOF.
 */

typedef struct mFile {
    unsigned char *data;	/* mmencoded source string */
    int c;			/* bits left over from previous character */
    int state;			/* decoder state (0-4 or GIF_DONE) */
    int length;			/* Total amount of bytes in data */
} MFile;

#include "tkInt.h"
#include "tkPort.h"

/*
 * Non-ASCII encoding support:
 * Most data in a GIF image is binary and is treated as such. However, a few
 * key bits are stashed in ASCII. If we try to compare those pieces to the
 * char they represent, it will fail on any non-ASCII (eg, EBCDIC) system. To
 * accomodate these systems, we test against the numeric value of the ASCII
 * characters instead of the characters themselves. This is encoding
 * independant.
 */

static CONST char GIF87a[] = {			/* ASCII GIF87a */
    0x47, 0x49, 0x46, 0x38, 0x37, 0x61, 0x00
};
static CONST char GIF89a[] = {			/* ASCII GIF89a */
    0x47, 0x49, 0x46, 0x38, 0x39, 0x61, 0x00
};
#define GIF_TERMINATOR	0x3b			/* ASCII ; */
#define GIF_EXTENSION	0x21			/* ASCII ! */
#define GIF_START	0x2c			/* ASCII , */

#define INLINE_DATA_BINARY	((const char *) 0x01)
#define INLINE_DATA_BASE64	((const char *) 0x02)

/*
 *			HACK ALERT!!  HACK ALERT!!  HACK ALERT!!
 * This code is hard-wired for reading from files. In order to read from a
 * data stream, we'll trick fread so we can reuse the same code. 0==from file;
 * 1==from base64 encoded data; 2==from binary data
 */

typedef struct {
    const char *fromData;
    unsigned char workingBuffer[280];
    struct {
	int bytes;
	int done;
	unsigned int window;
	int bitsInWindow;
	unsigned char *c;
    } reader;
} GIFImageConfig;

/*
 * The format record for the GIF file format:
 */

static int	FileMatchGIF _ANSI_ARGS_((Tcl_Channel chan, CONST char *fileName,
		    Tcl_Obj *format, int *widthPtr, int *heightPtr,
		    Tcl_Interp *interp));
static int	FileReadGIF _ANSI_ARGS_((Tcl_Interp *interp, Tcl_Channel chan,
		    CONST char *fileName, Tcl_Obj *format,
		    Tk_PhotoHandle imageHandle, int destX, int destY,
		    int width, int height, int srcX, int srcY));
static int	StringMatchGIF _ANSI_ARGS_(( Tcl_Obj *dataObj, Tcl_Obj *format,
		    int *widthPtr, int *heightPtr, Tcl_Interp *interp));
static int	StringReadGIF _ANSI_ARGS_((Tcl_Interp *interp, Tcl_Obj *dataObj,
		    Tcl_Obj *format, Tk_PhotoHandle imageHandle,
		    int destX, int destY, int width, int height,
		    int srcX, int srcY));
static int 	FileWriteGIF _ANSI_ARGS_((Tcl_Interp *interp,
		    CONST char *filename, Tcl_Obj *format,
		    Tk_PhotoImageBlock *blockPtr));
static int	CommonWriteGIF _ANSI_ARGS_((Tcl_Interp *interp,
		    Tcl_Channel handle, Tcl_Obj *format,
		    Tk_PhotoImageBlock *blockPtr));

Tk_PhotoImageFormat tkImgFmtGIF = {
    "gif",		/* name */
    FileMatchGIF,	/* fileMatchProc */
    StringMatchGIF,	/* stringMatchProc */
    FileReadGIF,	/* fileReadProc */
    StringReadGIF,	/* stringReadProc */
    FileWriteGIF,	/* fileWriteProc */
    NULL,		/* stringWriteProc */
};

#define INTERLACE		0x40
#define LOCALCOLORMAP		0x80
#define BitSet(byte, bit)	(((byte) & (bit)) == (bit))
#define MAXCOLORMAPSIZE		256
#define CM_RED			0
#define CM_GREEN		1
#define CM_BLUE			2
#define CM_ALPHA		3
#define MAX_LWZ_BITS		12
#define LM_to_uint(a,b)		(((b)<<8)|(a))

/*
 * Prototypes for local procedures defined in this file:
 */

static int	DoExtension _ANSI_ARGS_((GIFImageConfig *gifConfPtr,
		    Tcl_Channel chan, int label, unsigned char *buffer,
		    int *transparent));
static int	GetCode _ANSI_ARGS_((Tcl_Channel chan, int code_size, int flag,
		    GIFImageConfig *gifConfPtr));
static int	GetDataBlock _ANSI_ARGS_((GIFImageConfig *gifConfPtr,
		    Tcl_Channel chan, unsigned char *buf));
static int	ReadColorMap _ANSI_ARGS_((GIFImageConfig *gifConfPtr,
		    Tcl_Channel chan, int number,
		    unsigned char buffer[MAXCOLORMAPSIZE][4]));
static int	ReadGIFHeader _ANSI_ARGS_((GIFImageConfig *gifConfPtr,
		    Tcl_Channel chan, int *widthPtr, int *heightPtr));
static int	ReadImage _ANSI_ARGS_((GIFImageConfig *gifConfPtr,
		    Tcl_Interp *interp, char *imagePtr, Tcl_Channel chan,
		    int len, int rows, unsigned char cmap[MAXCOLORMAPSIZE][4],
		    int width, int height, int srcX, int srcY, int interlace,
		    int transparent));

/*
 * these are for the BASE64 image reader code only
 */

static int	Fread _ANSI_ARGS_((GIFImageConfig *gifConfPtr,
		    unsigned char *dst, size_t size, size_t count,
		    Tcl_Channel chan));
static int	Mread _ANSI_ARGS_((unsigned char *dst, size_t size,
		    size_t count, MFile *handle));
static int	Mgetc _ANSI_ARGS_((MFile *handle));
static int	char64 _ANSI_ARGS_((int c));
static void	mInit _ANSI_ARGS_((unsigned char *string, MFile *handle,
		    int length));

/*
 *----------------------------------------------------------------------
 *
 * FileMatchGIF --
 *
 *	This procedure is invoked by the photo image type to see if a file
 *	contains image data in GIF format.
 *
 * Results:
 *	The return value is 1 if the first characters in file f look like GIF
 *	data, and 0 otherwise.
 *
 * Side effects:
 *	The access position in f may change.
 *
 *----------------------------------------------------------------------
 */

static int
FileMatchGIF(chan, fileName, format, widthPtr, heightPtr, interp)
    Tcl_Channel chan;		/* The image file, open for reading. */
    CONST char *fileName;	/* The name of the image file. */
    Tcl_Obj *format;		/* User-specified format object, or NULL. */
    int *widthPtr, *heightPtr;	/* The dimensions of the image are returned
				 * here if the file is a valid raw GIF file. */
    Tcl_Interp *interp;		/* not used */
{
    GIFImageConfig gifConf;

    memset(&gifConf, 0, sizeof(GIFImageConfig));
    return ReadGIFHeader(&gifConf, chan, widthPtr, heightPtr);
}

/*
 *----------------------------------------------------------------------
 *
 * FileReadGIF --
 *
 *	This procedure is called by the photo image type to read GIF format
 *	data from a file and write it into a given photo image.
 *
 * Results:
 *	A standard TCL completion code.  If TCL_ERROR is returned then an
 *	error message is left in the interp's result.
 *
 * Side effects:
 *	The access position in file f is changed, and new data is added to the
 *	image given by imageHandle.
 *
 *----------------------------------------------------------------------
 */

static int
FileReadGIF(interp, chan, fileName, format, imageHandle, destX, destY,
	width, height, srcX, srcY)
    Tcl_Interp *interp;		/* Interpreter to use for reporting errors. */
    Tcl_Channel chan;		/* The image file, open for reading. */
    CONST char *fileName;	/* The name of the image file. */
    Tcl_Obj *format;		/* User-specified format object, or NULL. */
    Tk_PhotoHandle imageHandle;	/* The photo image to write into. */
    int destX, destY;		/* Coordinates of top-left pixel in photo
				 * image to be written to. */
    int width, height;		/* Dimensions of block of photo image to be
				 * written to. */
    int srcX, srcY;		/* Coordinates of top-left pixel to be used in
				 * image being read. */
{
    int fileWidth, fileHeight;
    int nBytes, index = 0, argc = 0, i;
    Tcl_Obj **objv;
    Tk_PhotoImageBlock block;
    unsigned char buf[100];
    unsigned char *trashBuffer = NULL;
    int bitPixel;
    unsigned char colorMap[MAXCOLORMAPSIZE][4];
    int transparent = -1;
    static CONST char *optionStrings[] = {
	"-index",	NULL
    };
    GIFImageConfig gifConf, *gifConfPtr = &gifConf;

    /*
     * Decode the magic used to convey when we're sourcing data from a string
     * source and not a file.
     */

    memset(gifConfPtr, 0, sizeof(GIFImageConfig));
    if (fileName == INLINE_DATA_BINARY || fileName == INLINE_DATA_BASE64) {
	gifConfPtr->fromData = fileName;
	fileName = "inline data";
    }

    if (format && Tcl_ListObjGetElements(interp, format,
	    &argc, &objv) != TCL_OK) {
	return TCL_ERROR;
    }
    for (i = 1; i < argc; i++) {
	if (Tcl_GetIndexFromObj(interp, objv[i], optionStrings, "option name",
		0, &nBytes) != TCL_OK) {
	    return TCL_ERROR;
	}
	if (i == (argc-1)) {
	    Tcl_AppendResult(interp, "no value given for \"",
		    Tcl_GetString(objv[i]), "\" option", (char *) NULL);
	    return TCL_ERROR;
	}
	if (Tcl_GetIntFromObj(interp, objv[++i], &index) != TCL_OK) {
	    return TCL_ERROR;
	}
    }
    if (!ReadGIFHeader(gifConfPtr, chan, &fileWidth, &fileHeight)) {
    	Tcl_AppendResult(interp, "couldn't read GIF header from file \"",
		fileName, "\"", NULL);
	return TCL_ERROR;
    }
    if ((fileWidth <= 0) || (fileHeight <= 0)) {
	Tcl_AppendResult(interp, "GIF image file \"", fileName,
		"\" has dimension(s) <= 0", (char *) NULL);
	return TCL_ERROR;
    }

    if (Fread(gifConfPtr, buf, 1, 3, chan) != 3) {
	return TCL_OK;
    }
    bitPixel = 2<<(buf[0]&0x07);

    if (BitSet(buf[0], LOCALCOLORMAP)) {	/* Global Colormap */
	if (!ReadColorMap(gifConfPtr, chan, bitPixel, colorMap)) {
	    Tcl_AppendResult(interp, "error reading color map",
		    (char *) NULL);
	    return TCL_ERROR;
	}
    }

    if ((srcX + width) > fileWidth) {
	width = fileWidth - srcX;
    }
    if ((srcY + height) > fileHeight) {
	height = fileHeight - srcY;
    }
    if ((width <= 0) || (height <= 0)
	    || (srcX >= fileWidth) || (srcY >= fileHeight)) {
	return TCL_OK;
    }

    if (Tk_PhotoExpand(interp, imageHandle,
	    destX + width, destY + height) != TCL_OK) {
	return TCL_ERROR;
    }

    block.width = width;
    block.height = height;
    block.pixelSize = 4;
    block.pitch = block.pixelSize * block.width;
    block.offset[0] = 0;
    block.offset[1] = 1;
    block.offset[2] = 2;
    block.offset[3] = 3;
    block.pixelPtr = NULL;

    while (1) {
	if (Fread(gifConfPtr, buf, 1, 1, chan) != 1) {
	    /*
	     * Premature end of image.
	     */

	    Tcl_AppendResult(interp,
		    "premature end of image data for this index",
		    (char *) NULL);
	    goto error;
	}

	if (buf[0] == GIF_TERMINATOR) {
	    /*
	     * GIF terminator.
	     */

	    Tcl_AppendResult(interp, "no image data for this index",
		    (char *) NULL);
	    goto error;
	}

	if (buf[0] == GIF_EXTENSION) {
	    /*
	     * This is a GIF extension.
	     */

	    if (Fread(gifConfPtr, buf, 1, 1, chan) != 1) {
		Tcl_SetResult(interp,
			"error reading extension function code in GIF image",
			TCL_STATIC);
		goto error;
	    }
	    if (DoExtension(gifConfPtr, chan, buf[0],
		    gifConfPtr->workingBuffer, &transparent) < 0) {
		Tcl_SetResult(interp, "error reading extension in GIF image",
			TCL_STATIC);
		goto error;
	    }
	    continue;
	}

	if (buf[0] != GIF_START) {
	    /*
	     * Not a valid start character; ignore it.
	     */
	    continue;
	}

	if (Fread(gifConfPtr, buf, 1, 9, chan) != 9) {
	    Tcl_SetResult(interp,
		    "couldn't read left/top/width/height in GIF image",
		    TCL_STATIC);
	    goto error;
	}

	fileWidth = LM_to_uint(buf[4], buf[5]);
	fileHeight = LM_to_uint(buf[6], buf[7]);

	bitPixel = 1<<((buf[8]&0x07)+1);

	if (index--) {
	    /*
	     * This is not the image we want to read: skip it.
	     */
	    if (BitSet(buf[8], LOCALCOLORMAP)) {
		if (!ReadColorMap(gifConfPtr, chan, bitPixel, colorMap)) {
		    Tcl_AppendResult(interp,
			    "error reading color map", (char *) NULL);
		    goto error;
		}
	    }

	    /*
	     * If we've not yet allocated a trash buffer, do so now.
	     */
	    if (trashBuffer == NULL) {
		nBytes = fileWidth * fileHeight * 3;
		trashBuffer = (unsigned char *) ckalloc((unsigned int) nBytes);
	    }

	    /*
	     * Slurp!  Process the data for this image and stuff it in a trash
	     * buffer.
	     *
	     * Yes, it might be more efficient here to *not* store the data
	     * (we're just going to throw it away later). However, I elected
	     * to implement it this way for good reasons. First, I wanted to
	     * avoid duplicating the (fairly complex) LWZ decoder in
	     * ReadImage. Fine, you say, why didn't you just modify it to
	     * allow the use of a NULL specifier for the output buffer? I
	     * tried that, but it negatively impacted the performance of what
	     * I think will be the common case: reading the first image in the
	     * file. Rather than marginally improve the speed of the less
	     * frequent case, I chose to maintain high performance for the
	     * common case.
	     */

	    if (ReadImage(gifConfPtr, interp, (char *)trashBuffer, chan,
		    fileWidth, fileHeight, colorMap, 0, 0, 0, 0, 0,
		    -1) != TCL_OK) {
		goto error;
	    }
	    continue;
	}

	if (BitSet(buf[8], LOCALCOLORMAP)) {
	    if (!ReadColorMap(gifConfPtr, chan, bitPixel, colorMap)) {
		    Tcl_AppendResult(interp, "error reading color map", 
			    (char *) NULL);
		    goto error;
	    }
	}

	index = LM_to_uint(buf[0], buf[1]);
	srcX -= index;
	if (srcX<0) {
	    destX -= srcX; width += srcX;
	    srcX = 0;
	}

	if (width > fileWidth) {
	    width = fileWidth;
	}

	index = LM_to_uint(buf[2], buf[3]);
	srcY -= index;
	if (index > srcY) {
	    destY -= srcY; height += srcY;
	    srcY = 0;
	}
	if (height > fileHeight) {
	    height = fileHeight;
	}

	if ((width <= 0) || (height <= 0)) {
	    block.pixelPtr = 0;
	    goto noerror;
	}

	block.width = width;
	block.height = height;
	block.pixelSize = (transparent>=0) ? 4 : 3;
	block.offset[3] = (transparent>=0) ? 3 : 0;
	block.pitch = block.pixelSize * fileWidth;
	nBytes = block.pitch * fileHeight;
	block.pixelPtr = (unsigned char *) ckalloc((unsigned) nBytes);

	if (ReadImage(gifConfPtr, interp, (char *) block.pixelPtr, chan,
		fileWidth, fileHeight, colorMap, fileWidth, fileHeight, srcX,
		srcY, BitSet(buf[8], INTERLACE), transparent) != TCL_OK) {
	    goto error;
	}
	break;
    }

    if (Tk_PhotoPutBlock(interp, imageHandle, &block, destX, destY,
	    width, height, TK_PHOTO_COMPOSITE_SET) != TCL_OK) {
	goto error;
    }

  noerror:
    /*
     * If a trash buffer has been allocated, free it now.
     */
    if (trashBuffer != NULL) {
	ckfree((char *)trashBuffer);
    }
    if (block.pixelPtr) {
	ckfree((char *) block.pixelPtr);
    }
    Tcl_AppendResult(interp, tkImgFmtGIF.name, (char *) NULL);
    return TCL_OK;

  error:
    /*
     * If a trash buffer has been allocated, free it now.
     */
    if (trashBuffer != NULL) {
	ckfree((char *)trashBuffer);
    }
    if (block.pixelPtr) {
	ckfree((char *) block.pixelPtr);
    }
    return TCL_ERROR;
}

/*
 *----------------------------------------------------------------------
 *
 * StringMatchGIF --
 *
 *	This procedure is invoked by the photo image type to see if an object
 *	contains image data in GIF format.
 *
 * Results:
 *	The return value is 1 if the first characters in the data are like GIF
 *	data, and 0 otherwise.
 *
 * Side effects:
 *	The size of the image is placed in widthPtr and heightPtr.
 *
 *----------------------------------------------------------------------
 */

static int
StringMatchGIF(dataObj, format, widthPtr, heightPtr, interp)
    Tcl_Obj *dataObj;		/* the object containing the image data */
    Tcl_Obj *format;		/* the image format object, or NULL */
    int *widthPtr;		/* where to put the string width */
    int *heightPtr;		/* where to put the string height */
    Tcl_Interp *interp;		/* not used */
{
    unsigned char *data, header[10];
    int got, length;
    MFile handle;

    data = Tcl_GetByteArrayFromObj(dataObj, &length);

    /*
     * Header is a minimum of 10 bytes.
     */
    if (length < 10) {
	return 0;
    }

    /*
     * Check whether the data is Base64 encoded.
     */

    if ((strncmp(GIF87a, (char *) data, 6) != 0) && 
	    (strncmp(GIF89a, (char *) data, 6) != 0)) {
	/*
	 * Try interpreting the data as Base64 encoded
	 */
	mInit((unsigned char *) data, &handle, length);
	got = Mread(header, 10, 1, &handle);
	if (got != 10
		|| ((strncmp(GIF87a, (char *) header, 6) != 0)
		&& (strncmp(GIF89a, (char *) header, 6) != 0))) {
	    return 0;
	}
    } else {
	memcpy((VOID *) header, (VOID *) data, 10);
    }
    *widthPtr = LM_to_uint(header[6], header[7]);
    *heightPtr = LM_to_uint(header[8], header[9]);
    return 1;
}

/*
 *----------------------------------------------------------------------
 *
 * StringReadGIF --
 *
 *	This procedure is called by the photo image type to read GIF format
 *	data from an object, optionally base64 encoded, and give it to the
 *	photo image.
 *
 * Results:
 *	A standard TCL completion code.  If TCL_ERROR is returned then an
 *	error message is left in the interp's result.
 *
 * Side effects:
 *	New data is added to the image given by imageHandle.  This procedure
 *	calls FileReadGIF by redefining the operation of fprintf temporarily.
 *
 *----------------------------------------------------------------------
 */

static int
StringReadGIF(interp, dataObj, format, imageHandle,
	destX, destY, width, height, srcX, srcY)
    Tcl_Interp *interp;		/* interpreter for reporting errors in */
    Tcl_Obj *dataObj;		/* object containing the image */
    Tcl_Obj *format;		/* format object, or NULL */
    Tk_PhotoHandle imageHandle;	/* the image to write this data into */
    int destX, destY;		/* The rectangular region of the */
    int width, height;		/* image to copy */
    int srcX, srcY;
{
    MFile handle;
    int length;
    char *data = (char *) Tcl_GetByteArrayFromObj(dataObj, &length);

    mInit((unsigned char *)data, &handle, length);

    if (strncmp(GIF87a, data, 6) && strncmp(GIF89a, data, 6)) {
	/*
	 * Check whether the data is Base64 encoded by doing a
	 * character-by-charcter comparison with the binary-format headers;
	 * BASE64-encoded never matches (matching the other way is harder
	 * because of potential padding).
	 */

	return FileReadGIF(interp, (Tcl_Channel) &handle, INLINE_DATA_BASE64,
		format, imageHandle, destX, destY, width, height, srcX, srcY);
    } else {
	return FileReadGIF(interp, (Tcl_Channel) &handle, INLINE_DATA_BINARY,
		format, imageHandle, destX, destY, width, height, srcX, srcY);
    }
}

/*
 *----------------------------------------------------------------------
 *
 * ReadGIFHeader --
 *
 *	This procedure reads the GIF header from the beginning of a GIF file
 *	and returns the dimensions of the image.
 *
 * Results:
 *	The return value is 1 if file "f" appears to start with a valid GIF
 *	header, 0 otherwise.  If the header is valid, then *widthPtr and
 *	*heightPtr are modified to hold the dimensions of the image.
 *
 * Side effects:
 *	The access position in f advances.
 *
 *----------------------------------------------------------------------
 */

static int
ReadGIFHeader(gifConfPtr, chan, widthPtr, heightPtr)
    GIFImageConfig *gifConfPtr;
    Tcl_Channel chan;		/* Image file to read the header from */
    int *widthPtr, *heightPtr;	/* The dimensions of the image are returned
				 * here. */
{
    unsigned char buf[7];

    if ((Fread(gifConfPtr, buf, 1, 6, chan) != 6)
	    || ((strncmp(GIF87a, (char *) buf, 6) != 0)
	    && (strncmp(GIF89a, (char *) buf, 6) != 0))) {
	return 0;
    }

    if (Fread(gifConfPtr, buf, 1, 4, chan) != 4) {
	return 0;
    }

    *widthPtr = LM_to_uint(buf[0], buf[1]);
    *heightPtr = LM_to_uint(buf[2], buf[3]);
    return 1;
}

/*
 *-----------------------------------------------------------------
 * The code below is copied from the giftoppm program and modified
 * just slightly.
 *-----------------------------------------------------------------
 */

static int
ReadColorMap(gifConfPtr, chan, number, buffer)
    GIFImageConfig *gifConfPtr;
    Tcl_Channel chan;
    int number;
    unsigned char buffer[MAXCOLORMAPSIZE][4];
{
    int i;
    unsigned char rgb[3];

    for (i = 0; i < number; ++i) {
	if (Fread(gifConfPtr, rgb, sizeof(rgb), 1, chan) <= 0) {
	    return 0;
	}

	if (buffer) {
	    buffer[i][CM_RED] = rgb[0];
	    buffer[i][CM_GREEN] = rgb[1];
	    buffer[i][CM_BLUE] = rgb[2];
	    buffer[i][CM_ALPHA] = 255;
	}
    }
    return 1;
}

static int
DoExtension(gifConfPtr, chan, label, buf, transparent)
    GIFImageConfig *gifConfPtr;
    Tcl_Channel chan;
    int label;
    unsigned char *buf;
    int *transparent;
{
    int count;

    switch (label) {
    case 0x01:		/* Plain Text Extension */
	break;

    case 0xff:		/* Application Extension */
	break;

    case 0xfe:		/* Comment Extension */
	do {
	    count = GetDataBlock(gifConfPtr, chan, (unsigned char*) buf);
	} while (count > 0);
	return count;

    case 0xf9:		/* Graphic Control Extension */
	count = GetDataBlock(gifConfPtr, chan, (unsigned char*) buf);
	if (count < 0) {
	    return 1;
	}
	if ((buf[0] & 0x1) != 0) {
	    *transparent = buf[3];
	}

	do {
	    count = GetDataBlock(gifConfPtr, chan, (unsigned char*) buf);
	} while (count > 0);
	return count;
    }

    do {
	count = GetDataBlock(gifConfPtr, chan, (unsigned char*) buf);
    } while (count > 0);
    return count;
}

static int
GetDataBlock(gifConfPtr, chan, buf)
    GIFImageConfig *gifConfPtr;
    Tcl_Channel chan;
    unsigned char *buf;
{
    unsigned char count;

    if (Fread(gifConfPtr, &count, 1, 1, chan) <= 0) {
	return -1;
    }

    if ((count != 0) && (Fread(gifConfPtr, buf, count, 1, chan) <= 0)) {
	return -1;
    }

    return count;
}

/*
 *----------------------------------------------------------------------
 *
 * ReadImage --
 *
 *	Process a GIF image from a given source, with a given height, width,
 *	transparency, etc.
 *
 *	This code is based on the code found in the ImageMagick GIF decoder,
 *	which is (c) 2000 ImageMagick Studio.
 *
 *	Some thoughts on our implementation:
 *	It sure would be nice if ReadImage didn't take 11 parameters! I think
 *	that if we were smarter, we could avoid doing that.
 *
 *	Possible further optimizations: we could pull the GetCode function
 *	directly into ReadImage, which would improve our speed.
 *
 * Results:
 *	Processes a GIF image and loads the pixel data into a memory array.
 *
 * Side effects:
 *	None.
 *
 *----------------------------------------------------------------------
 */

static int
ReadImage(gifConfPtr, interp, imagePtr, chan, len, rows, cmap,
	width, height, srcX, srcY, interlace, transparent)
    GIFImageConfig *gifConfPtr;
    Tcl_Interp *interp;
    char *imagePtr;
    Tcl_Channel chan;
    int len, rows;
    unsigned char cmap[MAXCOLORMAPSIZE][4];
    int width, height;
    int srcX, srcY;
    int interlace;
    int transparent;
{
    unsigned char initialCodeSize;
    int v;
    int xpos = 0, ypos = 0, pass = 0, i;
    register char *pixelPtr;
    CONST static int interlaceStep[] = { 8, 8, 4, 2 };
    CONST static int interlaceStart[] = { 0, 4, 2, 1 };
    unsigned short prefix[(1 << MAX_LWZ_BITS)];
    unsigned char append[(1 << MAX_LWZ_BITS)];
    unsigned char stack[(1 << MAX_LWZ_BITS)*2];
    register unsigned char *top;
    int codeSize, clearCode, inCode, endCode, oldCode, maxCode;
    int code, firstCode;

    /*
     * Initialize the decoder
     */
    if (Fread(gifConfPtr, &initialCodeSize, 1, 1, chan) <= 0) {
	Tcl_AppendResult(interp, "error reading GIF image: ",
		Tcl_PosixError(interp), (char *) NULL);
	return TCL_ERROR;
    }
    if (transparent != -1) {
	cmap[transparent][CM_RED] = 0;
	cmap[transparent][CM_GREEN] = 0;
	cmap[transparent][CM_BLUE] = 0;
	cmap[transparent][CM_ALPHA] = 0;
    }

    pixelPtr = imagePtr;

    /*
     * Initialize the decoder.
     *
     * Set values for "special" numbers:
     * clear code	reset the decoder
     * end code		stop decoding
     * code size	size of the next code to retrieve
     * max code		next available table position
     */
    clearCode = 1 << (int) initialCodeSize;
    endCode = clearCode + 1;
    codeSize = (int) initialCodeSize + 1;
    maxCode = clearCode + 2;
    oldCode = -1;
    firstCode = -1;
    
    memset((void *)prefix, 0, (1 << MAX_LWZ_BITS) * sizeof(short));
    memset((void *)append, 0, (1 << MAX_LWZ_BITS) * sizeof(char));
    for (i = 0; i < clearCode; i++) {
	append[i] = i;
    }
    top = stack;

    GetCode(chan, 0, 1, gifConfPtr);

    /*
     * Read until we finish the image
     */
    for (i = 0, ypos = 0; i < rows; i++) {
	for (xpos = 0; xpos < len; ) {

	    if (top == stack) {
		/*
		 * Bummer -- our stack is empty.  Now we have to work!
		 */
		code = GetCode(chan, codeSize, 0, gifConfPtr);
		if (code < 0) {
		    return TCL_OK;
		}

		if (code > maxCode || code == endCode) {
		    /*
		     * If we're doing things right, we should never receive a
		     * code that is greater than our current maximum code.  If
		     * we do, bail, because our decoder does not yet have that
		     * code set up.
		     *
		     * If the code is the magic endCode value, quit.
		     */
		    return TCL_OK;
		}

		if (code == clearCode) {
		    /*
		     * Reset the decoder.
		     */
		    codeSize = initialCodeSize + 1;
		    maxCode = clearCode + 2;
		    oldCode = -1;
		    continue;
		}
		
		if (oldCode == -1) {
		    /*
		     * Last pass reset the decoder, so the first code we see
		     * must be a singleton. Seed the stack with it, and set up
		     * the old/first code pointers for insertion into the
		     * string table. We can't just roll this into the
		     * clearCode test above, because at that point we have not
		     * yet read the next code.
		     */
		    *top++ = append[code];
		    oldCode = code;
		    firstCode = code;
		    continue;
		}
		
		inCode = code;

		if (code == maxCode) {
		    /*
		     * maxCode is always one bigger than our highest assigned
		     * code. If the code we see is equal to maxCode, then we
		     * are about to add a new string to the table. ???
		     */
		    *top++ = firstCode;
		    code = oldCode;
		}

		while (code > clearCode) {
		    /*
		     * Populate the stack by tracing the string in the string
		     * table from its tail to its head
		     */
		    *top++ = append[code];
		    code = prefix[code];
		}
		firstCode = append[code];

		/*
		 * If there's no more room in our string table, quit.
		 * Otherwise, add a new string to the table
		 */
		if (maxCode >= (1 << MAX_LWZ_BITS)) {
		    return TCL_OK;
		}

		/*
		 * Push the head of the string onto the stack.
		 */
		*top++ = firstCode;

		/*
		 * Add a new string to the string table
		 */
		prefix[maxCode] = oldCode;
		append[maxCode] = firstCode;
		maxCode++;

		/*
		 * maxCode tells us the maximum code value we can accept. If
		 * we see that we need more bits to represent it than we are
		 * requesting from the unpacker, we need to increase the
		 * number we ask for.
		 */

		if ((maxCode >= (1 << codeSize))
			&& (maxCode < (1<<MAX_LWZ_BITS))) {
		    codeSize++;
		}
		oldCode = inCode;
	    }

	    /*
	     * Pop the next color index off the stack.
	     */

	    v = *(--top);
	    if (v < 0) {
		return TCL_OK;
	    }

	    /* 
	     * If pixelPtr is null, we're skipping this image (presumably
	     * there are more in the file and we will be called to read one of
	     * them later)
	     */

	    *pixelPtr++ = cmap[v][CM_RED];
	    *pixelPtr++ = cmap[v][CM_GREEN];
	    *pixelPtr++ = cmap[v][CM_BLUE];
	    if (transparent >= 0) {
		*pixelPtr++ = cmap[v][CM_ALPHA];
	    }
	    xpos++;

	}

	/*
	 * If interlacing, the next ypos is not just +1
	 */
	if (interlace) {
	    ypos += interlaceStep[pass];
	    while (ypos >= height) {
		pass++;
		if (pass > 3) {
		    return TCL_OK;
		}
		ypos = interlaceStart[pass];
	    }
	} else {
	    ypos++;
	}
	pixelPtr = imagePtr + (ypos) * len * ((transparent>=0)?4:3);
    }
    return TCL_OK;
}

/*
 *----------------------------------------------------------------------
 *
 * GetCode --
 *
 *	Extract the next compression code from the file.  In GIF's, the
 *	compression codes are between 3 and 12 bits long and are then packed
 *	into 8 bit bytes, left to right, for example:
 *		bbbaaaaa
 *		dcccccbb
 *		eeeedddd
 *		...
 *	We use a byte buffer read from the file and a sliding window to unpack
 *	the bytes.  Thanks to ImageMagick for the sliding window idea.
 *	args:  chan	    the channel to read from
 *	       code_size    size of the code to extract
 *	       flag	    boolean indicating whether the extractor should be
 *			    reset or not
 *
 * Results:
 *	code		    the next compression code
 *
 * Side effects:
 *	May consume more input from chan.
 *
 *----------------------------------------------------------------------
 */

static int
GetCode(chan, code_size, flag, gifConfPtr)
    Tcl_Channel chan;
    int code_size;
    int flag;
    GIFImageConfig *gifConfPtr;
{
    int ret;
    
    if (flag) {
	/*
	 * Initialize the decoder.
	 */
	gifConfPtr->reader.bitsInWindow = 0;
	gifConfPtr->reader.bytes = 0;
	gifConfPtr->reader.window = 0;
	gifConfPtr->reader.done = 0;
	gifConfPtr->reader.c = NULL;
	return 0;
    }

    while (gifConfPtr->reader.bitsInWindow < code_size) {
	/*
	 * Not enough bits in our window to cover the request.
	 */
	if (gifConfPtr->reader.done) {
	    return -1;
	}
	if (gifConfPtr->reader.bytes == 0) {
	    /*
	     * Not enough bytes in our buffer to add to the window.
	     */
	    gifConfPtr->reader.bytes =
		    GetDataBlock(gifConfPtr, chan, gifConfPtr->workingBuffer);
	    gifConfPtr->reader.c = gifConfPtr->reader.workingBuffer;
	    if (gifConfPtr->reader.bytes <= 0) {
		gifConfPtr->reader.done = 1;
		break;
	    }
	}
	/*
	 * Tack another byte onto the window, see if that's enough.
	 */
	gifConfPtr->reader.window +=
		(*gifConfPtr->reader.c) << gifConfPtr->reader.bitsInWindow;
	gifConfPtr->reader.c++;
	gifConfPtr->reader.bitsInWindow += 8;
	gifConfPtr->reader.bytes--;
    }

    /*
     * The next code will always be the last code_size bits of the window.
     */
    ret = gifConfPtr->reader.window & ((1 << code_size) - 1);
    
    /*
     * Shift data in the window to put the next code at the end.
     */
    gifConfPtr->reader.window >>= code_size;
    gifConfPtr->reader.bitsInWindow -= code_size;
    return ret;
}

/*
 *----------------------------------------------------------------------
 *
 * Minit -- --
 *
 *	This procedure initializes a base64 decoder handle
 *
 * Results:
 *	None
 *
 * Side effects:
 *	The base64 handle is initialized
 *
 *----------------------------------------------------------------------
 */

static void
mInit(string, handle, length)
   unsigned char *string;	/* string containing initial mmencoded data */
   MFile *handle;		/* mmdecode "file" handle */
   int length;			/* Number of bytes in string */
{
   handle->data = string;
   handle->state = 0;
   handle->c = 0;
   handle->length = length;
}

/*
 *----------------------------------------------------------------------
 *
 * Mread --
 *
 *	This procedure is invoked by the GIF file reader as a temporary
 *	replacement for "fread", to get GIF data out of a string (using
 *	Mgetc).
 *
 * Results:
 *	The return value is the number of characters "read"
 *
 * Side effects:
 *	The base64 handle will change state.
 *
 *----------------------------------------------------------------------
 */

static int
Mread(dst, chunkSize, numChunks, handle)  
   unsigned char *dst;	/* where to put the result */
   size_t chunkSize;	/* size of each transfer */
   size_t numChunks;	/* number of chunks */
   MFile *handle;	/* mmdecode "file" handle */
{
   register int i, c;
   int count = chunkSize * numChunks;

   for (i=0; i<count && (c=Mgetc(handle)) != GIF_DONE; i++) {
       *dst++ = c;
   }
   return i;
}

/*
 * Get the next decoded character from an mmencode handle.
 * This causes at least 1 character to be "read" from the encoded string
 */

/*
 *----------------------------------------------------------------------
 *
 * Mgetc --
 *
 *	This procedure decodes and returns the next byte from a base64 encoded
 *	string.
 *
 * Results:
 *	The next byte (or GIF_DONE) is returned.
 *
 * Side effects:
 *	The base64 handle will change state.
 *
 *----------------------------------------------------------------------
 */

static int
Mgetc(handle)
    MFile *handle;		/* Handle containing decoder data and state */
{
    int c;
    int result = 0;		/* Initialization needed only to prevent gcc
				 * compiler warning. */

    if (handle->state == GIF_DONE) {
	return GIF_DONE;
    }

    do {
	if (handle->length-- <= 0) {
	    return GIF_DONE;
	}
	c = char64(*handle->data);
	handle->data++;
    } while (c == GIF_SPACE);

    if (c>GIF_SPECIAL) {
	handle->state = GIF_DONE;
	return handle->c;
    }

    switch (handle->state++) {
    case 0:
	handle->c = c<<2;
	result = Mgetc(handle);
	break;
    case 1:
	result = handle->c | (c>>4);
	handle->c = (c&0xF)<<4;
	break;
    case 2:
	result = handle->c | (c>>2);
	handle->c = (c&0x3) << 6;
	break;
    case 3:
	result = handle->c | c;
	handle->state = 0;
	break;
    }
    return result;
}

/*
 *----------------------------------------------------------------------
 *
 * char64 --
 *
 *	This procedure converts a base64 ascii character into its binary
 *	equivalent.  This code is a slightly modified version of the char64
 *	proc in N. Borenstein's metamail decoder.
 *
 * Results:
 *	The binary value, or an error code.
 *
 * Side effects:
 *	None.
 *
 *----------------------------------------------------------------------
 */

static int
char64(c)
    int c;
{
    switch(c) {
    case 'A': return 0;  case 'B': return 1;  case 'C': return 2;
    case 'D': return 3;  case 'E': return 4;  case 'F': return 5;
    case 'G': return 6;  case 'H': return 7;  case 'I': return 8;
    case 'J': return 9;  case 'K': return 10; case 'L': return 11;
    case 'M': return 12; case 'N': return 13; case 'O': return 14;
    case 'P': return 15; case 'Q': return 16; case 'R': return 17;
    case 'S': return 18; case 'T': return 19; case 'U': return 20;
    case 'V': return 21; case 'W': return 22; case 'X': return 23;
    case 'Y': return 24; case 'Z': return 25; case 'a': return 26;
    case 'b': return 27; case 'c': return 28; case 'd': return 29;
    case 'e': return 30; case 'f': return 31; case 'g': return 32;
    case 'h': return 33; case 'i': return 34; case 'j': return 35;
    case 'k': return 36; case 'l': return 37; case 'm': return 38;
    case 'n': return 39; case 'o': return 40; case 'p': return 41;
    case 'q': return 42; case 'r': return 43; case 's': return 44;
    case 't': return 45; case 'u': return 46; case 'v': return 47;
    case 'w': return 48; case 'x': return 49; case 'y': return 50;
    case 'z': return 51; case '0': return 52; case '1': return 53;
    case '2': return 54; case '3': return 55; case '4': return 56;
    case '5': return 57; case '6': return 58; case '7': return 59;
    case '8': return 60; case '9': return 61; case '+': return 62;
    case '/': return 63;

    case ' ': case '\t': case '\n': case '\r': case '\f':
	return GIF_SPACE;
    case '=':
	return GIF_PAD;
    case '\0':
	return GIF_DONE;
    default:
	return GIF_BAD;
    }
}

/*
 *----------------------------------------------------------------------
 *
 * Fread --
 *
 *	This procedure calls either fread or Mread to read data from a file or
 *	a base64 encoded string.
 *
 * Results: - same as POSIX fread() or Tcl Tcl_Read()
 *
 *----------------------------------------------------------------------
 */

static int
Fread(gifConfPtr, dst, hunk, count, chan)
    GIFImageConfig *gifConfPtr;
    unsigned char *dst;		/* where to put the result */
    size_t hunk, count;		/* how many */
    Tcl_Channel chan;
{
    MFile *handle;

    if (gifConfPtr->fromData == INLINE_DATA_BASE64) {
	return Mread(dst, hunk, count, (MFile *) chan);
    }

    if (gifConfPtr->fromData == INLINE_DATA_BINARY) {
	handle = (MFile *) chan;
	if (handle->length <= 0 || (size_t)handle->length < hunk*count) {
	    return -1;
	}
	memcpy((VOID *)dst, (VOID *) handle->data, (size_t) (hunk * count));
	handle->data += hunk * count;
	return (int)(hunk * count);
    }

    /*
     * Otherwise we've got a real file to read.
     */
    return Tcl_Read(chan, (char *) dst, (int) (hunk * count));
}

/*
 * ChanWriteGIF - writes a image in GIF format.
 *-------------------------------------------------------------------------
 * Author:			Lolo
 *				Engeneering Projects Area 
 *				Department of Mining 
 *				University of Oviedo
 * e-mail			zz11425958@zeus.etsimo.uniovi.es
 *				lolo@pcsig22.etsimo.uniovi.es
 * Date:			Fri September 20 1996
 *
 * Modified for transparency handling (gif89a) and miGIF compression
 * by Jan Nijtmans <j.nijtmans@chello.nl>
 *
 *----------------------------------------------------------------------
 * FileWriteGIF-
 *
 *	This procedure is called by the photo image type to write GIF format
 *	data from a photo image into a given file
 *
 * Results:
 *	A standard TCL completion code.  If TCL_ERROR is returned then an
 *	error message is left in interp->result.
 *
 *----------------------------------------------------------------------
 */

/*
 * Types, defines and variables needed to write and compress a GIF.
 */

typedef int (* ifunptr) _ANSI_ARGS_((ClientData clientData));

#define LSB(a)			((unsigned char) (((short)(a)) & 0x00FF))
#define MSB(a)			((unsigned char) (((short)(a)) >> 8))

#define GIFBITS 12
#define HSIZE  5003		/* 80% occupancy */

typedef struct {
    int ssize;
    int csize;
    int rsize;
    unsigned char *pixelo;
    int pixelSize;
    int pixelPitch;
    int greenOffset;
    int blueOffset;
    int alphaOffset;
    int num;
    unsigned char mapa[MAXCOLORMAPSIZE][3];
} GifWriterState;

/*
 * Definition of new functions to write GIFs
 */

static int	color _ANSI_ARGS_((GifWriterState *statePtr,
		    int red, int green, int blue,
		    unsigned char mapa[MAXCOLORMAPSIZE][3]));
static void	compress _ANSI_ARGS_((int initBits, Tcl_Channel handle,
		    ifunptr readValue, ClientData clientData));
static int	nuevo _ANSI_ARGS_((GifWriterState *statePtr,
		    int red, int green, int blue,
		    unsigned char mapa[MAXCOLORMAPSIZE][3]));
static void	savemap _ANSI_ARGS_((GifWriterState *statePtr,
		    Tk_PhotoImageBlock *blockPtr,
		    unsigned char mapa[MAXCOLORMAPSIZE][3]));
static int	ReadValue _ANSI_ARGS_((ClientData clientData));

static int
FileWriteGIF(interp, filename, format, blockPtr)
    Tcl_Interp *interp;		/* Interpreter to use for reporting errors. */
    CONST char	*filename;
    Tcl_Obj	*format;
    Tk_PhotoImageBlock *blockPtr;
{
    Tcl_Channel chan = NULL;
    int result;

    chan = Tcl_OpenFileChannel(interp, (char *) filename, "w", 0644);
    if (!chan) {
	return TCL_ERROR;
    }
    if (Tcl_SetChannelOption(interp, chan, "-translation",
	    "binary") != TCL_OK) {
	Tcl_Close(NULL, chan);
	return TCL_ERROR;
    }

    result = CommonWriteGIF(interp, chan, format, blockPtr);

    if (Tcl_Close(interp, chan) == TCL_ERROR) {
	return TCL_ERROR;
    }
    return result;
}

static int
CommonWriteGIF(interp, handle, format, blockPtr)
    Tcl_Interp *interp;
    Tcl_Channel handle;
    Tcl_Obj *format;
    Tk_PhotoImageBlock *blockPtr;
{
    GifWriteState state, *statePtr = &state;
    int resolution;
    long width, height, x;
    unsigned char c;
    unsigned int top, left;

    top = 0;
    left = 0;

    memset(statePtr, 0, sizeof(state));

    statePtr->pixelSize = blockPtr->pixelSize;
    statePtr->greenOffset = blockPtr->offset[1]-blockPtr->offset[0];
    statePtr->blueOffset = blockPtr->offset[2]-blockPtr->offset[0];
    statePtr->alphaOffset = blockPtr->offset[0];
    if (statePtr->alphaOffset < blockPtr->offset[2]) {
	statePtr->alphaOffset = blockPtr->offset[2];
    }
    if (++statePtr->alphaOffset < statePtr->pixelSize) {
	statePtr->alphaOffset -= blockPtr->offset[0];
    } else {
	statePtr->alphaOffset = 0;
    }

    Tcl_Write(handle, (char *) (statePtr->alphaOffset ? GIF89a : GIF87a), 6);

    for (x=0 ; x<MAXCOLORMAPSIZE ; x++) {
	statePtr->mapa[x][CM_RED] = 255;
	statePtr->mapa[x][CM_GREEN] = 255;
	statePtr->mapa[x][CM_BLUE] = 255;
    }


    width = blockPtr->width;
    height = blockPtr->height;
    statePtr->pixelo = blockPtr->pixelPtr + blockPtr->offset[0];
    statePtr->pixelPitch = blockPtr->pitch;
    savemap(statePtr, blockPtr, statePtr->mapa);
    if (statePtr->num >= MAXCOLORMAPSIZE) {
	Tcl_AppendResult(interp, "too many colors", (char *) NULL);
	return TCL_ERROR;
    }
    if (statePtr->num<2) {
	statePtr->num = 2;
    }
    c = LSB(width);
    Tcl_Write(handle, (char *) &c, 1);
    c = MSB(width);
    Tcl_Write(handle, (char *) &c, 1);
    c = LSB(height);
    Tcl_Write(handle, (char *) &c, 1);
    c = MSB(height);
    Tcl_Write(handle, (char *) &c, 1);

    resolution = 0;
    while (statePtr->num >> resolution) {
	resolution++;
    }
    c = 111 + resolution * 17;
    Tcl_Write(handle, (char *) &c, 1);

    statePtr->num = 1 << resolution;

    /*
     * background color
     */

    c = 0;
    Tcl_Write(handle, (char *) &c, 1);

    /*
     * zero for future expansion.
     */

    Tcl_Write(handle, (char *) &c, 1);

    for (x=0 ; x<statePtr->num ; x++) {
	c = statePtr->mapa[x][CM_RED];
	Tcl_Write(handle, (char *) &c, 1);
	c = statePtr->mapa[x][CM_GREEN];
	Tcl_Write(handle, (char *) &c, 1);
	c = statePtr->mapa[x][CM_BLUE];
	Tcl_Write(handle, (char *) &c, 1);
    }

    /*
     * Write out extension for transparent colour index, if necessary.
     */

    if (statePtr->alphaOffset) {
	c = GIF_EXTENSION;
	Tcl_Write(handle, (char *) &c, 1);
	Tcl_Write(handle, "\371\4\1\0\0\0", 7);
    }

    c = GIF_START;
    Tcl_Write(handle, (char *) &c, 1);
    c = LSB(top);
    Tcl_Write(handle, (char *) &c, 1);
    c = MSB(top);
    Tcl_Write(handle, (char *) &c, 1);
    c = LSB(left);
    Tcl_Write(handle, (char *) &c, 1);
    c = MSB(left);
    Tcl_Write(handle, (char *) &c, 1);

    c = LSB(width);
    Tcl_Write(handle, (char *) &c, 1);
    c = MSB(width);
    Tcl_Write(handle, (char *) &c, 1);

    c = LSB(height);
    Tcl_Write(handle, (char *) &c, 1);
    c = MSB(height);
    Tcl_Write(handle, (char *) &c, 1);

    c = 0;
    Tcl_Write(handle, (char *) &c, 1);
    c = resolution;
    Tcl_Write(handle, (char *) &c, 1);

    statePtr->ssize = statePtr->rsize = blockPtr->width;
    statePtr->csize = blockPtr->height;
    compress(resolution+1, handle, ReadValue, (ClientData) statePtr);

    c = 0; 
    Tcl_Write(handle, (char *) &c, 1);
    c = GIF_TERMINATOR;
    Tcl_Write(handle, (char *) &c, 1);

    return TCL_OK;	
}

static int
color(statePtr, red, green, blue, mapa)
    GifWriterState *statePtr;
    int red;
    int green;
    int blue;
    unsigned char mapa[MAXCOLORMAPSIZE][3];
{
    int x;
    for (x=(statePtr->alphaOffset != 0) ; x<=MAXCOLORMAPSIZE ; x++) {
	if ((mapa[x][CM_RED] == red) && (mapa[x][CM_GREEN] == green) &&
		(mapa[x][CM_BLUE] == blue)) {
	    return x;
	}
    }
    return -1;
}

static int
nuevo(statePtr, red, green, blue, mapa)
    GifWriterState *statePtr;
    int red, green, blue;
    unsigned char mapa[MAXCOLORMAPSIZE][3];
{
    int x = (statePtr->alphaOffset != 0);
    for (; x<=statePtr->num ; x++) {
	if ((mapa[x][CM_RED] == red) && (mapa[x][CM_GREEN] == green) &&
		(mapa[x][CM_BLUE] == blue)) {
	    return 0;
	}
    }
    return 1;
}

static void
savemap(statePtr, blockPtr, mapa)
    GifWriterState *statePtr;
    Tk_PhotoImageBlock *blockPtr;
    unsigned char mapa[MAXCOLORMAPSIZE][3];
{
    unsigned char *colores;
    int x, y;
    unsigned char red, green, blue;

    if (statePtr->alphaOffset) {
	statePtr->num = 0;
	mapa[0][CM_RED] = 0xd9;
	mapa[0][CM_GREEN] = 0xd9;
	mapa[0][CM_BLUE] = 0xd9;
    } else {
	statePtr->num = -1;
    }

    for(y=0 ; y<blockPtr->height ; y++) {
	colores = blockPtr->pixelPtr + blockPtr->offset[0] + y*blockPtr->pitch;
	for(x=0 ; x<blockPtr->width ; x++) {
	    if (!statePtr->alphaOffset || colores[statePtr->alphaOffset]!=0) {
		red = colores[0];
		green = colores[statePtr->greenOffset];
		blue = colores[statePtr->blueOffset];
		if (nuevo(statePtr, red, green, blue, mapa)) {
		    statePtr->num++;
		    if (statePtr->num >= MAXCOLORMAPSIZE) {
			return;
		    }
		    mapa[statePtr->num][CM_RED] = red;
		    mapa[statePtr->num][CM_GREEN] = green;
		    mapa[statePtr->num][CM_BLUE] = blue;
		}
	    }
	    colores += statePtr->pixelSize;
	}
    }
    return;
}

static int
ReadValue(clientData)
    ClientData clientData;
{
    GifWriterState *statePtr = (GifWriterState *) clientData;
    unsigned int col;

    if (statePtr->csize == 0) {
	return EOF;
    }
    if (statePtr->alphaOffset && statePtr->pixelo[statePtr->alphaOffset]==0) {
	col = 0;
    } else {
	col = color(statePtr, statePtr->pixelo[0],
		statePtr->pixelo[statePtr->greenOffset],
		statePtr->pixelo[statePtr->blueOffset], statePtr->mapa);
    }
    statePtr->pixelo += statePtr->pixelSize;
    if (--statePtr->ssize <= 0) {
	statePtr->ssize = statePtr->rsize;
	statePtr->csize--;
	statePtr->pixelo += statePtr->pixelPitch
		- (statePtr->rsize * statePtr->pixelSize);
    }

    return col;
}

/*
 *-----------------------------------------------------------------------
 *
 * miGIF Compression - mouse and ivo's GIF-compatible compression
 *
 *		-run length encoding compression routines-
 *
 * Copyright (C) 1998 Hutchison Avenue Software Corporation
 *		 http://www.hasc.com
 *		 info@hasc.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 software is provided "AS IS." The Hutchison Avenue
 * Software Corporation disclaims all warranties, either express or implied,
 * including but not limited to implied warranties of merchantability and
 * fitness for a particular purpose, with respect to this code and
 * accompanying documentation.
 * 
 * The miGIF compression routines do not, strictly speaking, generate files
 * conforming to the GIF spec, since the image data is not LZW-compressed
 * (this is the point: in order to avoid transgression of the Unisys patent on
 * the LZW algorithm.) However, miGIF generates data streams that any
 * reasonably sane LZW decompresser will decompress to what we want.
 *
 * miGIF compression uses run length encoding. It compresses horizontal runs
 * of pixels of the same color. This type of compression gives good results on
 * images with many runs, for example images with lines, text and solid shapes
 * on a solid-colored background. It gives little or no compression on images
 * with few runs, for example digital or scanned photos.
 *
 *				 der Mouse
 *			mouse@rodents.montreal.qc.ca
 *	      7D C8 61 52 5D E7 2D 39  4E F1 31 3E E8 B3 27 4B
 *
 *			       ivo@hasc.com
 *
 * The Graphics Interchange Format(c) is the Copyright property of CompuServe
 * Incorporated. GIF(sm) is a Service Mark property of CompuServe Incorporated.
 *
 *-----------------------------------------------------------------------
 */

typedef struct {
    int runlengthPixel;
    int runlengthBaseCode;
    int runlengthCount;
    int runlengthTablePixel;
    int runlengthTableMax;
    int justCleared;
    int outputBits;
    int outputBitsInit;
    int outputCount;
    int outputBump;
    int outputBumpInit;
    int outputClear;
    int outputClearInit;
    int maxOcodes;
    int codeClear;
    int codeEOF;
    unsigned int obuf;
    int obits;
    Tcl_Channel ofile;
    unsigned char oblock[256];
    int oblen;
} miGIFState_t;

/*
 * Used only when debugging GIF compression code
 */
/* #define MIGIF_DEBUGGING_ENVARS */

#ifdef MIGIF_DEBUGGING_ENVARS

/*
 * This debugging code is _absolutely_ not thread-safe. It's also not normally
 * enabled either.
 */

static int verboseSet = 0;
static int verbose;
#define MIGIF_VERBOSE (verboseSet?verbose:setVerbose())
#define DEBUGMSG(printfArgs) if (MIGIF_VERBOSE) { printf printfArgs; }

static int
setVerbose(void)
{
    verbose = !!getenv("MIGIF_VERBOSE");
    verboseSet = 1;
    return verbose;
}

static const char *
binformat(v, nbits)
    unsigned int v;
    int nbits;
{
    static char bufs[8][64];
    static int bhand = 0;
    unsigned int bit;
    int bno;
    char *bp;

    bhand--;
    if (bhand < 0) {
	bhand = (sizeof(bufs) / sizeof(bufs[0])) - 1;
    }
    bp = &bufs[bhand][0];
    for (bno=nbits-1,bit=((unsigned int)1)<<bno ; bno>=0 ; bno--,bit>>=1) {
	*bp++ = (v & bit) ? '1' : '0';
	if (((bno&3) == 0) && (bno != 0)) {
	    *bp++ = '.';
	}
    }
    *bp = '\0';
    return &bufs[bhand][0];
}
#else /* !MIGIF_DEBUGGING_ENVARS */
#define DEBUGMSG(printfArgs) /* do nothing */
#endif

static void
writeBlock(statePtr)
    miGIFState_t *statePtr;
{
    int i;
    unsigned char c;

#ifdef MIGIF_DEBUGGING_ENVARS
    if (MIGIF_VERBOSE) {
	printf("writeBlock %d:", statePtr->oblen);
	for (i=0 ; i<statePtr->oblen ; i++) {
	    printf(" %02x", statePtr->oblock[i]);
	}
	printf("\n");
    }
#endif
    c = statePtr->oblen;
    Tcl_Write(statePtr->ofile, (char *) &c, 1);
    Tcl_Write(statePtr->ofile, (char *) &statePtr->oblock[0], statePtr->oblen);
    statePtr->oblen = 0;
}

static void
blockOut(statePtr, c)
    miGIFState_t *statePtr;
    unsigned char c;
{
    DEBUGMSG(("blockOut %s\n", binformat(c, 8)));
    statePtr->oblock[statePtr->oblen++] = c;
    if (statePtr->oblen >= 255) {
	writeBlock(statePtr);
    }
}

static void
blockFlush(statePtr)
    miGIFState_t *statePtr;
{
    DEBUGMSG(("blockFlush\n"));
    if (statePtr->oblen > 0) {
	writeBlock(statePtr);
    }
}

static void
output(statePtr, val)
    miGIFState_t *statePtr;
    int val;
{
    DEBUGMSG(("output %s [%s %d %d]\n", binformat(val, statePtr->outputBits),
	    binformat(statePtr->obuf, statePtr->obits), statePtr->obits,
	    statePtr->outputBits));
    statePtr->obuf |= val << statePtr->obits;
    statePtr->obits += statePtr->outputBits;
    while (statePtr->obits >= 8) {
	blockOut(statePtr, statePtr->obuf&0xff);
	statePtr->obuf >>= 8;
	statePtr->obits -= 8;
    }
    DEBUGMSG(("output leaving [%s %d]\n",
	    binformat(statePtr->obuf, statePtr->obits), statePtr->obits));
}

static void
outputFlush(statePtr)
    miGIFState_t *statePtr;
{
    DEBUGMSG(("outputFlush\n"));
    if (statePtr->obits > 0) {
	blockOut(statePtr, statePtr->obuf);
    }
    blockFlush(statePtr);
}

static void
didClear(statePtr)
    miGIFState_t *statePtr;
{
    DEBUGMSG(("didClear\n"));
    statePtr->outputBits = statePtr->outputBitsInit;
    statePtr->outputBump = statePtr->outputBumpInit;
    statePtr->outputClear = statePtr->outputClearInit;
    statePtr->outputCount = 0;
    statePtr->runlengthTableMax = 0;
    statePtr->justCleared = 1;
}

static void
outputPlain(statePtr, c)
    miGIFState_t *statePtr;
    int c;
{
    DEBUGMSG(("outputPlain %s\n", binformat(c, statePtr->outputBits)));
    statePtr->justCleared = 0;
    output(statePtr, c);
    statePtr->outputCount++;
    if (statePtr->outputCount >= statePtr->outputBump) {
	statePtr->outputBits++;
	statePtr->outputBump += 1 << (statePtr->outputBits - 1);
    }
    if (statePtr->outputCount >= statePtr->outputClear) {
	output(statePtr, statePtr->codeClear);
	didClear(statePtr);
    }
}

static unsigned int
isqrt(x)
    unsigned int x;
{
    unsigned int r;
    unsigned int v;

    if (x < 2) {
	return x;
    }
    for (v=x,r=1 ; v ; v>>=2,r<<=1);
    while (1) {
	v = ((x / r) + r) / 2;
	if (v==r || v==r+1) {
	    return r;
	}
	r = v;
    }
}

static unsigned int
computeTriangleCount(count, nrepcodes)
    unsigned int count;
    unsigned int nrepcodes;
{
    unsigned int perrep;
    unsigned int cost;

    cost = 0;
    perrep = (nrepcodes * (nrepcodes+1)) / 2;
    while (count >= perrep) {
	cost += nrepcodes;
	count -= perrep;
    }
    if (count > 0) {
	unsigned int n;
	n = isqrt(count);
	while (n*(n+1) >= 2*count) {
	    n--;
	}
	while (n*(n+1) < 2*count) {
	    n++;
	}
	cost += n;
    }
    return cost;
}

static void
maxOutputClear(statePtr)
    miGIFState_t *statePtr;
{
    statePtr->outputClear = statePtr->maxOcodes;
}

static void
resetOutputClear(statePtr)
    miGIFState_t *statePtr;
{
    statePtr->outputClear = statePtr->outputClearInit;
    if (statePtr->outputCount >= statePtr->outputClear) {
	output(statePtr, statePtr->codeClear);
	didClear(statePtr);
    }
}

static void
runlengthFlushFromClear(statePtr, count)
    miGIFState_t *statePtr;
    int count;
{
    int n;

    DEBUGMSG(("runlengthFlushFromClear %d\n", count));
    maxOutputClear(statePtr);
    statePtr->runlengthTablePixel = statePtr->runlengthPixel;
    n = 1;
    while (count > 0) {
	if (n == 1) {
	    statePtr->runlengthTableMax = 1;
	    outputPlain(statePtr, statePtr->runlengthPixel);
	    count--;
	} else if (count >= n) {
	    statePtr->runlengthTableMax = n;
	    outputPlain(statePtr, statePtr->runlengthBaseCode+n-2);
	    count -= n;
	} else if (count == 1) {
	    statePtr->runlengthTableMax++;
	    outputPlain(statePtr, statePtr->runlengthPixel);
	    count = 0;
	} else {
	    statePtr->runlengthTableMax++;
	    outputPlain(statePtr, statePtr->runlengthBaseCode+count-2);
	    count = 0;
	}
	if (statePtr->outputCount == 0) {
	    n = 1;
	} else {
	    n++;
	}
    }
    resetOutputClear(statePtr);
    DEBUGMSG(("runlengthFlushFromClear leaving tableMax=%d\n",
	    statePtr->runlengthTableMax));
}

static void
runlengthFlushClearOrRep(statePtr, count)
    miGIFState_t *statePtr;
    int count;
{
    int withclr;

    DEBUGMSG(("runlengthFlushClearOrRep %d\n", count));
    withclr = 1 + computeTriangleCount(count, statePtr->maxOcodes);
    if (withclr < count) {
	output(statePtr, statePtr->codeClear);
	didClear(statePtr);
	runlengthFlushFromClear(statePtr, count);
    } else {
	for (; count>0 ; count--) {
	    outputPlain(statePtr, statePtr->runlengthPixel);
	}
    }
}

static void
runlengthFlushWithTable(statePtr, count)
    miGIFState_t *statePtr;
    int count;
{
    int repmax;
    int repleft;
    int leftover;

    DEBUGMSG(("runlengthFlushWithTable %d\n", count));
    repmax = count / statePtr->runlengthTableMax;
    leftover = count % statePtr->runlengthTableMax;
    repleft = (leftover ? 1 : 0);
    if (statePtr->outputCount+repmax+repleft > statePtr->maxOcodes) {
	repmax = statePtr->maxOcodes - statePtr->outputCount;
	leftover = count - (repmax * statePtr->runlengthTableMax);
	repleft = 1 + computeTriangleCount(leftover, statePtr->maxOcodes);
    }
    DEBUGMSG(("runlengthFlushWithTable repmax=%d leftover=%d repleft=%d\n",
	    repmax, leftover, repleft));
    if (1+(int)computeTriangleCount(count, statePtr->maxOcodes)
	    < repmax+repleft) {
	output(statePtr, statePtr->codeClear);
	didClear(statePtr);
	runlengthFlushFromClear(statePtr, count);
	return;
    }
    maxOutputClear(statePtr);
    for (; repmax>0 ; repmax--) {
	outputPlain(statPtr,
		statePtr->runlengthBaseCode + statePtr->runlengthTableMax - 2);
    }
    if (leftover) {
	if (statePtr->justCleared) {
	    runlengthFlushFromClear(statePtr, leftover);
	} else if (leftover == 1) {
	    outputPlain(statePtr, statePtr->runlengthPixel);
	} else {
	    outputPlain(statePtr, statePtr->runlengthBaseCode + leftover - 2);
	}
    }
    resetOutputClear(statePtr);
}

static void
runlengthFlush(statePtr)
    miGIFState_t *statePtr;
{
    DEBUGMSG(("runlengthFlush [ %d %d\n", statePtr->runlengthCount,
	    statePtr->runlengthPixel));
    if (statePtr->runlengthCount == 1) {
	outputPlain(statePtr, statePtr->runlengthPixel);
	statePtr->runlengthCount = 0;
	DEBUGMSG(("runlengthFlush ]\n"));
	return;
    }
    if (statePtr->justCleared) {
	runlengthFlushFromClear(statePtr, statePtr->runlengthCount);
    } else if ((statePtr->runlengthTableMax < 2)
	    || (statePtr->runlengthTablePixel != statePtr->runlengthPixel)) {
	runlengthFlushClearOrRep(statePtr, statePtr->runlengthCount);
    } else {
	runlengthFlushWithTable(statePtr, statePtr->runlengthCount);
    }
    DEBUGMSG(("runlengthFlush ]\n"));
    statePtr->runlengthCount = 0;
}

static void
compress(initBits, handle, readValue, clientData)
    int initBits;
    Tcl_Channel handle;
    ifunptr readValue;
    ClientData clientData;
{
    int c;
    miGIFState_t state, *statePtr = &state;

    memset(statePtr, 0, sizeof(state));

    statePtr->ofile = handle;
    statePtr->obuf = 0;
    statePtr->obits = 0;
    statePtr->oblen = 0;
    statePtr->codeClear = 1 << (initBits - 1);
    statePtr->codeEOF = statePtr->codeClear + 1;
    statePtr->runlengthBaseCode = statePtr->codeEOF + 1;
    statePtr->outputBumpInit = (1 << (initBits - 1)) - 1;
    /*
     * For images with a lot of runs, making outputClearInit larger will give
     * better compression.
     */
    statePtr->outputClearInit =
	    (initBits <= 3) ? 9 : (statePtr->outputBumpInit-1);
#ifdef MIGIF_DEBUGGING_ENVARS
    {
	const char *ocienv;
	ocienv = getenv("MIGIF_OUT_CLEAR_INIT");
	if (ocienv) {
	    statePtr->outputClearInit = atoi(ocienv);
	    DEBUGMSG(("[overriding outputClearInit to %d]\n",
		    statePtr->outputClearInit));
	}
    }
#endif
    statePtr->outputBitsInit = initBits;
    statePtr->maxOcodes =
	    (1 << GIFBITS) - ((1 << (statePtr->outputBitsInit - 1)) + 3);
    didClear(statePtr);
    output(statePtr, statePtr->codeClear);
    statePtr->runlengthCount = 0;
    while (1) {
	c = readValue(clientData);
	if (statePtr->runlengthCount>0 && statePtr->runlengthPixel!=c) {
	    runlengthFlush(statePtr);
	}
	if (c == EOF) {
	    break;
	}
	if (statePtr->runlengthPixel == c) {
	    statePtr->runlengthCount++;
	} else {
	    statePtr->runlengthPixel = c;
	    statePtr->runlengthCount = 1;
	}
    }
    output(statePtr, statePtr->codeEOF);
    outputFlush(statePtr);
}

/*
 *-----------------------------------------------------------------------
 *
 * End of miGIF section  - See copyright notice at start of section.
 *
 *-----------------------------------------------------------------------
 */