xref: /PHP-7.0/ext/gd/libgd/gd_gif_out.c (revision 8dc5ffa4)
1 #include <stdio.h>
2 #include <math.h>
3 #include <string.h>
4 #include <stdlib.h>
5 #include "gd.h"
6 
7 /* Code drawn from ppmtogif.c, from the pbmplus package
8 **
9 ** Based on GIFENCOD by David Rowley <mgardi@watdscu.waterloo.edu>. A
10 ** Lempel-Zim compression based on "compress".
11 **
12 ** Modified by Marcel Wijkstra <wijkstra@fwi.uva.nl>
13 **
14 ** Copyright (C) 1989 by Jef Poskanzer.
15 **
16 ** Permission to use, copy, modify, and distribute this software and its
17 ** documentation for any purpose and without fee is hereby granted, provided
18 ** that the above copyright notice appear in all copies and that both that
19 ** copyright notice and this permission notice appear in supporting
20 ** documentation.  This software is provided "as is" without express or
21 ** implied warranty.
22 **
23 ** The Graphics Interchange Format(c) is the Copyright property of
24 ** CompuServe Incorporated.  GIF(sm) is a Service Mark property of
25 ** CompuServe Incorporated.
26 */
27 
28 /*
29  * a code_int must be able to hold 2**GIFBITS values of type int, and also -1
30  */
31 typedef int             code_int;
32 
33 #ifdef SIGNED_COMPARE_SLOW
34 typedef unsigned long int count_int;
35 typedef unsigned short int count_short;
36 #else /*SIGNED_COMPARE_SLOW*/
37 typedef long int          count_int;
38 #endif /*SIGNED_COMPARE_SLOW*/
39 
40 /* 2.0.28: threadsafe */
41 
42 #define maxbits GIFBITS
43 
44 /* should NEVER generate this code */
45 #define maxmaxcode ((code_int)1 << GIFBITS)
46 
47 #define HSIZE  5003            /* 80% occupancy */
48 #define hsize HSIZE            /* Apparently invariant, left over from
49 					compress */
50 
51 typedef struct {
52 	int Width, Height;
53 	int curx, cury;
54 	long CountDown;
55 	int Pass;
56 	int Interlace;
57         int n_bits;                        /* number of bits/code */
58         code_int maxcode;                  /* maximum code, given n_bits */
59         count_int htab [HSIZE];
60         unsigned short codetab [HSIZE];
61 	code_int free_ent;                  /* first unused entry */
62 	/*
63 	 * block compression parameters -- after all codes are used up,
64 	 * and compression rate changes, start over.
65 	 */
66 	int clear_flg;
67 	int offset;
68 	long int in_count;            /* length of input */
69 	long int out_count;           /* # of codes output (for debugging) */
70 
71 	int g_init_bits;
72 	gdIOCtx * g_outfile;
73 
74 	int ClearCode;
75 	int EOFCode;
76 	unsigned long cur_accum;
77 	int cur_bits;
78         /*
79          * Number of characters so far in this 'packet'
80          */
81         int a_count;
82         /*
83          * Define the storage for the packet accumulator
84          */
85         char accum[ 256 ];
86 } GifCtx;
87 
88 static int gifPutWord(int w, gdIOCtx *out);
89 static int colorstobpp(int colors);
90 static void BumpPixel (GifCtx *ctx);
91 static int GIFNextPixel (gdImagePtr im, GifCtx *ctx);
92 static void GIFEncode (gdIOCtxPtr fp, int GWidth, int GHeight, int GInterlace, int Background, int Transparent, int BitsPerPixel, int *Red, int *Green, int *Blue, gdImagePtr im);
93 static void compress (int init_bits, gdIOCtx *outfile, gdImagePtr im, GifCtx *ctx);
94 static void output (code_int code, GifCtx *ctx);
95 static void cl_block (GifCtx *ctx);
96 static void cl_hash (register count_int chsize, GifCtx *ctx);
97 static void char_init (GifCtx *ctx);
98 static void char_out (int c, GifCtx *ctx);
99 static void flush_char (GifCtx *ctx);
gdImageGifPtr(gdImagePtr im,int * size)100 void * gdImageGifPtr (gdImagePtr im, int *size)
101 {
102   void *rv;
103   gdIOCtx *out = gdNewDynamicCtx (2048, NULL);
104   gdImageGifCtx (im, out);
105   rv = gdDPExtractData (out, size);
106   out->gd_free (out);
107   return rv;
108 }
109 
gdImageGif(gdImagePtr im,FILE * outFile)110 void gdImageGif (gdImagePtr im, FILE * outFile)
111 {
112   gdIOCtx *out = gdNewFileCtx (outFile);
113   gdImageGifCtx (im, out);
114   out->gd_free (out);
115 }
116 
gdImageGifCtx(gdImagePtr im,gdIOCtxPtr out)117 void gdImageGifCtx(gdImagePtr im, gdIOCtxPtr out)
118 {
119 	gdImagePtr pim = 0, tim = im;
120 	int interlace, BitsPerPixel;
121 	interlace = im->interlace;
122 	if (im->trueColor) {
123 		/* Expensive, but the only way that produces an
124 			acceptable result: mix down to a palette
125 			based temporary image. */
126 		pim = gdImageCreatePaletteFromTrueColor(im, 1, 256);
127 		if (!pim) {
128 			return;
129 		}
130 		tim = pim;
131 	}
132 	BitsPerPixel = colorstobpp(tim->colorsTotal);
133 	/* All set, let's do it. */
134 	GIFEncode(
135 		out, tim->sx, tim->sy, tim->interlace, 0, tim->transparent, BitsPerPixel,
136 		tim->red, tim->green, tim->blue, tim);
137 	if (pim) {
138 		/* Destroy palette based temporary image. */
139 		gdImageDestroy(	pim);
140 	}
141 }
142 
143 static int
colorstobpp(int colors)144 colorstobpp(int colors)
145 {
146     int bpp = 0;
147 
148     if ( colors <= 2 )
149         bpp = 1;
150     else if ( colors <= 4 )
151         bpp = 2;
152     else if ( colors <= 8 )
153         bpp = 3;
154     else if ( colors <= 16 )
155         bpp = 4;
156     else if ( colors <= 32 )
157         bpp = 5;
158     else if ( colors <= 64 )
159         bpp = 6;
160     else if ( colors <= 128 )
161         bpp = 7;
162     else if ( colors <= 256 )
163         bpp = 8;
164     return bpp;
165     }
166 
167 /*****************************************************************************
168  *
169  * GIFENCODE.C    - GIF Image compression interface
170  *
171  * GIFEncode( FName, GHeight, GWidth, GInterlace, Background, Transparent,
172  *            BitsPerPixel, Red, Green, Blue, gdImagePtr )
173  *
174  *****************************************************************************/
175 
176 #define TRUE 1
177 #define FALSE 0
178 /*
179  * Bump the 'curx' and 'cury' to point to the next pixel
180  */
181 static void
BumpPixel(GifCtx * ctx)182 BumpPixel(GifCtx *ctx)
183 {
184         /*
185          * Bump the current X position
186          */
187         ++(ctx->curx);
188 
189         /*
190          * If we are at the end of a scan line, set curx back to the beginning
191          * If we are interlaced, bump the cury to the appropriate spot,
192          * otherwise, just increment it.
193          */
194         if( ctx->curx == ctx->Width ) {
195                 ctx->curx = 0;
196 
197                 if( !ctx->Interlace )
198                         ++(ctx->cury);
199                 else {
200                      switch( ctx->Pass ) {
201 
202                        case 0:
203                           ctx->cury += 8;
204                           if( ctx->cury >= ctx->Height ) {
205                                 ++(ctx->Pass);
206                                 ctx->cury = 4;
207                           }
208                           break;
209 
210                        case 1:
211                           ctx->cury += 8;
212                           if( ctx->cury >= ctx->Height ) {
213                                 ++(ctx->Pass);
214                                 ctx->cury = 2;
215                           }
216                           break;
217 
218                        case 2:
219                           ctx->cury += 4;
220                           if( ctx->cury >= ctx->Height ) {
221                              ++(ctx->Pass);
222                              ctx->cury = 1;
223                           }
224                           break;
225 
226                        case 3:
227                           ctx->cury += 2;
228                           break;
229                         }
230                 }
231         }
232 }
233 
234 /*
235  * Return the next pixel from the image
236  */
237 static int
GIFNextPixel(gdImagePtr im,GifCtx * ctx)238 GIFNextPixel(gdImagePtr im, GifCtx *ctx)
239 {
240         int r;
241 
242         if( ctx->CountDown == 0 )
243                 return EOF;
244 
245         --(ctx->CountDown);
246 
247         r = gdImageGetPixel(im, ctx->curx, ctx->cury);
248 
249         BumpPixel(ctx);
250 
251         return r;
252 }
253 
254 /* public */
255 
256 static void
GIFEncode(gdIOCtxPtr fp,int GWidth,int GHeight,int GInterlace,int Background,int Transparent,int BitsPerPixel,int * Red,int * Green,int * Blue,gdImagePtr im)257 GIFEncode(gdIOCtxPtr fp, int GWidth, int GHeight, int GInterlace, int Background, int Transparent, int BitsPerPixel, int *Red, int *Green, int *Blue, gdImagePtr im)
258 {
259         int B;
260         int RWidth, RHeight;
261         int LeftOfs, TopOfs;
262         int Resolution;
263         int ColorMapSize;
264         int InitCodeSize;
265         int i;
266 		GifCtx ctx;
267 
268 		memset(&ctx, 0, sizeof(ctx));
269         ctx.Interlace = GInterlace;
270 		ctx.in_count = 1;
271 
272         ColorMapSize = 1 << BitsPerPixel;
273 
274         RWidth = ctx.Width = GWidth;
275         RHeight = ctx.Height = GHeight;
276         LeftOfs = TopOfs = 0;
277 
278         Resolution = BitsPerPixel;
279 
280         /*
281          * Calculate number of bits we are expecting
282          */
283         ctx.CountDown = (long)ctx.Width * (long)ctx.Height;
284 
285         /*
286          * Indicate which pass we are on (if interlace)
287          */
288         ctx.Pass = 0;
289 
290         /*
291          * The initial code size
292          */
293         if( BitsPerPixel <= 1 )
294                 InitCodeSize = 2;
295         else
296                 InitCodeSize = BitsPerPixel;
297 
298         /*
299          * Set up the current x and y position
300          */
301         ctx.curx = ctx.cury = 0;
302 
303         /*
304          * Write the Magic header
305          */
306         gdPutBuf(Transparent < 0 ? "GIF87a" : "GIF89a", 6, fp );
307 
308         /*
309          * Write out the screen width and height
310          */
311         gifPutWord( RWidth, fp );
312         gifPutWord( RHeight, fp );
313 
314         /*
315          * Indicate that there is a global colour map
316          */
317         B = 0x80;       /* Yes, there is a color map */
318 
319         /*
320          * OR in the resolution
321          */
322         B |= (Resolution - 1) << 5;
323 
324         /*
325          * OR in the Bits per Pixel
326          */
327         B |= (BitsPerPixel - 1);
328 
329         /*
330          * Write it out
331          */
332         gdPutC( B, fp );
333 
334         /*
335          * Write out the Background colour
336          */
337         gdPutC( Background, fp );
338 
339         /*
340          * Byte of 0's (future expansion)
341          */
342         gdPutC( 0, fp );
343 
344         /*
345          * Write out the Global Colour Map
346          */
347         for( i=0; i<ColorMapSize; ++i ) {
348                 gdPutC( Red[i], fp );
349                 gdPutC( Green[i], fp );
350                 gdPutC( Blue[i], fp );
351         }
352 
353 	/*
354 	 * Write out extension for transparent colour index, if necessary.
355 	 */
356 	if ( Transparent >= 0 ) {
357 	    gdPutC( '!', fp );
358 	    gdPutC( 0xf9, fp );
359 	    gdPutC( 4, fp );
360 	    gdPutC( 1, fp );
361 	    gdPutC( 0, fp );
362 	    gdPutC( 0, fp );
363 	    gdPutC( (unsigned char) Transparent, fp );
364 	    gdPutC( 0, fp );
365 	}
366 
367         /*
368          * Write an Image separator
369          */
370         gdPutC( ',', fp );
371 
372         /*
373          * Write the Image header
374          */
375 
376         gifPutWord( LeftOfs, fp );
377         gifPutWord( TopOfs, fp );
378         gifPutWord( ctx.Width, fp );
379         gifPutWord( ctx.Height, fp );
380 
381         /*
382          * Write out whether or not the image is interlaced
383          */
384         if( ctx.Interlace )
385                 gdPutC( 0x40, fp );
386         else
387                 gdPutC( 0x00, fp );
388 
389         /*
390          * Write out the initial code size
391          */
392         gdPutC( InitCodeSize, fp );
393 
394         /*
395          * Go and actually compress the data
396          */
397         compress( InitCodeSize+1, fp, im, &ctx );
398 
399         /*
400          * Write out a Zero-length packet (to end the series)
401          */
402         gdPutC( 0, fp );
403 
404         /*
405          * Write the GIF file terminator
406          */
407         gdPutC( ';', fp );
408 }
409 
410 /***************************************************************************
411  *
412  *  GIFCOMPR.C       - GIF Image compression routines
413  *
414  *  Lempel-Ziv compression based on 'compress'.  GIF modifications by
415  *  David Rowley (mgardi@watdcsu.waterloo.edu)
416  *
417  ***************************************************************************/
418 
419 /*
420  * General DEFINEs
421  */
422 
423 #define GIFBITS    12
424 
425 #ifdef NO_UCHAR
426  typedef char   char_type;
427 #else /*NO_UCHAR*/
428  typedef        unsigned char   char_type;
429 #endif /*NO_UCHAR*/
430 
431 /*
432  *
433  * GIF Image compression - modified 'compress'
434  *
435  * Based on: compress.c - File compression ala IEEE Computer, June 1984.
436  *
437  * By Authors:  Spencer W. Thomas       (decvax!harpo!utah-cs!utah-gr!thomas)
438  *              Jim McKie               (decvax!mcvax!jim)
439  *              Steve Davies            (decvax!vax135!petsd!peora!srd)
440  *              Ken Turkowski           (decvax!decwrl!turtlevax!ken)
441  *              James A. Woods          (decvax!ihnp4!ames!jaw)
442  *              Joe Orost               (decvax!vax135!petsd!joe)
443  *
444  */
445 #include <ctype.h>
446 
447 #define ARGVAL() (*++(*argv) || (--argc && *++argv))
448 
449 #ifdef COMPATIBLE               /* But wrong! */
450 # define MAXCODE(n_bits)        ((code_int) 1 << (n_bits) - 1)
451 #else /*COMPATIBLE*/
452 # define MAXCODE(n_bits)        (((code_int) 1 << (n_bits)) - 1)
453 #endif /*COMPATIBLE*/
454 
455 #define HashTabOf(i)       ctx->htab[i]
456 #define CodeTabOf(i)    ctx->codetab[i]
457 
458 
459 /*
460  * To save much memory, we overlay the table used by compress() with those
461  * used by decompress().  The tab_prefix table is the same size and type
462  * as the codetab.  The tab_suffix table needs 2**GIFBITS characters.  We
463  * get this from the beginning of htab.  The output stack uses the rest
464  * of htab, and contains characters.  There is plenty of room for any
465  * possible stack (stack used to be 8000 characters).
466  */
467 
468 #define tab_prefixof(i) CodeTabOf(i)
469 #define tab_suffixof(i)        ((char_type*)(htab))[i]
470 #define de_stack               ((char_type*)&tab_suffixof((code_int)1<<GIFBITS))
471 
472 /*
473  * compress stdin to stdout
474  *
475  * Algorithm:  use open addressing double hashing (no chaining) on the
476  * prefix code / next character combination.  We do a variant of Knuth's
477  * algorithm D (vol. 3, sec. 6.4) along with G. Knott's relatively-prime
478  * secondary probe.  Here, the modular division first probe is gives way
479  * to a faster exclusive-or manipulation.  Also do block compression with
480  * an adaptive reset, whereby the code table is cleared when the compression
481  * ratio decreases, but after the table fills.  The variable-length output
482  * codes are re-sized at this point, and a special CLEAR code is generated
483  * for the decompressor.  Late addition:  construct the table according to
484  * file size for noticeable speed improvement on small files.  Please direct
485  * questions about this implementation to ames!jaw.
486  */
487 
488 static void
489 output(code_int code, GifCtx *ctx);
490 
491 static void
compress(int init_bits,gdIOCtxPtr outfile,gdImagePtr im,GifCtx * ctx)492 compress(int init_bits, gdIOCtxPtr outfile, gdImagePtr im, GifCtx *ctx)
493 {
494     register long fcode;
495     register code_int i /* = 0 */;
496     register int c;
497     register code_int ent;
498     register code_int disp;
499     register code_int hsize_reg;
500     register int hshift;
501 
502     /*
503      * Set up the globals:  g_init_bits - initial number of bits
504      *                      g_outfile   - pointer to output file
505      */
506     ctx->g_init_bits = init_bits;
507     ctx->g_outfile = outfile;
508 
509     /*
510      * Set up the necessary values
511      */
512     ctx->offset = 0;
513     ctx->out_count = 0;
514     ctx->clear_flg = 0;
515     ctx->in_count = 1;
516     ctx->maxcode = MAXCODE(ctx->n_bits = ctx->g_init_bits);
517 
518     ctx->ClearCode = (1 << (init_bits - 1));
519     ctx->EOFCode = ctx->ClearCode + 1;
520     ctx->free_ent = ctx->ClearCode + 2;
521 
522     char_init(ctx);
523 
524     ent = GIFNextPixel( im, ctx );
525 
526     hshift = 0;
527     for ( fcode = (long) hsize;  fcode < 65536L; fcode *= 2L )
528         ++hshift;
529     hshift = 8 - hshift;                /* set hash code range bound */
530 
531     hsize_reg = hsize;
532     cl_hash( (count_int) hsize_reg, ctx );            /* clear hash table */
533 
534     output( (code_int)ctx->ClearCode, ctx );
535 
536 #ifdef SIGNED_COMPARE_SLOW
537     while ( (c = GIFNextPixel( im )) != (unsigned) EOF ) {
538 #else /*SIGNED_COMPARE_SLOW*/
539     while ( (c = GIFNextPixel( im, ctx )) != EOF ) {  /* } */
540 #endif /*SIGNED_COMPARE_SLOW*/
541 
542         ++(ctx->in_count);
543 
544         fcode = (long) (((long) c << maxbits) + ent);
545         i = (((code_int)c << hshift) ^ ent);    /* xor hashing */
546 
547         if ( HashTabOf (i) == fcode ) {
548             ent = CodeTabOf (i);
549             continue;
550         } else if ( (long)HashTabOf (i) < 0 )      /* empty slot */
551             goto nomatch;
552         disp = hsize_reg - i;           /* secondary hash (after G. Knott) */
553         if ( i == 0 )
554             disp = 1;
555 probe:
556         if ( (i -= disp) < 0 )
557             i += hsize_reg;
558 
559         if ( HashTabOf (i) == fcode ) {
560             ent = CodeTabOf (i);
561             continue;
562         }
563         if ( (long)HashTabOf (i) > 0 )
564             goto probe;
565 nomatch:
566         output ( (code_int) ent, ctx );
567         ++(ctx->out_count);
568         ent = c;
569 #ifdef SIGNED_COMPARE_SLOW
570         if ( (unsigned) ctx->free_ent < (unsigned) maxmaxcode) {
571 #else /*SIGNED_COMPARE_SLOW*/
572         if ( ctx->free_ent < maxmaxcode ) {  /* } */
573 #endif /*SIGNED_COMPARE_SLOW*/
574             CodeTabOf (i) = ctx->free_ent++; /* code -> hashtable */
575             HashTabOf (i) = fcode;
576         } else
577                 cl_block(ctx);
578     }
579     /*
580      * Put out the final code.
581      */
582     output( (code_int)ent, ctx );
583     ++(ctx->out_count);
584     output( (code_int) ctx->EOFCode, ctx );
585 }
586 
587 /*****************************************************************
588  * TAG( output )
589  *
590  * Output the given code.
591  * Inputs:
592  *      code:   A n_bits-bit integer.  If == -1, then EOF.  This assumes
593  *              that n_bits =< (long)wordsize - 1.
594  * Outputs:
595  *      Outputs code to the file.
596  * Assumptions:
597  *      Chars are 8 bits long.
598  * Algorithm:
599  *      Maintain a GIFBITS character long buffer (so that 8 codes will
600  * fit in it exactly).  Use the VAX insv instruction to insert each
601  * code in turn.  When the buffer fills up empty it and start over.
602  */
603 
604 static const unsigned long masks[] = { 0x0000, 0x0001, 0x0003, 0x0007, 0x000F,
605                                   0x001F, 0x003F, 0x007F, 0x00FF,
606                                   0x01FF, 0x03FF, 0x07FF, 0x0FFF,
607                                   0x1FFF, 0x3FFF, 0x7FFF, 0xFFFF };
608 
609 
610 /* Arbitrary value to mark output is done.  When we see EOFCode, then we don't
611  * expect to see any more data.  If we do (e.g. corrupt image inputs), cur_bits
612  * might be negative, so flag it to return early.
613  */
614 #define CUR_BITS_FINISHED -1000
615 
616 
617 static void
618 output(code_int code, GifCtx *ctx)
619 {
620 	if (ctx->cur_bits == CUR_BITS_FINISHED) {
621 		return;
622 	}
623 
624     ctx->cur_accum &= masks[ ctx->cur_bits ];
625 
626     if( ctx->cur_bits > 0 )
627         ctx->cur_accum |= ((long)code << ctx->cur_bits);
628     else
629         ctx->cur_accum = code;
630 
631     ctx->cur_bits += ctx->n_bits;
632 
633     while( ctx->cur_bits >= 8 ) {
634         char_out( (unsigned int)(ctx->cur_accum & 0xff), ctx );
635         ctx->cur_accum >>= 8;
636         ctx->cur_bits -= 8;
637     }
638 
639     /*
640      * If the next entry is going to be too big for the code size,
641      * then increase it, if possible.
642      */
643    if ( ctx->free_ent > ctx->maxcode || ctx->clear_flg ) {
644 
645             if( ctx->clear_flg ) {
646 
647                 ctx->maxcode = MAXCODE (ctx->n_bits = ctx->g_init_bits);
648                 ctx->clear_flg = 0;
649 
650             } else {
651 
652                 ++(ctx->n_bits);
653                 if ( ctx->n_bits == maxbits )
654                     ctx->maxcode = maxmaxcode;
655                 else
656                     ctx->maxcode = MAXCODE(ctx->n_bits);
657             }
658         }
659 
660     if( code == ctx->EOFCode ) {
661         /*
662          * At EOF, write the rest of the buffer.
663          */
664         while( ctx->cur_bits > 0 ) {
665                 char_out( (unsigned int)(ctx->cur_accum & 0xff), ctx);
666                 ctx->cur_accum >>= 8;
667                 ctx->cur_bits -= 8;
668         }
669 
670 		/* Flag that it's done to prevent re-entry. */
671 		ctx->cur_bits = CUR_BITS_FINISHED;
672 
673         flush_char(ctx);
674     }
675 }
676 
677 /*
678  * Clear out the hash table
679  */
680 static void
681 cl_block (GifCtx *ctx)             /* table clear for block compress */
682 {
683 
684         cl_hash ( (count_int) hsize, ctx );
685         ctx->free_ent = ctx->ClearCode + 2;
686         ctx->clear_flg = 1;
687 
688         output( (code_int)ctx->ClearCode, ctx);
689 }
690 
691 static void
692 cl_hash(register count_int chsize, GifCtx *ctx)          /* reset code table */
693 
694 {
695 
696         register count_int *htab_p = ctx->htab+chsize;
697 
698         register long i;
699         register long m1 = -1;
700 
701         i = chsize - 16;
702         do {                            /* might use Sys V memset(3) here */
703                 *(htab_p-16) = m1;
704                 *(htab_p-15) = m1;
705                 *(htab_p-14) = m1;
706                 *(htab_p-13) = m1;
707                 *(htab_p-12) = m1;
708                 *(htab_p-11) = m1;
709                 *(htab_p-10) = m1;
710                 *(htab_p-9) = m1;
711                 *(htab_p-8) = m1;
712                 *(htab_p-7) = m1;
713                 *(htab_p-6) = m1;
714                 *(htab_p-5) = m1;
715                 *(htab_p-4) = m1;
716                 *(htab_p-3) = m1;
717                 *(htab_p-2) = m1;
718                 *(htab_p-1) = m1;
719                 htab_p -= 16;
720         } while ((i -= 16) >= 0);
721 
722         for ( i += 16; i > 0; --i )
723                 *--htab_p = m1;
724 }
725 
726 /******************************************************************************
727  *
728  * GIF Specific routines
729  *
730  ******************************************************************************/
731 
732 /*
733  * Set up the 'byte output' routine
734  */
735 static void
736 char_init(GifCtx *ctx)
737 {
738         ctx->a_count = 0;
739 }
740 
741 /*
742  * Add a character to the end of the current packet, and if it is 254
743  * characters, flush the packet to disk.
744  */
745 static void
746 char_out(int c, GifCtx *ctx)
747 {
748         ctx->accum[ ctx->a_count++ ] = c;
749         if( ctx->a_count >= 254 )
750                 flush_char(ctx);
751 }
752 
753 /*
754  * Flush the packet to disk, and reset the accumulator
755  */
756 static void
757 flush_char(GifCtx *ctx)
758 {
759         if( ctx->a_count > 0 ) {
760                 gdPutC( ctx->a_count, ctx->g_outfile );
761                 gdPutBuf( ctx->accum, ctx->a_count, ctx->g_outfile );
762                 ctx->a_count = 0;
763         }
764 }
765 
766 static int gifPutWord(int w, gdIOCtx *out)
767 {
768 	/* Byte order is little-endian */
769 	gdPutC(w & 0xFF, out);
770 	gdPutC((w >> 8) & 0xFF, out);
771 	return 0;
772 }
773 
774 
775