summaryrefslogtreecommitdiff
path: root/src/gif.c
blob: ec5c185b6ad19c006d5ce17f945a4224e7c068a5 (plain)
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
61
62
63
64
65
66
67
68
69
70
71
72
73
74
75
76
77
78
79
80
81
82
83
84
85
86
87
88
89
90
91
92
93
94
95
96
97
98
99
100
101
102
103
104
105
106
107
108
109
110
111
112
113
114
115
116
117
118
119
120
121
122
123
124
125
126
127
128
129
130
131
132
133
134
135
136
137
138
139
140
141
142
143
144
145
146
147
148
149
150
151
152
153
154
155
156
157
158
159
160
161
162
163
164
165
166
167
168
169
170
171
172
173
174
175
176
177
178
179
180
181
182
183
184
185
186
187
188
189
190
191
192
193
194
195
196
197
198
199
200
201
202
203
204
205
206
207
208
209
210
211
212
213
214
215
216
217
218
219
220
221
222
223
224
225
226
227
228
229
230
231
232
233
234
235
236
237
238
239
240
241
242
243
244
245
246
247
248
249
250
251
252
253
254
255
256
257
258
259
260
261
262
263
264
265
266
267
268
269
270
271
272
273
274
275
276
277
278
279
280
281
282
283
284
285
286
287
288
289
290
291
292
293
294
295
296
297
298
299
300
301
302
303
304
305
306
307
308
309
310
311
312
313
314
315
316
317
318
319
320
321
322
323
324
325
326
327
328
329
330
331
332
333
334
335
336
337
338
339
340
341
342
343
344
345
346
347
348
349
350
351
352
353
354
355
356
357
358
359
360
361
362
363
364
365
366
367
368
369
370
371
372
373
374
375
376
377
378
379
380
381
382
383
384
385
386
387
388
389
390
391
392
393
394
395
396
397
398
399
400
401
402
403
404
405
406
407
408
409
410
411
412
413
414
415
416
417
418
419
420
421
422
423
424
425
426
427
428
429
430
431
432
433
434
435
436
437
438
439
440
441
442
443
444
445
446
447
448
449
450
451
452
453
454
455
456
457
458
459
460
461
462
463
464
465
466
467
468
469
470
471
472
473
474
475
476
477
478
479
480
481
482
483
484
485
486
487
488
489
490
491
492
493
494
495
496
497
498
499
500
501
502
503
504
505
506
507
508
509
510
511
512
513
514
515
516
517
518
519
520
521
522
523
524
525
526
527
528
529
530
531
532
533
534
535
536
537
538
539
540
541
542
543
544
545
546
547
548
549
550
551
552
553
554
555
556
557
558
559
560
561
562
563
564
565
566
567
568
569
570
571
572
573
574
575
576
577
578
579
580
581
582
583
584
585
586
587
588
589
590
591
592
593
594
595
596
597
598
599
600
601
602
603
604
605
606
607
608
609
610
611
612
613
614
615
616
617
618
619
620
621
622
623
624
625
626
627
628
629
630
631
632
633
634
635
636
637
638
639
640
641
642
643
644
645
646
647
648
649
650
651
652
653
654
655
656
657
658
659
660
661
662
663
664
665
666
667
668
669
670
671
672
673
674
675
676
677
678
679
680
681
682
683
684
685
686
687
688
689
690
691
692
693
694
695
696
697
698
699
700
701
702
703
704
705
706
707
708
709
710
711
712
713
714
715
716
717
718
719
720
721
722
723
724
725
726
727
728
729
730
731
732
733
734
735
736
737
738
739
740
741
742
743
744
745
746
747
748
749
750
751
752
753
754
755
756
757
758
759
760
761
762
763
764
765
766
767
768
769
770
771
772
773
774
775
776
777
778
779
780
781
782
783
784
785
786
787
788
789
790
791
792
793
794
795
796
797
798
799
800
801
802
803
804
805
806
807
808
809
810
811
812
813
814
815
816
817
818
819
820
821
822
823
824
825
826
827
828
829
830
831
832
833
834
835
836
837
838
839
840
841
842
843
844
845
846
847
848
849
850
851
852
853
854
855
856
857
858
859
860
861
862
863
864
865
866
867
868
869
870
871
872
873
874
875
876
877
878
879
880
881
882
883
884
885
886
887
888
889
890
891
892
893
894
895
896
897
898
899
900
901
902
903
904
905
906
907
908
909
910
911
912
913
914
915
916
917
918
919
920
921
922
923
924
925
926
927
928
929
930
931
932
933
934
935
936
937
938
939
940
941
942
943
944
945
946
947
948
949
950
951
952
953
954
955
956
957
958
959
960
961
962
963
964
965
966
967
968
969
970
971
972
973
974
975
976
977
978
979
980
981
982
983
984
985
986
987
988
989
990
991
992
993
994
995
996
997
998
999
1000
1001
1002
1003
1004
1005
1006
1007
1008
1009
1010
1011
1012
1013
1014
1015
1016
1017
1018
1019
1020
1021
1022
1023
1024
1025
1026
1027
1028
1029
1030
1031
1032
1033
1034
1035
1036
1037
1038
1039
1040
1041
1042
1043
1044
1045
1046
1047
1048
1049
1050
1051
1052
1053
1054
1055
1056
1057
1058
1059
1060
1061
1062
1063
1064
1065
1066
1067
1068
1069
1070
1071
1072
1073
1074
1075
1076
1077
1078
1079
1080
1081
1082
1083
1084
1085
1086
1087
1088
1089
1090
1091
1092
1093
1094
1095
1096
1097
1098
1099
1100
1101
1102
1103
1104
1105
1106
1107
1108
1109
1110
1111
1112
1113
1114
1115
1116
1117
1118
1119
1120
1121
1122
1123
1124
1125
1126
1127
1128
1129
1130
1131
1132
1133
1134
1135
1136
1137
1138
1139
1140
1141
1142
1143
1144
1145
1146
1147
1148
1149
1150
1151
1152
1153
1154
1155
1156
1157
1158
1159
1160
1161
1162
1163
1164
1165
1166
1167
1168
1169
1170
1171
1172
1173
1174
1175
1176
1177
1178
1179
1180
1181
1182
1183
1184
1185
1186
1187
1188
1189
1190
1191
1192
1193
1194
1195
1196
1197
1198
1199
1200
1201
1202
1203
1204
1205
1206
1207
1208
1209
1210
1211
1212
1213
1214
1215
1216
1217
1218
1219
1220
1221
1222
1223
1224
1225
1226
1227
1228
1229
1230
1231
1232
1233
1234
1235
1236
1237
1238
1239
1240
1241
1242
1243
1244
1245
1246
1247
1248
1249
1250
1251
1252
1253
1254
1255
1256
1257
1258
1259
1260
1261
1262
1263
1264
1265
1266
1267
1268
1269
1270
1271
1272
1273
1274
1275
1276
1277
1278
1279
1280
1281
1282
1283
1284
1285
1286
1287
1288
1289
1290
1291
1292
1293
1294
1295
1296
1297
1298
1299
1300
1301
1302
1303
1304
1305
1306
1307
1308
1309
1310
1311
1312
1313
1314
1315
1316
1317
1318
1319
1320
1321
1322
1323
1324
1325
1326
1327
1328
1329
1330
1331
1332
1333
1334
1335
1336
1337
1338
1339
1340
1341
1342
1343
1344
1345
1346
1347
1348
1349
1350
1351
1352
1353
1354
1355
1356
1357
1358
1359
1360
1361
1362
1363
1364
1365
1366
1367
1368
1369
1370
1371
1372
1373
1374
1375
1376
1377
1378
1379
1380
1381
1382
1383
1384
1385
1386
1387
1388
1389
1390
1391
1392
1393
1394
1395
1396
1397
1398
1399
1400
1401
1402
1403
1404
1405
1406
1407
1408
1409
1410
1411
1412
1413
1414
1415
1416
1417
1418
1419
1420
1421
1422
1423
1424
1425
1426
1427
1428
1429
1430
1431
1432
1433
1434
1435
1436
1437
1438
1439
1440
1441
1442
1443
1444
1445
1446
1447
1448
1449
1450
1451
1452
1453
1454
1455
1456
1457
1458
1459
1460
1461
1462
1463
1464
1465
1466
1467
/*
 * Copyright 2004 Richard Wilson <richard.wilson@netsurf-browser.org>
 * Copyright 2008 Sean Fox <dyntryx@gmail.com>
 * Copyright 2013-2021 Michael Drake <tlsa@netsurf-browser.org>
 *
 * This file is part of NetSurf's libnsgif, http://www.netsurf-browser.org/
 * Licenced under the MIT License,
 *                http://www.opensource.org/licenses/mit-license.php
 */

#include <assert.h>
#include <stdint.h>
#include <stdlib.h>
#include <string.h>
#include <stdbool.h>

#include "lzw.h"
#include "nsgif.h"

/**
 *
 * \file
 * \brief GIF image decoder
 *
 * The GIF format is thoroughly documented; a full description can be found at
 * http://www.w3.org/Graphics/GIF/spec-gif89a.txt
 *
 * \todo Plain text and comment extensions should be implemented.
 */

/** Maximum colour table size */
#define NSGIF_MAX_COLOURS 256

/** Internal flag that the colour table needs to be processed */
#define NSGIF_PROCESS_COLOURS 0xaa000000

/** Internal flag that a frame is invalid/unprocessed */
#define NSGIF_INVALID_FRAME -1

/** Transparent colour */
#define NSGIF_TRANSPARENT_COLOUR 0x00

/** No transparency */
#define NSGIF_NO_TRANSPARENCY (0xFFFFFFFFu)

enum nsgif_disposal {
	NSGIF_DISPOSAL_UNSPECIFIED,
	NSGIF_DISPOSAL_NONE,
	NSGIF_DISPOSAL_RESTORE_BG,
	NSGIF_DISPOSAL_RESTORE_PREV,
	NSGIF_DISPOSAL_RESTORE_QUIRK, /**< Alias for NSGIF_DISPOSAL_RESTORE_PREV. */
};

/* GIF Flags */
#define NSGIF_COLOUR_TABLE_MASK 0x80
#define NSGIF_COLOUR_TABLE_SIZE_MASK 0x07
#define NSGIF_BLOCK_TERMINATOR 0x00
#define NSGIF_TRAILER 0x3b

/**
 * Convert an LZW result code to equivalent GIF result code.
 *
 * \param[in]  l_res  LZW response code.
 * \return GIF result code.
 */
static nsgif_result nsgif__error_from_lzw(lzw_result l_res)
{
	static const nsgif_result g_res[] = {
		[LZW_OK]        = NSGIF_OK,
		[LZW_OK_EOD]    = NSGIF_END_OF_FRAME,
		[LZW_NO_MEM]    = NSGIF_INSUFFICIENT_MEMORY,
		[LZW_NO_DATA]   = NSGIF_INSUFFICIENT_DATA,
		[LZW_EOI_CODE]  = NSGIF_FRAME_DATA_ERROR,
		[LZW_BAD_ICODE] = NSGIF_FRAME_DATA_ERROR,
		[LZW_BAD_CODE]  = NSGIF_FRAME_DATA_ERROR,
	};
	assert(l_res != LZW_BAD_PARAM);
	assert(l_res != LZW_NO_COLOUR);
	return g_res[l_res];
}

/**
 * Updates the sprite memory size
 *
 * \param gif The animation context
 * \param width The width of the sprite
 * \param height The height of the sprite
 * \return NSGIF_INSUFFICIENT_MEMORY for a memory error NSGIF_OK for success
 */
static nsgif_result nsgif__initialise_sprite(
		struct nsgif_animation *gif,
		uint32_t width,
		uint32_t height)
{
	/* Already allocated? */
	if (gif->frame_image) {
		return NSGIF_OK;
	}

	assert(gif->bitmap.create);
	gif->frame_image = gif->bitmap.create(width, height);
	if (gif->frame_image == NULL) {
		return NSGIF_INSUFFICIENT_MEMORY;
	}

	return NSGIF_OK;
}

/**
 * Helper to get the rendering bitmap for a gif.
 *
 * \param[in]  gif  The gif object we're decoding.
 * \return Client pixel buffer for rendering into.
 */
static inline uint32_t* nsgif__bitmap_get(
		struct nsgif_animation *gif)
{
	nsgif_result ret;

	/* Make sure we have a buffer to decode to. */
	ret = nsgif__initialise_sprite(gif, gif->width, gif->height);
	if (ret != NSGIF_OK) {
		return NULL;
	}

	/* Get the frame data */
	assert(gif->bitmap.get_buffer);
	return (uint32_t *)gif->bitmap.get_buffer(gif->frame_image);
}

/**
 * Helper to tell the client that their bitmap was modified.
 *
 * \param[in]  gif  The gif object we're decoding.
 */
static inline void nsgif__bitmap_modified(
		const struct nsgif_animation *gif)
{
	if (gif->bitmap.modified) {
		gif->bitmap.modified(gif->frame_image);
	}
}

/**
 * Helper to tell the client that whether the bitmap is opaque.
 *
 * \param[in]  gif    The gif object we're decoding.
 * \param[in]  frame  The frame that has been decoded.
 */
static inline void nsgif__bitmap_set_opaque(
		const struct nsgif_animation *gif,
		const struct nsgif_frame *frame)
{
	if (gif->bitmap.set_opaque) {
		gif->bitmap.set_opaque(
				gif->frame_image, frame->opaque);
	}
}

/**
 * Helper to get the client to determine if the bitmap is opaque.
 *
 * \todo: We don't really need to get the client to do this for us.
 *
 * \param[in]  gif    The gif object we're decoding.
 * \return true if the bitmap is opaque, false otherwise.
 */
static inline bool nsgif__bitmap_get_opaque(
		const struct nsgif_animation *gif)
{
	if (gif->bitmap.test_opaque) {
		return gif->bitmap.test_opaque(
				gif->frame_image);
	}

	return false;
}

static void nsgif__record_frame(
		struct nsgif_animation *gif,
		const uint32_t *bitmap)
{
	bool need_alloc = gif->prev_frame == NULL;
	uint32_t *prev_frame;

	if (gif->decoded_frame == NSGIF_INVALID_FRAME ||
	    gif->decoded_frame == gif->prev_index) {
		/* No frame to copy, or already have this frame recorded. */
		return;
	}

	bitmap = nsgif__bitmap_get(gif);
	if (bitmap == NULL) {
		return;
	}

	if (gif->prev_frame != NULL &&
	    gif->width * gif->height > gif->prev_width * gif->prev_height) {
		need_alloc = true;
	}

	if (need_alloc) {
		prev_frame = realloc(gif->prev_frame,
				gif->width * gif->height * 4);
		if (prev_frame == NULL) {
			return;
		}
	} else {
		prev_frame = gif->prev_frame;
	}

	memcpy(prev_frame, bitmap, gif->width * gif->height * 4);

	gif->prev_frame  = prev_frame;
	gif->prev_width  = gif->width;
	gif->prev_height = gif->height;
	gif->prev_index  = gif->decoded_frame;
}

static nsgif_result nsgif__recover_frame(
		const struct nsgif_animation *gif,
		uint32_t *bitmap)
{
	const uint32_t *prev_frame = gif->prev_frame;
	unsigned height = gif->height < gif->prev_height ? gif->height : gif->prev_height;
	unsigned width  = gif->width  < gif->prev_width  ? gif->width  : gif->prev_width;

	if (prev_frame == NULL) {
		return NSGIF_FRAME_DATA_ERROR;
	}

	for (unsigned y = 0; y < height; y++) {
		memcpy(bitmap, prev_frame, width * 4);

		bitmap += gif->width;
		prev_frame += gif->prev_width;
	}

	return NSGIF_OK;
}

/**
 * Get the next line for GIF decode.
 *
 * Note that the step size must be initialised to 24 at the start of the frame
 * (when y == 0).  This is because of the first two passes of the frame have
 * the same step size of 8, and the step size is used to determine the current
 * pass.
 *
 * \param[in]     height     Frame height in pixels.
 * \param[in,out] y          Current row, starting from 0, updated on exit.
 * \param[in,out] step       Current step starting with 24, updated on exit.
 * \return true if there is a row to process, false at the end of the frame.
 */
static inline bool nsgif__deinterlace(uint32_t height, uint32_t *y, uint8_t *step)
{
	*y += *step & 0xf;

	if (*y < height) return true;

	switch (*step) {
	case 24: *y = 4; *step = 8; if (*y < height) return true;
	         /* Fall through. */
	case  8: *y = 2; *step = 4; if (*y < height) return true;
	         /* Fall through. */
	case  4: *y = 1; *step = 2; if (*y < height) return true;
	         /* Fall through. */
	default:
		break;
	}

	return false;
}

/**
 * Get the next line for GIF decode.
 *
 * \param[in]     interlace  Non-zero if the frame is not interlaced.
 * \param[in]     height     Frame height in pixels.
 * \param[in,out] y          Current row, starting from 0, updated on exit.
 * \param[in,out] step       Current step starting with 24, updated on exit.
 * \return true if there is a row to process, false at the end of the frame.
 */
static inline bool nsgif__next_row(uint32_t interlace,
		uint32_t height, uint32_t *y, uint8_t *step)
{
	if (!interlace) {
		return (++*y != height);
	} else {
		return nsgif__deinterlace(height, y, step);
	}
}

/**
 * Get any frame clip adjustment for the image extent.
 *
 * \param[in]  frame_off  Frame's X or Y offset.
 * \param[in]  frame_dim  Frame width or height.
 * \param[in]  image_ext  Image width or height constraint.
 * \return the amount the frame needs to be clipped to fit the image in given
 *         dimension.
 */
static inline uint32_t gif__clip(
		uint32_t frame_off,
		uint32_t frame_dim,
		uint32_t image_ext)
{
	uint32_t frame_ext = frame_off + frame_dim;

	if (frame_ext <= image_ext) {
		return 0;
	}

	return frame_ext - image_ext;
}

/**
 * Perform any jump over decoded data, to accommodate clipped portion of frame.
 *
 * \param[in,out] skip       Number of pixels of data to jump.
 * \param[in,out] available  Number of pixels of data currently available.
 * \param[in,out] pos        Position in decoded pixel value data.
 */
static inline void gif__jump_data(
		uint32_t *skip,
		uint32_t *available,
		const uint8_t **pos)
{
	uint32_t jump = (*skip < *available) ? *skip : *available;

	*skip -= jump;
	*available -= jump;
	*pos += jump;
}

static nsgif_result nsgif__decode_complex(
		struct nsgif_animation *gif,
		uint32_t width,
		uint32_t height,
		uint32_t offset_x,
		uint32_t offset_y,
		uint32_t interlace,
		const uint8_t *data,
		uint32_t transparency_index,
		uint32_t *restrict frame_data,
		uint32_t *restrict colour_table)
{
	lzw_result res;
	nsgif_result ret = NSGIF_OK;
	uint32_t clip_x = gif__clip(offset_x, width, gif->width);
	uint32_t clip_y = gif__clip(offset_y, height, gif->height);
	const uint8_t *uncompressed;
	uint32_t available = 0;
	uint8_t step = 24;
	uint32_t skip = 0;
	uint32_t y = 0;

	if (offset_x >= gif->width ||
	    offset_y >= gif->height) {
		return NSGIF_OK;
	}

	width -= clip_x;
	height -= clip_y;

	if (width == 0 || height == 0) {
		return NSGIF_OK;
	}

	/* Initialise the LZW decoding */
	res = lzw_decode_init(gif->lzw_ctx, data[0],
			gif->nsgif_data, gif->buffer_size,
			data + 1 - gif->nsgif_data);
	if (res != LZW_OK) {
		return nsgif__error_from_lzw(res);
	}

	do {
		uint32_t x;
		uint32_t *frame_scanline;

		frame_scanline = frame_data + offset_x +
				(y + offset_y) * gif->width;

		x = width;
		while (x > 0) {
			unsigned row_available;
			while (available == 0) {
				if (res != LZW_OK) {
					/* Unexpected end of frame, try to recover */
					if (res == LZW_OK_EOD) {
						ret = NSGIF_OK;
					} else {
						ret = nsgif__error_from_lzw(res);
					}
					return ret;
				}
				res = lzw_decode(gif->lzw_ctx,
						&uncompressed, &available);

				if (available == 0) {
					return NSGIF_OK;
				}
				gif__jump_data(&skip, &available, &uncompressed);
			}

			row_available = x < available ? x : available;
			x -= row_available;
			available -= row_available;
			if (transparency_index > 0xFF) {
				while (row_available-- > 0) {
					*frame_scanline++ =
						colour_table[*uncompressed++];
				}
			} else {
				while (row_available-- > 0) {
					register uint32_t colour;
					colour = *uncompressed++;
					if (colour != transparency_index) {
						*frame_scanline =
							colour_table[colour];
					}
					frame_scanline++;
				}
			}
		}

		skip = clip_x;
		gif__jump_data(&skip, &available, &uncompressed);
	} while (nsgif__next_row(interlace, height, &y, &step));

	return ret;
}

static nsgif_result nsgif__decode_simple(
		struct nsgif_animation *gif,
		uint32_t height,
		uint32_t offset_y,
		const uint8_t *data,
		uint32_t transparency_index,
		uint32_t *restrict frame_data,
		uint32_t *restrict colour_table)
{
	uint32_t pixels = gif->width * height;
	uint32_t written = 0;
	nsgif_result ret = NSGIF_OK;
	lzw_result res;

	if (offset_y >= gif->height) {
		return NSGIF_OK;
	}

	height -= gif__clip(offset_y, height, gif->height);

	if (height == 0) {
		return NSGIF_OK;
	}

	/* Initialise the LZW decoding */
	res = lzw_decode_init_map(gif->lzw_ctx, data[0],
			transparency_index, colour_table,
			gif->nsgif_data, gif->buffer_size,
			data + 1 - gif->nsgif_data);
	if (res != LZW_OK) {
		return nsgif__error_from_lzw(res);
	}

	frame_data += (offset_y * gif->width);

	while (pixels > 0) {
		res = lzw_decode_map(gif->lzw_ctx,
				frame_data, pixels, &written);
		pixels -= written;
		frame_data += written;
		if (res != LZW_OK) {
			/* Unexpected end of frame, try to recover */
			if (res == LZW_OK_EOD) {
				ret = NSGIF_OK;
			} else {
				ret = nsgif__error_from_lzw(res);
			}
			break;
		}
	}

	if (pixels == 0) {
		ret = NSGIF_OK;
	}

	return ret;
}

static inline nsgif_result nsgif__decode(
		struct nsgif_animation *gif,
		struct nsgif_frame *frame,
		const uint8_t *data,
		uint32_t *restrict frame_data)
{
	enum {
		GIF_MASK_INTERLACE = 0x40,
	};

	nsgif_result ret;
	uint32_t width = frame->redraw.w;
	uint32_t height = frame->redraw.h;
	uint32_t offset_x = frame->redraw.x;
	uint32_t offset_y = frame->redraw.y;
	uint32_t interlace = frame->flags & GIF_MASK_INTERLACE;
	uint32_t transparency_index = frame->transparency_index;
	uint32_t *restrict colour_table = gif->colour_table;

	if (interlace == false && width == gif->width && offset_x == 0) {
		ret = nsgif__decode_simple(gif, height, offset_y,
				data, transparency_index,
				frame_data, colour_table);
	} else {
		ret = nsgif__decode_complex(gif, width, height,
				offset_x, offset_y, interlace,
				data, transparency_index,
				frame_data, colour_table);
	}

	return ret;
}

/**
 * Restore a GIF to the background colour.
 *
 * \param[in] gif     The gif object we're decoding.
 * \param[in] frame   The frame to clear, or NULL.
 * \param[in] bitmap  The bitmap to clear the frame in.
 */
static void nsgif__restore_bg(
		struct nsgif_animation *gif,
		struct nsgif_frame *frame,
		uint32_t *bitmap)
{
	if (frame == NULL) {
		memset(bitmap, NSGIF_TRANSPARENT_COLOUR,
				gif->width * gif->height * sizeof(*bitmap));
	} else {
		uint32_t width = frame->redraw.w;
		uint32_t height = frame->redraw.h;
		uint32_t offset_x = frame->redraw.x;
		uint32_t offset_y = frame->redraw.y;

		width -= gif__clip(offset_x, width, gif->width);
		height -= gif__clip(offset_y, height, gif->height);

		if (frame->display == false || width == 0) {
			return;
		}

		if (frame->transparency) {
			for (uint32_t y = 0; y < height; y++) {
				uint32_t *scanline = bitmap + offset_x +
						(offset_y + y) * gif->width;
				memset(scanline, NSGIF_TRANSPARENT_COLOUR,
						width * sizeof(*bitmap));
			}
		} else {
			for (uint32_t y = 0; y < height; y++) {
				uint32_t *scanline = bitmap + offset_x +
						(offset_y + y) * gif->width;
				for (uint32_t x = 0; x < width; x++) {
					scanline[x] = gif->bg_colour;
				}
			}
		}
	}
}

static nsgif_result nsgif__update_bitmap(
		struct nsgif_animation *gif,
		struct nsgif_frame *frame,
		const uint8_t *data,
		uint32_t frame_idx)
{
	nsgif_result ret;
	uint32_t *bitmap;

	gif->decoded_frame = frame_idx;

	bitmap = nsgif__bitmap_get(gif);
	if (bitmap == NULL) {
		return NSGIF_INSUFFICIENT_MEMORY;
	}

	/* Handle any bitmap clearing/restoration required before decoding this
	 * frame. */
	if (frame_idx == 0 || gif->decoded_frame == NSGIF_INVALID_FRAME) {
		nsgif__restore_bg(gif, NULL, bitmap);

	} else {
		struct nsgif_frame *prev = &gif->frames[frame_idx - 1];

		if (prev->disposal_method == NSGIF_DISPOSAL_RESTORE_BG) {
			nsgif__restore_bg(gif, prev, bitmap);

		} else if (prev->disposal_method == NSGIF_DISPOSAL_RESTORE_PREV) {
			ret = nsgif__recover_frame(gif, bitmap);
			if (ret != NSGIF_OK) {
				nsgif__restore_bg(gif, prev, bitmap);
			}
		}
	}

	if (frame->disposal_method == NSGIF_DISPOSAL_RESTORE_PREV) {
		/* Store the previous frame for later restoration */
		nsgif__record_frame(gif, bitmap);
	}

	ret = nsgif__decode(gif, frame, data, bitmap);

	nsgif__bitmap_modified(gif);

	if (!frame->decoded) {
		frame->opaque = nsgif__bitmap_get_opaque(gif);
		frame->decoded = true;
	}
	nsgif__bitmap_set_opaque(gif, frame);

	return ret;
}

/**
 * Parse the graphic control extension
 *
 * \param[in] frame  The gif object we're decoding.
 * \param[in] data   The data to decode.
 * \param[in] len    Byte length of data.
 * \return NSGIF_INSUFFICIENT_DATA if more data is needed,
 *         NSGIF_OK for success.
 */
static nsgif_result nsgif__parse_extension_graphic_control(
		struct nsgif_frame *frame,
		const uint8_t *data,
		size_t len)
{
	enum {
		GIF_MASK_TRANSPARENCY = 0x01,
		GIF_MASK_DISPOSAL     = 0x1c,
	};

	/* 6-byte Graphic Control Extension is:
	 *
	 *  +0  CHAR    Graphic Control Label
	 *  +1  CHAR    Block Size
	 *  +2  CHAR    __Packed Fields__
	 *              3BITS   Reserved
	 *              3BITS   Disposal Method
	 *              1BIT    User Input Flag
	 *              1BIT    Transparent Color Flag
	 *  +3  SHORT   Delay Time
	 *  +5  CHAR    Transparent Color Index
	 */
	if (len < 6) {
		return NSGIF_INSUFFICIENT_DATA;
	}

	frame->frame_delay = data[3] | (data[4] << 8);
	if (data[2] & GIF_MASK_TRANSPARENCY) {
		frame->transparency = true;
		frame->transparency_index = data[5];
	}

	frame->disposal_method = ((data[2] & GIF_MASK_DISPOSAL) >> 2);
	/* I have encountered documentation and GIFs in the
	 * wild that use 0x04 to restore the previous frame,
	 * rather than the officially documented 0x03.  I
	 * believe some (older?)  software may even actually
	 * export this way.  We handle this as a type of
	 * "quirks" mode. */
	if (frame->disposal_method == NSGIF_DISPOSAL_RESTORE_QUIRK) {
		frame->disposal_method = NSGIF_DISPOSAL_RESTORE_PREV;
	}

	/* if we are clearing the background then we need to
	 * redraw enough to cover the previous frame too. */
	frame->redraw_required =
			frame->disposal_method == NSGIF_DISPOSAL_RESTORE_BG ||
			frame->disposal_method == NSGIF_DISPOSAL_RESTORE_PREV;

	return NSGIF_OK;
}

/**
 * Parse the application extension
 *
 * \param[in] gif   The gif object we're decoding.
 * \param[in] data  The data to decode.
 * \param[in] len   Byte length of data.
 * \return NSGIF_INSUFFICIENT_DATA if more data is needed,
 *         NSGIF_OK for success.
 */
static nsgif_result nsgif__parse_extension_application(
		struct nsgif_animation *gif,
		const uint8_t *data,
		size_t len)
{
	/* 14-byte+ Application Extension is:
	 *
	 *  +0    CHAR    Application Extension Label
	 *  +1    CHAR    Block Size
	 *  +2    8CHARS  Application Identifier
	 *  +10   3CHARS  Appl. Authentication Code
	 *  +13   1-256   Application Data (Data sub-blocks)
	 */
	if (len < 17) {
		return NSGIF_INSUFFICIENT_DATA;
	}

	if ((data[1] == 0x0b) &&
	    (strncmp((const char *)data + 2, "NETSCAPE2.0", 11) == 0) &&
	    (data[13] == 0x03) && (data[14] == 0x01)) {
		gif->loop_count = data[15] | (data[16] << 8);
	}

	return NSGIF_OK;
}

/**
 * Parse the frame's extensions
 *
 * \param[in] gif     The gif object we're decoding.
 * \param[in] frame   The frame to parse extensions for.
 * \param[in] pos     Current position in data, updated on exit.
 * \param[in] decode  Whether to decode or skip over the extension.
 * \return NSGIF_INSUFFICIENT_DATA if more data is needed,
 *         NSGIF_OK for success.
 */
static nsgif_result nsgif__parse_frame_extensions(
		struct nsgif_animation *gif,
		struct nsgif_frame *frame,
		const uint8_t **pos,
		bool decode)
{
	enum {
		GIF_EXT_INTRODUCER      = 0x21,
		GIF_EXT_GRAPHIC_CONTROL = 0xf9,
		GIF_EXT_COMMENT         = 0xfe,
		GIF_EXT_PLAIN_TEXT      = 0x01,
		GIF_EXT_APPLICATION     = 0xff,
	};
	const uint8_t *nsgif_data = *pos;
	const uint8_t *nsgif_end = gif->nsgif_data + gif->buffer_size;
	int nsgif_bytes = nsgif_end - nsgif_data;

	/* Initialise the extensions */
	while (nsgif_bytes > 0 && nsgif_data[0] == GIF_EXT_INTRODUCER) {
		bool block_step = true;
		nsgif_result ret;

		nsgif_data++;
		nsgif_bytes--;

		if (nsgif_bytes == 0) {
			return NSGIF_INSUFFICIENT_DATA;
		}

		/* Switch on extension label */
		switch (nsgif_data[0]) {
		case GIF_EXT_GRAPHIC_CONTROL:
			if (decode) {
				ret = nsgif__parse_extension_graphic_control(
						frame, nsgif_data, nsgif_bytes);
				if (ret != NSGIF_OK) {
					return ret;
				}
			}
			break;

		case GIF_EXT_APPLICATION:
			if (decode) {
				ret = nsgif__parse_extension_application(
						gif, nsgif_data, nsgif_bytes);
				if (ret != NSGIF_OK) {
					return ret;
				}
			}
			break;

		case GIF_EXT_COMMENT:
			/* Move the pointer to the first data sub-block Skip 1
			 * byte for the extension label. */
			++nsgif_data;
			block_step = false;
			break;

		default:
			break;
		}

		if (block_step) {
			/* Move the pointer to the first data sub-block Skip 2
			 * bytes for the extension label and size fields Skip
			 * the extension size itself
			 */
			if (nsgif_bytes < 2) {
				return NSGIF_INSUFFICIENT_DATA;
			}
			nsgif_data += 2 + nsgif_data[1];
		}

		/* Repeatedly skip blocks until we get a zero block or run out
		 * of data.  This data is ignored by this gif decoder. */
		while (nsgif_data < nsgif_end && nsgif_data[0] != NSGIF_BLOCK_TERMINATOR) {
			nsgif_data += nsgif_data[0] + 1;
			if (nsgif_data >= nsgif_end) {
				return NSGIF_INSUFFICIENT_DATA;
			}
		}
		nsgif_data++;
		nsgif_bytes = nsgif_end - nsgif_data;
	}

	if (nsgif_data > nsgif_end) {
		nsgif_data = nsgif_end;
	}

	/* Set buffer position and return */
	*pos = nsgif_data;
	return NSGIF_OK;
}

/**
 * Parse a GIF Image Descriptor.
 *
 * The format is:
 *
 *  +0   CHAR   Image Separator (0x2c)
 *  +1   SHORT  Image Left Position
 *  +3   SHORT  Image Top Position
 *  +5   SHORT  Width
 *  +7   SHORT  Height
 *  +9   CHAR   __Packed Fields__
 *              1BIT    Local Colour Table Flag
 *              1BIT    Interlace Flag
 *              1BIT    Sort Flag
 *              2BITS   Reserved
 *              3BITS   Size of Local Colour Table
 *
 * \param[in] gif     The gif object we're decoding.
 * \param[in] frame   The frame to parse an image descriptor for.
 * \param[in] pos     Current position in data, updated on exit.
 * \param[in] decode  Whether to decode the image descriptor.
 * \return NSGIF_OK on success, appropriate error otherwise.
 */
static nsgif_result nsgif__parse_image_descriptor(
		struct nsgif_animation *gif,
		struct nsgif_frame *frame,
		const uint8_t **pos,
		bool decode)
{
	const uint8_t *data = *pos;
	size_t len = gif->nsgif_data + gif->buffer_size - data;
	enum {
		NSGIF_IMAGE_DESCRIPTOR_LEN = 10u,
		NSGIF_IMAGE_SEPARATOR      = 0x2Cu,
	};

	assert(gif != NULL);
	assert(frame != NULL);

	if (len < NSGIF_IMAGE_DESCRIPTOR_LEN) {
		return NSGIF_INSUFFICIENT_DATA;
	}

	if (decode) {
		unsigned x, y, w, h;

		if (data[0] != NSGIF_IMAGE_SEPARATOR) {
			return NSGIF_FRAME_DATA_ERROR;
		}

		x = data[1] | (data[2] << 8);
		y = data[3] | (data[4] << 8);
		w = data[5] | (data[6] << 8);
		h = data[7] | (data[8] << 8);
		frame->flags = data[9];

		frame->redraw.x = x;
		frame->redraw.y = y;
		frame->redraw.w = w;
		frame->redraw.h = h;

		/* Allow first frame to grow image dimensions. */
		if (gif->frame_count == 0) {
			if (x + w > gif->width) {
				gif->width = x + w;
			}
			if (y + h > gif->height) {
				gif->height = y + h;
			}
		}
	}

	*pos += NSGIF_IMAGE_DESCRIPTOR_LEN;
	return NSGIF_OK;
}

/**
 * Extract a GIF colour table into a LibNSGIF colour table buffer.
 *
 * \param[in] gif                   The gif object we're decoding.
 * \param[in] colour_table          The colour table to populate.
 * \param[in] colour_table_entries  The number of colour table entries.
 * \param[in] pos                   Current position in data, updated on exit.
 * \param[in] decode                Whether to decode the colour table.
 * \return NSGIF_OK on success, appropriate error otherwise.
 */
static nsgif_result nsgif__colour_table_extract(
		struct nsgif_animation *gif,
		uint32_t *colour_table,
		size_t colour_table_entries,
		const uint8_t **pos,
		bool decode)
{
	const uint8_t *data = *pos;
	size_t len = gif->nsgif_data + gif->buffer_size - data;

	if (len < colour_table_entries * 3) {
		return NSGIF_INSUFFICIENT_DATA;
	}

	if (decode) {
		int count = colour_table_entries;
		uint8_t *entry = (uint8_t *)colour_table;

		while (count--) {
			/* Gif colour map contents are r,g,b.
			 *
			 * We want to pack them bytewise into the
			 * colour table, such that the red component
			 * is in byte 0 and the alpha component is in
			 * byte 3.
			 */

			*entry++ = *data++; /* r */
			*entry++ = *data++; /* g */
			*entry++ = *data++; /* b */
			*entry++ = 0xff;    /* a */
		}
	}

	*pos += colour_table_entries * 3;
	return NSGIF_OK;
}

/**
 * Get a frame's colour table.
 *
 * Sets up gif->colour_table for the frame.
 *
 * \param[in] gif     The gif object we're decoding.
 * \param[in] frame   The frame to get the colour table for.
 * \param[in] pos     Current position in data, updated on exit.
 * \param[in] decode  Whether to decode the colour table.
 * \return NSGIF_OK on success, appropriate error otherwise.
 */
static nsgif_result nsgif__parse_colour_table(
		struct nsgif_animation *gif,
		struct nsgif_frame *frame,
		const uint8_t **pos,
		bool decode)
{
	nsgif_result ret;

	assert(gif != NULL);
	assert(frame != NULL);

	if ((frame->flags & NSGIF_COLOUR_TABLE_MASK) == 0) {
		gif->colour_table = gif->global_colour_table;
		return NSGIF_OK;
	}

	ret = nsgif__colour_table_extract(gif, gif->local_colour_table,
			2 << (frame->flags & NSGIF_COLOUR_TABLE_SIZE_MASK),
			pos, decode);
	if (ret != NSGIF_OK) {
		return ret;
	}

	gif->colour_table = gif->local_colour_table;
	return NSGIF_OK;
}

/**
 * Parse the image data for a gif frame.
 *
 * Sets up gif->colour_table for the frame.
 *
 * \param[in] gif     The gif object we're decoding.
 * \param[in] frame   The frame to parse image data for.
 * \param[in] pos     Current position in data, updated on exit.
 * \param[in] decode  Whether to decode the image data.
 * \return NSGIF_OK on success, appropriate error otherwise.
 */
static nsgif_result nsgif__parse_image_data(
		struct nsgif_animation *gif,
		struct nsgif_frame *frame,
		const uint8_t **pos,
		bool decode)
{
	const uint8_t *data = *pos;
	size_t len = gif->nsgif_data + gif->buffer_size - data;
	uint32_t frame_idx = frame - gif->frames;
	uint8_t minimum_code_size;
	nsgif_result ret;

	assert(gif != NULL);
	assert(frame != NULL);

	if (!decode) {
		gif->frame_count_partial = frame_idx + 1;
	}

	/* Ensure sufficient data remains.  A gif trailer or a minimum lzw code
	 * followed by a gif trailer is treated as OK, although without any
	 * image data. */
	switch (len) {
		default: if (data[0] == NSGIF_TRAILER) return NSGIF_OK;
			break;
		case 2: if (data[1] == NSGIF_TRAILER) return NSGIF_OK;
			/* Fall through. */
		case 1: if (data[0] == NSGIF_TRAILER) return NSGIF_OK;
			/* Fall through. */
		case 0: return NSGIF_INSUFFICIENT_DATA;
	}

	minimum_code_size = data[0];
	if (minimum_code_size >= LZW_CODE_MAX) {
		return NSGIF_DATA_ERROR;
	}

	if (decode) {
		ret = nsgif__update_bitmap(gif, frame, data, frame_idx);
	} else {
		uint32_t block_size = 0;

		/* Skip the minimum code size. */
		data++;
		len--;

		while (block_size != 1) {
			if (len < 1) return NSGIF_INSUFFICIENT_DATA;
			block_size = data[0] + 1;
			/* Check if the frame data runs off the end of the file	*/
			if (block_size > len) {
				block_size = len;
				return NSGIF_OK;
			}

			len -= block_size;
			data += block_size;
		}

		gif->frame_count = frame_idx + 1;
		gif->frames[frame_idx].display = true;
		*pos = data;

		/* Check if we've finished */
		if (len < 1) {
			return NSGIF_INSUFFICIENT_DATA;
		} else {
			if (data[0] == NSGIF_TRAILER) {
				return NSGIF_OK;
			}
		}

		return NSGIF_WORKING;
	}

	return ret;
}

static struct nsgif_frame *nsgif__get_frame(
		struct nsgif_animation *gif,
		uint32_t frame_idx)
{
	struct nsgif_frame *frame;

	if (gif->frame_holders > frame_idx) {
		frame = &gif->frames[frame_idx];
	} else {
		/* Allocate more memory */
		size_t count = frame_idx + 1;
		struct nsgif_frame *temp;

		temp = realloc(gif->frames, count * sizeof(*frame));
		if (temp == NULL) {
			return NULL;
		}
		gif->frames = temp;
		gif->frame_holders = count;

		frame = &gif->frames[frame_idx];

		frame->transparency = false;
		frame->transparency_index = NSGIF_NO_TRANSPARENCY;
		frame->frame_pointer = gif->buffer_position;
		frame->redraw_required = false;
		frame->disposal_method = 0;
		frame->frame_delay = 100;
		frame->display = false;
		frame->decoded = false;
	}

	return frame;
}

/**
 * Attempts to initialise the next frame
 *
 * \param[in] gif       The animation context
 * \param[in] frame_idx The frame number to decode.
 * \param[in] decode    Whether to decode the graphical image data.
 * \return error code
 *         - NSGIF_INSUFFICIENT_DATA reached unexpected end of source data.
 *         - NSGIF_FRAME_DATA_ERROR for GIF frame data error
 *         - NSGIF_INSUFFICIENT_MEMORY for insufficient memory to process
 *         - NSGIF_DATA_ERROR for GIF error (invalid frame header)
 *         - NSGIF_OK for successful decoding
 *         - NSGIF_WORKING for successful decoding if more frames are expected
*/
static nsgif_result nsgif__process_frame(
		struct nsgif_animation *gif,
		uint32_t frame_idx,
		bool decode)
{
	nsgif_result ret;
	const uint8_t *pos;
	const uint8_t *end;
	struct nsgif_frame *frame;

	frame = nsgif__get_frame(gif, frame_idx);
	if (frame == NULL) {
		return NSGIF_INSUFFICIENT_MEMORY;
	}

	end = gif->nsgif_data + gif->buffer_size;

	if (decode) {
		pos = gif->nsgif_data + frame->frame_pointer;

		/* Ensure this frame is supposed to be decoded */
		if (frame->display == false) {
			return NSGIF_OK;
		}

		/* Ensure the frame is in range to decode */
		if (frame_idx > gif->frame_count_partial) {
			return NSGIF_INSUFFICIENT_DATA;
		}

		/* Done if frame is already decoded */
		if ((int)frame_idx == gif->decoded_frame) {
			return NSGIF_OK;
		}
	} else {
		pos = (uint8_t *)(gif->nsgif_data + gif->buffer_position);

		/* Check if we've finished */
		if (pos < end && pos[0] == NSGIF_TRAILER) {
			return NSGIF_OK;
		}

		/* We could theoretically get some junk data that gives us
		 * millions of frames, so we ensure that we don't have a
		 * silly number. */
		if (frame_idx > 4096) {
			return NSGIF_FRAME_DATA_ERROR;
		}
	}

	ret = nsgif__parse_frame_extensions(gif, frame, &pos, !decode);
	if (ret != NSGIF_OK) {
		goto cleanup;
	}

	ret = nsgif__parse_image_descriptor(gif, frame, &pos, !decode);
	if (ret != NSGIF_OK) {
		goto cleanup;
	}

	ret = nsgif__parse_colour_table(gif, frame, &pos, decode);
	if (ret != NSGIF_OK) {
		goto cleanup;
	}

	ret = nsgif__parse_image_data(gif, frame, &pos, decode);
	if (ret != NSGIF_OK) {
		goto cleanup;
	}

cleanup:
	if (!decode) {
		gif->buffer_position = pos - gif->nsgif_data;
	}

	return ret;
}

/* exported function documented in libnsgif.h */
void nsgif_create(nsgif_animation *gif, nsgif_bitmap_cb_vt *bitmap)
{
	memset(gif, 0, sizeof(nsgif_animation));
	gif->bitmap = *bitmap;
	gif->decoded_frame = NSGIF_INVALID_FRAME;
	gif->prev_index = NSGIF_INVALID_FRAME;
}

/**
 * Read GIF header.
 *
 * 6-byte GIF file header is:
 *
 *  +0   3CHARS   Signature ('GIF')
 *  +3   3CHARS   Version ('87a' or '89a')
 *
 * \param[in]      gif     The GIF object we're decoding.
 * \param[in,out]  pos     The current buffer position, updated on success.
 * \param[in]      strict  Whether to require a known GIF version.
 * \return NSGIF_OK on success, appropriate error otherwise.
 */
static nsgif_result nsgif__parse_header(
		struct nsgif_animation *gif,
		const uint8_t **pos,
		bool strict)
{
	const uint8_t *data = *pos;
	size_t len = gif->nsgif_data + gif->buffer_size - data;

	if (len < 6) {
		return NSGIF_INSUFFICIENT_DATA;
	}

	if (strncmp((const char *) data, "GIF", 3) != 0) {
		return NSGIF_DATA_ERROR;
	}
	data += 3;

	if (strict == true) {
		if ((strncmp((const char *) data, "87a", 3) != 0) &&
		    (strncmp((const char *) data, "89a", 3) != 0)) {
			return NSGIF_DATA_ERROR;
		}
	}
	data += 3;

	*pos = data;
	return NSGIF_OK;
}

/**
 * Read Logical Screen Descriptor.
 *
 * 7-byte Logical Screen Descriptor is:
 *
 *  +0   SHORT   Logical Screen Width
 *  +2   SHORT   Logical Screen Height
 *  +4   CHAR    __Packed Fields__
 *               1BIT    Global Colour Table Flag
 *               3BITS   Colour Resolution
 *               1BIT    Sort Flag
 *               3BITS   Size of Global Colour Table
 *  +5   CHAR    Background Colour Index
 *  +6   CHAR    Pixel Aspect Ratio
 *
 * \param[in]      gif     The GIF object we're decoding.
 * \param[in,out]  pos     The current buffer position, updated on success.
 * \return NSGIF_OK on success, appropriate error otherwise.
 */
static nsgif_result nsgif__parse_logical_screen_descriptor(
		struct nsgif_animation *gif,
		const uint8_t **pos)
{
	const uint8_t *data = *pos;
	size_t len = gif->nsgif_data + gif->buffer_size - data;

	if (len < 7) {
		return NSGIF_INSUFFICIENT_DATA;
	}

	gif->width = data[0] | (data[1] << 8);
	gif->height = data[2] | (data[3] << 8);
	gif->global_colours = data[4] & NSGIF_COLOUR_TABLE_MASK;
	gif->colour_table_size = 2 << (data[4] & NSGIF_COLOUR_TABLE_SIZE_MASK);
	gif->bg_index = data[5];
	gif->aspect_ratio = data[6];
	gif->loop_count = 1;

	*pos += 7;
	return NSGIF_OK;
}

/* exported function documented in libnsgif.h */
nsgif_result nsgif_initialise(nsgif_animation *gif, size_t size, const uint8_t *data)
{
	const uint8_t *nsgif_data;
	nsgif_result ret;

	/* Initialize values */
	gif->buffer_size = size;
	gif->nsgif_data = data;

	/* Get our current processing position */
	nsgif_data = gif->nsgif_data + gif->buffer_position;

	/* See if we should initialise the GIF */
	if (gif->buffer_position == 0) {
		/* We want everything to be NULL before we start so we've no
		 * chance of freeing bad pointers (paranoia)
		 */
		gif->frame_image = NULL;
		gif->frames = NULL;
		gif->frame_holders = 0;
		gif->local_colour_table = NULL;
		gif->global_colour_table = NULL;

		/* The caller may have been lazy and not reset any values */
		gif->frame_count = 0;
		gif->frame_count_partial = 0;
		gif->decoded_frame = NSGIF_INVALID_FRAME;

		ret = nsgif__parse_header(gif, &nsgif_data, false);
		if (ret != NSGIF_OK) {
			return ret;
		}

		ret = nsgif__parse_logical_screen_descriptor(gif, &nsgif_data);
		if (ret != NSGIF_OK) {
			return ret;
		}

		/* Remember we've done this now */
		gif->buffer_position = nsgif_data - gif->nsgif_data;

		/* Some broken GIFs report the size as the screen size they
		 * were created in. As such, we detect for the common cases and
		 * set the sizes as 0 if they are found which results in the
		 * GIF being the maximum size of the frames.
		 */
		if (((gif->width == 640) && (gif->height == 480)) ||
		    ((gif->width == 640) && (gif->height == 512)) ||
		    ((gif->width == 800) && (gif->height == 600)) ||
		    ((gif->width == 1024) && (gif->height == 768)) ||
		    ((gif->width == 1280) && (gif->height == 1024)) ||
		    ((gif->width == 1600) && (gif->height == 1200)) ||
		    ((gif->width == 0) || (gif->height == 0)) ||
		    ((gif->width > 2048) || (gif->height > 2048))) {
			gif->width = 1;
			gif->height = 1;
		}

		/* Allocate some data irrespective of whether we've got any
		 * colour tables. We always get the maximum size in case a GIF
		 * is lying to us. It's far better to give the wrong colours
		 * than to trample over some memory somewhere.
		*/
		gif->global_colour_table = calloc(NSGIF_MAX_COLOURS, sizeof(uint32_t));
		gif->local_colour_table = calloc(NSGIF_MAX_COLOURS, sizeof(uint32_t));
		if ((gif->global_colour_table == NULL) ||
		    (gif->local_colour_table == NULL)) {
			nsgif_finalise(gif);
			return NSGIF_INSUFFICIENT_MEMORY;
		}

		/* Set the first colour to a value that will never occur in
		 * reality so we know if we've processed it
		*/
		gif->global_colour_table[0] = NSGIF_PROCESS_COLOURS;

		/* Check if the GIF has no frame data (13-byte header + 1-byte
		 * termination block) Although generally useless, the GIF
		 * specification does not expressly prohibit this
		 */
		if (gif->buffer_size == gif->buffer_position + 1) {
			if (nsgif_data[0] == NSGIF_TRAILER) {
				return NSGIF_OK;
			}
		}
	}

	/*  Do the colour map if we haven't already. As the top byte is always
	 *  0xff or 0x00 depending on the transparency we know if it's been
	 *  filled in.
	 */
	if (gif->global_colour_table[0] == NSGIF_PROCESS_COLOURS) {
		/* Check for a global colour map signified by bit 7 */
		if (gif->global_colours) {
			ret = nsgif__colour_table_extract(gif,
					gif->global_colour_table,
					gif->colour_table_size,
					&nsgif_data, true);
			if (ret != NSGIF_OK) {
				return ret;
			}

			gif->buffer_position = (nsgif_data - gif->nsgif_data);
		} else {
			/* Create a default colour table with the first two
			 * colours as black and white
			 */
			uint32_t *entry = gif->global_colour_table;

			entry[0] = 0x00000000;
			/* Force Alpha channel to opaque */
			((uint8_t *) entry)[3] = 0xff;

			entry[1] = 0xffffffff;
		}

		if (gif->global_colours &&
		    gif->bg_index < gif->colour_table_size) {
			size_t bg_idx = gif->bg_index;
			gif->bg_colour = gif->global_colour_table[bg_idx];
		} else {
			gif->bg_colour = gif->global_colour_table[0];
		}
	}

	if (gif->lzw_ctx == NULL) {
		lzw_result res = lzw_context_create(
				(struct lzw_ctx **)&gif->lzw_ctx);
		if (res != LZW_OK) {
			return nsgif__error_from_lzw(res);
		}
	}

	/* Repeatedly try to initialise frames */
	do {
		ret = nsgif__process_frame(gif, gif->frame_count, false);
	} while (ret == NSGIF_WORKING);

	return ret;
}

/* exported function documented in libnsgif.h */
nsgif_result nsgif_decode_frame(nsgif_animation *gif, uint32_t frame)
{
	return nsgif__process_frame(gif, frame, true);
}

/* exported function documented in libnsgif.h */
void nsgif_finalise(nsgif_animation *gif)
{
	/* Release all our memory blocks */
	if (gif->frame_image) {
		assert(gif->bitmap.destroy);
		gif->bitmap.destroy(gif->frame_image);
	}

	gif->frame_image = NULL;
	free(gif->frames);
	gif->frames = NULL;
	free(gif->local_colour_table);
	gif->local_colour_table = NULL;
	free(gif->global_colour_table);
	gif->global_colour_table = NULL;

	free(gif->prev_frame);
	gif->prev_frame = NULL;

	lzw_context_destroy(gif->lzw_ctx);
	gif->lzw_ctx = NULL;
}