4
* Copyright (C) 1991-1997, Thomas G. Lane.
5
* This file is part of the Independent JPEG Group's software.
6
* For conditions of distribution and use, see the accompanying README file.
8
* This file contains Huffman entropy encoding routines.
10
* Much of the complexity here has to do with supporting output suspension.
11
* If the data destination module demands suspension, we want to be able to
12
* back up to the start of the current MCU. To do this, we copy state
13
* variables into local working storage, and update them back to the
14
* permanent JPEG objects only upon successful completion of an MCU.
17
#define JPEG_INTERNALS
20
#include "jchuff.h" /* Declarations shared with jcphuff.c */
23
/* Expanded entropy encoder object for Huffman encoding.
25
* The savable_state subrecord contains fields that change within an MCU,
26
* but must not be updated permanently until we complete the MCU.
30
INT32 put_buffer; /* current bit-accumulation buffer */
31
int put_bits; /* # of bits now in it */
32
int last_dc_val[MAX_COMPS_IN_SCAN]; /* last DC coef for each component */
35
/* This macro is to work around compilers with missing or broken
36
* structure assignment. You'll need to fix this code if you have
37
* such a compiler and you change MAX_COMPS_IN_SCAN.
40
#ifndef NO_STRUCT_ASSIGN
41
#define ASSIGN_STATE(dest,src) ((dest) = (src))
43
#if MAX_COMPS_IN_SCAN == 4
44
#define ASSIGN_STATE(dest,src) \
45
((dest).put_buffer = (src).put_buffer, \
46
(dest).put_bits = (src).put_bits, \
47
(dest).last_dc_val[0] = (src).last_dc_val[0], \
48
(dest).last_dc_val[1] = (src).last_dc_val[1], \
49
(dest).last_dc_val[2] = (src).last_dc_val[2], \
50
(dest).last_dc_val[3] = (src).last_dc_val[3])
56
struct jpeg_entropy_encoder pub; /* public fields */
58
savable_state saved; /* Bit buffer & DC state at start of MCU */
60
/* These fields are NOT loaded into local working state. */
61
unsigned int restarts_to_go; /* MCUs left in this restart interval */
62
int next_restart_num; /* next restart number to write (0-7) */
64
/* Pointers to derived tables (these workspaces have image lifespan) */
65
c_derived_tbl * dc_derived_tbls[NUM_HUFF_TBLS];
66
c_derived_tbl * ac_derived_tbls[NUM_HUFF_TBLS];
68
#ifdef ENTROPY_OPT_SUPPORTED /* Statistics tables for optimization */
69
long * dc_count_ptrs[NUM_HUFF_TBLS];
70
long * ac_count_ptrs[NUM_HUFF_TBLS];
72
} huff_entropy_encoder;
74
typedef huff_entropy_encoder * huff_entropy_ptr;
76
/* Working state while writing an MCU.
77
* This struct contains all the fields that are needed by subroutines.
81
JOCTET * next_output_byte; /* => next byte to write in buffer */
82
size_t free_in_buffer; /* # of byte spaces remaining in buffer */
83
savable_state cur; /* Current bit buffer & DC state */
84
j_compress_ptr cinfo; /* dump_buffer needs access to this */
88
/* Forward declarations */
89
METHODDEF(boolean) encode_mcu_huff JPP((j_compress_ptr cinfo,
90
JBLOCKROW *MCU_data));
91
METHODDEF(void) finish_pass_huff JPP((j_compress_ptr cinfo));
92
#ifdef ENTROPY_OPT_SUPPORTED
93
METHODDEF(boolean) encode_mcu_gather JPP((j_compress_ptr cinfo,
94
JBLOCKROW *MCU_data));
95
METHODDEF(void) finish_pass_gather JPP((j_compress_ptr cinfo));
100
* Initialize for a Huffman-compressed scan.
101
* If gather_statistics is TRUE, we do not output anything during the scan,
102
* just count the Huffman symbols used and generate Huffman code tables.
106
start_pass_huff (j_compress_ptr cinfo, boolean gather_statistics)
108
huff_entropy_ptr entropy = (huff_entropy_ptr) cinfo->entropy;
109
int ci, dctbl, actbl;
110
jpeg_component_info * compptr;
112
if (gather_statistics) {
113
#ifdef ENTROPY_OPT_SUPPORTED
114
entropy->pub.encode_mcu = encode_mcu_gather;
115
entropy->pub.finish_pass = finish_pass_gather;
117
ERREXIT(cinfo, JERR_NOT_COMPILED);
120
entropy->pub.encode_mcu = encode_mcu_huff;
121
entropy->pub.finish_pass = finish_pass_huff;
124
for (ci = 0; ci < cinfo->comps_in_scan; ci++) {
125
compptr = cinfo->cur_comp_info[ci];
126
dctbl = compptr->dc_tbl_no;
127
actbl = compptr->ac_tbl_no;
128
if (gather_statistics) {
129
#ifdef ENTROPY_OPT_SUPPORTED
130
/* Check for invalid table indexes */
131
/* (make_c_derived_tbl does this in the other path) */
132
if (dctbl < 0 || dctbl >= NUM_HUFF_TBLS)
133
ERREXIT1(cinfo, JERR_NO_HUFF_TABLE, dctbl);
134
if (actbl < 0 || actbl >= NUM_HUFF_TBLS)
135
ERREXIT1(cinfo, JERR_NO_HUFF_TABLE, actbl);
136
/* Allocate and zero the statistics tables */
137
/* Note that jpeg_gen_optimal_table expects 257 entries in each table! */
138
if (entropy->dc_count_ptrs[dctbl] == NULL)
139
entropy->dc_count_ptrs[dctbl] = (long *)
140
(*cinfo->mem->alloc_small) ((j_common_ptr) cinfo, JPOOL_IMAGE,
142
MEMZERO(entropy->dc_count_ptrs[dctbl], 257 * SIZEOF(long));
143
if (entropy->ac_count_ptrs[actbl] == NULL)
144
entropy->ac_count_ptrs[actbl] = (long *)
145
(*cinfo->mem->alloc_small) ((j_common_ptr) cinfo, JPOOL_IMAGE,
147
MEMZERO(entropy->ac_count_ptrs[actbl], 257 * SIZEOF(long));
150
/* Compute derived values for Huffman tables */
151
/* We may do this more than once for a table, but it's not expensive */
152
jpeg_make_c_derived_tbl(cinfo, TRUE, dctbl,
153
& entropy->dc_derived_tbls[dctbl]);
154
jpeg_make_c_derived_tbl(cinfo, FALSE, actbl,
155
& entropy->ac_derived_tbls[actbl]);
157
/* Initialize DC predictions to 0 */
158
entropy->saved.last_dc_val[ci] = 0;
161
/* Initialize bit buffer to empty */
162
entropy->saved.put_buffer = 0;
163
entropy->saved.put_bits = 0;
165
/* Initialize restart stuff */
166
entropy->restarts_to_go = cinfo->restart_interval;
167
entropy->next_restart_num = 0;
172
* Compute the derived values for a Huffman table.
173
* This routine also performs some validation checks on the table.
175
* Note this is also used by jcphuff.c.
179
jpeg_make_c_derived_tbl (j_compress_ptr cinfo, boolean isDC, int tblno,
180
c_derived_tbl ** pdtbl)
184
int p, i, l, lastp, si, maxsymbol;
186
unsigned int huffcode[257];
189
/* Note that huffsize[] and huffcode[] are filled in code-length order,
190
* paralleling the order of the symbols themselves in htbl->huffval[].
193
/* Find the input Huffman table */
194
if (tblno < 0 || tblno >= NUM_HUFF_TBLS)
195
ERREXIT1(cinfo, JERR_NO_HUFF_TABLE, tblno);
197
isDC ? cinfo->dc_huff_tbl_ptrs[tblno] : cinfo->ac_huff_tbl_ptrs[tblno];
199
ERREXIT1(cinfo, JERR_NO_HUFF_TABLE, tblno);
201
/* Allocate a workspace if we haven't already done so. */
203
*pdtbl = (c_derived_tbl *)
204
(*cinfo->mem->alloc_small) ((j_common_ptr) cinfo, JPOOL_IMAGE,
205
SIZEOF(c_derived_tbl));
208
/* Figure C.1: make table of Huffman code length for each symbol */
211
for (l = 1; l <= 16; l++) {
212
i = (int) htbl->bits[l];
213
if (i < 0 || p + i > 256) /* protect against table overrun */
214
ERREXIT(cinfo, JERR_BAD_HUFF_TABLE);
216
huffsize[p++] = (char) l;
221
/* Figure C.2: generate the codes themselves */
222
/* We also validate that the counts represent a legal Huffman code tree. */
227
while (huffsize[p]) {
228
while (((int) huffsize[p]) == si) {
229
huffcode[p++] = code;
232
/* code is now 1 more than the last code used for codelength si; but
233
* it must still fit in si bits, since no code is allowed to be all ones.
235
if (((INT32) code) >= (((INT32) 1) << si))
236
ERREXIT(cinfo, JERR_BAD_HUFF_TABLE);
241
/* Figure C.3: generate encoding tables */
242
/* These are code and size indexed by symbol value */
244
/* Set all codeless symbols to have code length 0;
245
* this lets us detect duplicate VAL entries here, and later
246
* allows emit_bits to detect any attempt to emit such symbols.
248
MEMZERO(dtbl->ehufsi, SIZEOF(dtbl->ehufsi));
250
/* This is also a convenient place to check for out-of-range
251
* and duplicated VAL entries. We allow 0..255 for AC symbols
252
* but only 0..15 for DC. (We could constrain them further
253
* based on data depth and mode, but this seems enough.)
255
maxsymbol = isDC ? 15 : 255;
257
for (p = 0; p < lastp; p++) {
258
i = htbl->huffval[p];
259
if (i < 0 || i > maxsymbol || dtbl->ehufsi[i])
260
ERREXIT(cinfo, JERR_BAD_HUFF_TABLE);
261
dtbl->ehufco[i] = huffcode[p];
262
dtbl->ehufsi[i] = huffsize[p];
267
/* Outputting bytes to the file */
269
/* Emit a byte, taking 'action' if must suspend. */
270
#define emit_byte(state,val,action) \
271
{ *(state)->next_output_byte++ = (JOCTET) (val); \
272
if (--(state)->free_in_buffer == 0) \
273
if (! dump_buffer(state)) \
278
dump_buffer (working_state * state)
279
/* Empty the output buffer; return TRUE if successful, FALSE if must suspend */
281
struct jpeg_destination_mgr * dest = state->cinfo->dest;
283
if (! (*dest->empty_output_buffer) (state->cinfo))
285
/* After a successful buffer dump, must reset buffer pointers */
286
state->next_output_byte = dest->next_output_byte;
287
state->free_in_buffer = dest->free_in_buffer;
292
/* Outputting bits to the file */
294
/* Only the right 24 bits of put_buffer are used; the valid bits are
295
* left-justified in this part. At most 16 bits can be passed to emit_bits
296
* in one call, and we never retain more than 7 bits in put_buffer
297
* between calls, so 24 bits are sufficient.
302
emit_bits (working_state * state, unsigned int code, int size)
303
/* Emit some bits; return TRUE if successful, FALSE if must suspend */
305
/* This routine is heavily used, so it's worth coding tightly. */
306
register INT32 put_buffer = (INT32) code;
307
register int put_bits = state->cur.put_bits;
309
/* if size is 0, caller used an invalid Huffman table entry */
311
ERREXIT(state->cinfo, JERR_HUFF_MISSING_CODE);
313
put_buffer &= (((INT32) 1)<<size) - 1; /* mask off any extra bits in code */
315
put_bits += size; /* new number of bits in buffer */
317
put_buffer <<= 24 - put_bits; /* align incoming bits */
319
put_buffer |= state->cur.put_buffer; /* and merge with old buffer contents */
321
while (put_bits >= 8) {
322
int c = (int) ((put_buffer >> 16) & 0xFF);
324
emit_byte(state, c, return FALSE);
325
if (c == 0xFF) { /* need to stuff a zero byte? */
326
emit_byte(state, 0, return FALSE);
332
state->cur.put_buffer = put_buffer; /* update state variables */
333
state->cur.put_bits = put_bits;
340
flush_bits (working_state * state)
342
if (! emit_bits(state, 0x7F, 7)) /* fill any partial byte with ones */
344
state->cur.put_buffer = 0; /* and reset bit-buffer to empty */
345
state->cur.put_bits = 0;
350
/* Encode a single block's worth of coefficients */
353
encode_one_block (working_state * state, JCOEFPTR block, int last_dc_val,
354
c_derived_tbl *dctbl, c_derived_tbl *actbl)
356
register int temp, temp2;
358
register int k, r, i;
360
/* Encode the DC coefficient difference per section F.1.2.1 */
362
temp = temp2 = block[0] - last_dc_val;
365
temp = -temp; /* temp is abs value of input */
366
/* For a negative input, want temp2 = bitwise complement of abs(input) */
367
/* This code assumes we are on a two's complement machine */
371
/* Find the number of bits needed for the magnitude of the coefficient */
377
/* Check for out-of-range coefficient values.
378
* Since we're encoding a difference, the range limit is twice as much.
380
if (nbits > MAX_COEF_BITS+1)
381
ERREXIT(state->cinfo, JERR_BAD_DCT_COEF);
383
/* Emit the Huffman-coded symbol for the number of bits */
384
if (! emit_bits(state, dctbl->ehufco[nbits], dctbl->ehufsi[nbits]))
387
/* Emit that number of bits of the value, if positive, */
388
/* or the complement of its magnitude, if negative. */
389
if (nbits) /* emit_bits rejects calls with size 0 */
390
if (! emit_bits(state, (unsigned int) temp2, nbits))
393
/* Encode the AC coefficients per section F.1.2.2 */
395
r = 0; /* r = run length of zeros */
397
for (k = 1; k < DCTSIZE2; k++) {
398
if ((temp = block[jpeg_natural_order[k]]) == 0) {
401
/* if run length > 15, must emit special run-length-16 codes (0xF0) */
403
if (! emit_bits(state, actbl->ehufco[0xF0], actbl->ehufsi[0xF0]))
410
temp = -temp; /* temp is abs value of input */
411
/* This code assumes we are on a two's complement machine */
415
/* Find the number of bits needed for the magnitude of the coefficient */
416
nbits = 1; /* there must be at least one 1 bit */
419
/* Check for out-of-range coefficient values */
420
if (nbits > MAX_COEF_BITS)
421
ERREXIT(state->cinfo, JERR_BAD_DCT_COEF);
423
/* Emit Huffman symbol for run length / number of bits */
424
i = (r << 4) + nbits;
425
if (! emit_bits(state, actbl->ehufco[i], actbl->ehufsi[i]))
428
/* Emit that number of bits of the value, if positive, */
429
/* or the complement of its magnitude, if negative. */
430
if (! emit_bits(state, (unsigned int) temp2, nbits))
437
/* If the last coef(s) were zero, emit an end-of-block code */
439
if (! emit_bits(state, actbl->ehufco[0], actbl->ehufsi[0]))
447
* Emit a restart marker & resynchronize predictions.
451
emit_restart (working_state * state, int restart_num)
455
if (! flush_bits(state))
458
emit_byte(state, 0xFF, return FALSE);
459
emit_byte(state, JPEG_RST0 + restart_num, return FALSE);
461
/* Re-initialize DC predictions to 0 */
462
for (ci = 0; ci < state->cinfo->comps_in_scan; ci++)
463
state->cur.last_dc_val[ci] = 0;
465
/* The restart counter is not updated until we successfully write the MCU. */
472
* Encode and output one MCU's worth of Huffman-compressed coefficients.
476
encode_mcu_huff (j_compress_ptr cinfo, JBLOCKROW *MCU_data)
478
huff_entropy_ptr entropy = (huff_entropy_ptr) cinfo->entropy;
481
jpeg_component_info * compptr;
483
/* Load up working state */
484
state.next_output_byte = cinfo->dest->next_output_byte;
485
state.free_in_buffer = cinfo->dest->free_in_buffer;
486
ASSIGN_STATE(state.cur, entropy->saved);
489
/* Emit restart marker if needed */
490
if (cinfo->restart_interval) {
491
if (entropy->restarts_to_go == 0)
492
if (! emit_restart(&state, entropy->next_restart_num))
496
/* Encode the MCU data blocks */
497
for (blkn = 0; blkn < cinfo->blocks_in_MCU; blkn++) {
498
ci = cinfo->MCU_membership[blkn];
499
compptr = cinfo->cur_comp_info[ci];
500
if (! encode_one_block(&state,
501
MCU_data[blkn][0], state.cur.last_dc_val[ci],
502
entropy->dc_derived_tbls[compptr->dc_tbl_no],
503
entropy->ac_derived_tbls[compptr->ac_tbl_no]))
505
/* Update last_dc_val */
506
state.cur.last_dc_val[ci] = MCU_data[blkn][0][0];
509
/* Completed MCU, so update state */
510
cinfo->dest->next_output_byte = state.next_output_byte;
511
cinfo->dest->free_in_buffer = state.free_in_buffer;
512
ASSIGN_STATE(entropy->saved, state.cur);
514
/* Update restart-interval state too */
515
if (cinfo->restart_interval) {
516
if (entropy->restarts_to_go == 0) {
517
entropy->restarts_to_go = cinfo->restart_interval;
518
entropy->next_restart_num++;
519
entropy->next_restart_num &= 7;
521
entropy->restarts_to_go--;
529
* Finish up at the end of a Huffman-compressed scan.
533
finish_pass_huff (j_compress_ptr cinfo)
535
huff_entropy_ptr entropy = (huff_entropy_ptr) cinfo->entropy;
538
/* Load up working state ... flush_bits needs it */
539
state.next_output_byte = cinfo->dest->next_output_byte;
540
state.free_in_buffer = cinfo->dest->free_in_buffer;
541
ASSIGN_STATE(state.cur, entropy->saved);
544
/* Flush out the last data */
545
if (! flush_bits(&state))
546
ERREXIT(cinfo, JERR_CANT_SUSPEND);
549
cinfo->dest->next_output_byte = state.next_output_byte;
550
cinfo->dest->free_in_buffer = state.free_in_buffer;
551
ASSIGN_STATE(entropy->saved, state.cur);
556
* Huffman coding optimization.
558
* We first scan the supplied data and count the number of uses of each symbol
559
* that is to be Huffman-coded. (This process MUST agree with the code above.)
560
* Then we build a Huffman coding tree for the observed counts.
561
* Symbols which are not needed at all for the particular image are not
562
* assigned any code, which saves space in the DHT marker as well as in
563
* the compressed data.
566
#ifdef ENTROPY_OPT_SUPPORTED
569
/* Process a single block's worth of coefficients */
572
htest_one_block (j_compress_ptr cinfo, JCOEFPTR block, int last_dc_val,
573
long dc_counts[], long ac_counts[])
579
/* Encode the DC coefficient difference per section F.1.2.1 */
581
temp = block[0] - last_dc_val;
585
/* Find the number of bits needed for the magnitude of the coefficient */
591
/* Check for out-of-range coefficient values.
592
* Since we're encoding a difference, the range limit is twice as much.
594
if (nbits > MAX_COEF_BITS+1)
595
ERREXIT(cinfo, JERR_BAD_DCT_COEF);
597
/* Count the Huffman symbol for the number of bits */
600
/* Encode the AC coefficients per section F.1.2.2 */
602
r = 0; /* r = run length of zeros */
604
for (k = 1; k < DCTSIZE2; k++) {
605
if ((temp = block[jpeg_natural_order[k]]) == 0) {
608
/* if run length > 15, must emit special run-length-16 codes (0xF0) */
614
/* Find the number of bits needed for the magnitude of the coefficient */
618
/* Find the number of bits needed for the magnitude of the coefficient */
619
nbits = 1; /* there must be at least one 1 bit */
622
/* Check for out-of-range coefficient values */
623
if (nbits > MAX_COEF_BITS)
624
ERREXIT(cinfo, JERR_BAD_DCT_COEF);
626
/* Count Huffman symbol for run length / number of bits */
627
ac_counts[(r << 4) + nbits]++;
633
/* If the last coef(s) were zero, emit an end-of-block code */
640
* Trial-encode one MCU's worth of Huffman-compressed coefficients.
641
* No data is actually output, so no suspension return is possible.
645
encode_mcu_gather (j_compress_ptr cinfo, JBLOCKROW *MCU_data)
647
huff_entropy_ptr entropy = (huff_entropy_ptr) cinfo->entropy;
649
jpeg_component_info * compptr;
651
/* Take care of restart intervals if needed */
652
if (cinfo->restart_interval) {
653
if (entropy->restarts_to_go == 0) {
654
/* Re-initialize DC predictions to 0 */
655
for (ci = 0; ci < cinfo->comps_in_scan; ci++)
656
entropy->saved.last_dc_val[ci] = 0;
657
/* Update restart state */
658
entropy->restarts_to_go = cinfo->restart_interval;
660
entropy->restarts_to_go--;
663
for (blkn = 0; blkn < cinfo->blocks_in_MCU; blkn++) {
664
ci = cinfo->MCU_membership[blkn];
665
compptr = cinfo->cur_comp_info[ci];
666
htest_one_block(cinfo, MCU_data[blkn][0], entropy->saved.last_dc_val[ci],
667
entropy->dc_count_ptrs[compptr->dc_tbl_no],
668
entropy->ac_count_ptrs[compptr->ac_tbl_no]);
669
entropy->saved.last_dc_val[ci] = MCU_data[blkn][0][0];
677
* Generate the best Huffman code table for the given counts, fill htbl.
678
* Note this is also used by jcphuff.c.
680
* The JPEG standard requires that no symbol be assigned a codeword of all
681
* one bits (so that padding bits added at the end of a compressed segment
682
* can't look like a valid code). Because of the canonical ordering of
683
* codewords, this just means that there must be an unused slot in the
684
* longest codeword length category. Section K.2 of the JPEG spec suggests
685
* reserving such a slot by pretending that symbol 256 is a valid symbol
686
* with count 1. In theory that's not optimal; giving it count zero but
687
* including it in the symbol set anyway should give a better Huffman code.
688
* But the theoretically better code actually seems to come out worse in
689
* practice, because it produces more all-ones bytes (which incur stuffed
690
* zero bytes in the final file). In any case the difference is tiny.
692
* The JPEG standard requires Huffman codes to be no more than 16 bits long.
693
* If some symbols have a very small but nonzero probability, the Huffman tree
694
* must be adjusted to meet the code length restriction. We currently use
695
* the adjustment method suggested in JPEG section K.2. This method is *not*
696
* optimal; it may not choose the best possible limited-length code. But
697
* typically only very-low-frequency symbols will be given less-than-optimal
698
* lengths, so the code is almost optimal. Experimental comparisons against
699
* an optimal limited-length-code algorithm indicate that the difference is
700
* microscopic --- usually less than a hundredth of a percent of total size.
701
* So the extra complexity of an optimal algorithm doesn't seem worthwhile.
705
jpeg_gen_optimal_table (j_compress_ptr cinfo, JHUFF_TBL * htbl, long freq[])
707
#define MAX_CLEN 32 /* assumed maximum initial code length */
708
UINT8 bits[MAX_CLEN+1]; /* bits[k] = # of symbols with code length k */
709
int codesize[257]; /* codesize[k] = code length of symbol k */
710
int others[257]; /* next symbol in current branch of tree */
715
/* This algorithm is explained in section K.2 of the JPEG standard */
717
MEMZERO(bits, SIZEOF(bits));
718
MEMZERO(codesize, SIZEOF(codesize));
719
for (i = 0; i < 257; i++)
720
others[i] = -1; /* init links to empty */
722
freq[256] = 1; /* make sure 256 has a nonzero count */
723
/* Including the pseudo-symbol 256 in the Huffman procedure guarantees
724
* that no real symbol is given code-value of all ones, because 256
725
* will be placed last in the largest codeword category.
728
/* Huffman's basic algorithm to assign optimal code lengths to symbols */
731
/* Find the smallest nonzero frequency, set c1 = its symbol */
732
/* In case of ties, take the larger symbol number */
735
for (i = 0; i <= 256; i++) {
736
if (freq[i] && freq[i] <= v) {
742
/* Find the next smallest nonzero frequency, set c2 = its symbol */
743
/* In case of ties, take the larger symbol number */
746
for (i = 0; i <= 256; i++) {
747
if (freq[i] && freq[i] <= v && i != c1) {
753
/* Done if we've merged everything into one frequency */
757
/* Else merge the two counts/trees */
758
freq[c1] += freq[c2];
761
/* Increment the codesize of everything in c1's tree branch */
763
while (others[c1] >= 0) {
768
others[c1] = c2; /* chain c2 onto c1's tree branch */
770
/* Increment the codesize of everything in c2's tree branch */
772
while (others[c2] >= 0) {
778
/* Now count the number of symbols of each code length */
779
for (i = 0; i <= 256; i++) {
781
/* The JPEG standard seems to think that this can't happen, */
782
/* but I'm paranoid... */
783
if (codesize[i] > MAX_CLEN)
784
ERREXIT(cinfo, JERR_HUFF_CLEN_OVERFLOW);
790
/* JPEG doesn't allow symbols with code lengths over 16 bits, so if the pure
791
* Huffman procedure assigned any such lengths, we must adjust the coding.
792
* Here is what the JPEG spec says about how this next bit works:
793
* Since symbols are paired for the longest Huffman code, the symbols are
794
* removed from this length category two at a time. The prefix for the pair
795
* (which is one bit shorter) is allocated to one of the pair; then,
796
* skipping the BITS entry for that prefix length, a code word from the next
797
* shortest nonzero BITS entry is converted into a prefix for two code words
801
for (i = MAX_CLEN; i > 16; i--) {
802
while (bits[i] > 0) {
803
j = i - 2; /* find length of new prefix to be used */
807
bits[i] -= 2; /* remove two symbols */
808
bits[i-1]++; /* one goes in this length */
809
bits[j+1] += 2; /* two new symbols in this length */
810
bits[j]--; /* symbol of this length is now a prefix */
814
/* Remove the count for the pseudo-symbol 256 from the largest codelength */
815
while (bits[i] == 0) /* find largest codelength still in use */
819
/* Return final symbol counts (only for lengths 0..16) */
820
MEMCOPY(htbl->bits, bits, SIZEOF(htbl->bits));
822
/* Return a list of the symbols sorted by code length */
823
/* It's not real clear to me why we don't need to consider the codelength
824
* changes made above, but the JPEG spec seems to think this works.
827
for (i = 1; i <= MAX_CLEN; i++) {
828
for (j = 0; j <= 255; j++) {
829
if (codesize[j] == i) {
830
htbl->huffval[p] = (UINT8) j;
836
/* Set sent_table FALSE so updated table will be written to JPEG file. */
837
htbl->sent_table = FALSE;
842
* Finish up a statistics-gathering pass and create the new Huffman tables.
846
finish_pass_gather (j_compress_ptr cinfo)
848
huff_entropy_ptr entropy = (huff_entropy_ptr) cinfo->entropy;
849
int ci, dctbl, actbl;
850
jpeg_component_info * compptr;
852
boolean did_dc[NUM_HUFF_TBLS];
853
boolean did_ac[NUM_HUFF_TBLS];
855
/* It's important not to apply jpeg_gen_optimal_table more than once
856
* per table, because it clobbers the input frequency counts!
858
MEMZERO(did_dc, SIZEOF(did_dc));
859
MEMZERO(did_ac, SIZEOF(did_ac));
861
for (ci = 0; ci < cinfo->comps_in_scan; ci++) {
862
compptr = cinfo->cur_comp_info[ci];
863
dctbl = compptr->dc_tbl_no;
864
actbl = compptr->ac_tbl_no;
865
if (! did_dc[dctbl]) {
866
htblptr = & cinfo->dc_huff_tbl_ptrs[dctbl];
867
if (*htblptr == NULL)
868
*htblptr = jpeg_alloc_huff_table((j_common_ptr) cinfo);
869
jpeg_gen_optimal_table(cinfo, *htblptr, entropy->dc_count_ptrs[dctbl]);
870
did_dc[dctbl] = TRUE;
872
if (! did_ac[actbl]) {
873
htblptr = & cinfo->ac_huff_tbl_ptrs[actbl];
874
if (*htblptr == NULL)
875
*htblptr = jpeg_alloc_huff_table((j_common_ptr) cinfo);
876
jpeg_gen_optimal_table(cinfo, *htblptr, entropy->ac_count_ptrs[actbl]);
877
did_ac[actbl] = TRUE;
883
#endif /* ENTROPY_OPT_SUPPORTED */
887
* Module initialization routine for Huffman entropy encoding.
891
jinit_huff_encoder (j_compress_ptr cinfo)
893
huff_entropy_ptr entropy;
896
entropy = (huff_entropy_ptr)
897
(*cinfo->mem->alloc_small) ((j_common_ptr) cinfo, JPOOL_IMAGE,
898
SIZEOF(huff_entropy_encoder));
899
cinfo->entropy = (struct jpeg_entropy_encoder *) entropy;
900
entropy->pub.start_pass = start_pass_huff;
902
/* Mark tables unallocated */
903
for (i = 0; i < NUM_HUFF_TBLS; i++) {
904
entropy->dc_derived_tbls[i] = entropy->ac_derived_tbls[i] = NULL;
905
#ifdef ENTROPY_OPT_SUPPORTED
906
entropy->dc_count_ptrs[i] = entropy->ac_count_ptrs[i] = NULL;
added
removed
jpeg/jchuff.c
