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view libs/libjpeg/jchuff.c @ 43:73813c1176de
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| author | John Tsiombikas <nuclear@member.fsf.org> |
|---|---|
| date | Fri, 14 Aug 2015 04:33:42 +0300 |
| parents | |
| children |
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1 /*
2 * jchuff.c
3 *
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.
7 *
8 * This file contains Huffman entropy encoding routines.
9 *
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.
15 */
17 #define JPEG_INTERNALS
18 #include "jinclude.h"
19 #include "jpeglib.h"
20 #include "jchuff.h" /* Declarations shared with jcphuff.c */
23 /* Expanded entropy encoder object for Huffman encoding.
24 *
25 * The savable_state subrecord contains fields that change within an MCU,
26 * but must not be updated permanently until we complete the MCU.
27 */
29 typedef struct {
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 */
33 } savable_state;
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.
38 */
40 #ifndef NO_STRUCT_ASSIGN
41 #define ASSIGN_STATE(dest,src) ((dest) = (src))
42 #else
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])
51 #endif
52 #endif
55 typedef struct {
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];
71 #endif
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.
78 */
80 typedef struct {
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 */
85 } working_state;
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));
96 #endif
99 /*
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.
103 */
105 METHODDEF(void)
106 start_pass_huff (j_compress_ptr cinfo, boolean gather_statistics)
107 {
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;
116 #else
117 ERREXIT(cinfo, JERR_NOT_COMPILED);
118 #endif
119 } else {
120 entropy->pub.encode_mcu = encode_mcu_huff;
121 entropy->pub.finish_pass = finish_pass_huff;
122 }
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,
141 257 * SIZEOF(long));
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,
146 257 * SIZEOF(long));
147 MEMZERO(entropy->ac_count_ptrs[actbl], 257 * SIZEOF(long));
148 #endif
149 } else {
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]);
156 }
157 /* Initialize DC predictions to 0 */
158 entropy->saved.last_dc_val[ci] = 0;
159 }
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;
168 }
171 /*
172 * Compute the derived values for a Huffman table.
173 * This routine also performs some validation checks on the table.
174 *
175 * Note this is also used by jcphuff.c.
176 */
178 GLOBAL(void)
179 jpeg_make_c_derived_tbl (j_compress_ptr cinfo, boolean isDC, int tblno,
180 c_derived_tbl ** pdtbl)
181 {
182 JHUFF_TBL *htbl;
183 c_derived_tbl *dtbl;
184 int p, i, l, lastp, si, maxsymbol;
185 char huffsize[257];
186 unsigned int huffcode[257];
187 unsigned int code;
189 /* Note that huffsize[] and huffcode[] are filled in code-length order,
190 * paralleling the order of the symbols themselves in htbl->huffval[].
191 */
193 /* Find the input Huffman table */
194 if (tblno < 0 || tblno >= NUM_HUFF_TBLS)
195 ERREXIT1(cinfo, JERR_NO_HUFF_TABLE, tblno);
196 htbl =
197 isDC ? cinfo->dc_huff_tbl_ptrs[tblno] : cinfo->ac_huff_tbl_ptrs[tblno];
198 if (htbl == NULL)
199 ERREXIT1(cinfo, JERR_NO_HUFF_TABLE, tblno);
201 /* Allocate a workspace if we haven't already done so. */
202 if (*pdtbl == NULL)
203 *pdtbl = (c_derived_tbl *)
204 (*cinfo->mem->alloc_small) ((j_common_ptr) cinfo, JPOOL_IMAGE,
205 SIZEOF(c_derived_tbl));
206 dtbl = *pdtbl;
208 /* Figure C.1: make table of Huffman code length for each symbol */
210 p = 0;
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);
215 while (i--)
216 huffsize[p++] = (char) l;
217 }
218 huffsize[p] = 0;
219 lastp = p;
221 /* Figure C.2: generate the codes themselves */
222 /* We also validate that the counts represent a legal Huffman code tree. */
224 code = 0;
225 si = huffsize[0];
226 p = 0;
227 while (huffsize[p]) {
228 while (((int) huffsize[p]) == si) {
229 huffcode[p++] = code;
230 code++;
231 }
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.
234 */
235 if (((INT32) code) >= (((INT32) 1) << si))
236 ERREXIT(cinfo, JERR_BAD_HUFF_TABLE);
237 code <<= 1;
238 si++;
239 }
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.
247 */
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.)
254 */
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];
263 }
264 }
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)) \
274 { action; } }
277 LOCAL(boolean)
278 dump_buffer (working_state * state)
279 /* Empty the output buffer; return TRUE if successful, FALSE if must suspend */
280 {
281 struct jpeg_destination_mgr * dest = state->cinfo->dest;
283 if (! (*dest->empty_output_buffer) (state->cinfo))
284 return FALSE;
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;
288 return TRUE;
289 }
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.
298 */
300 INLINE
301 LOCAL(boolean)
302 emit_bits (working_state * state, unsigned int code, int size)
303 /* Emit some bits; return TRUE if successful, FALSE if must suspend */
304 {
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 */
310 if (size == 0)
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);
327 }
328 put_buffer <<= 8;
329 put_bits -= 8;
330 }
332 state->cur.put_buffer = put_buffer; /* update state variables */
333 state->cur.put_bits = put_bits;
335 return TRUE;
336 }
339 LOCAL(boolean)
340 flush_bits (working_state * state)
341 {
342 if (! emit_bits(state, 0x7F, 7)) /* fill any partial byte with ones */
343 return FALSE;
344 state->cur.put_buffer = 0; /* and reset bit-buffer to empty */
345 state->cur.put_bits = 0;
346 return TRUE;
347 }
350 /* Encode a single block's worth of coefficients */
352 LOCAL(boolean)
353 encode_one_block (working_state * state, JCOEFPTR block, int last_dc_val,
354 c_derived_tbl *dctbl, c_derived_tbl *actbl)
355 {
356 register int temp, temp2;
357 register int nbits;
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;
364 if (temp < 0) {
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 */
368 temp2--;
369 }
371 /* Find the number of bits needed for the magnitude of the coefficient */
372 nbits = 0;
373 while (temp) {
374 nbits++;
375 temp >>= 1;
376 }
377 /* Check for out-of-range coefficient values.
378 * Since we're encoding a difference, the range limit is twice as much.
379 */
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]))
385 return FALSE;
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))
391 return FALSE;
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) {
399 r++;
400 } else {
401 /* if run length > 15, must emit special run-length-16 codes (0xF0) */
402 while (r > 15) {
403 if (! emit_bits(state, actbl->ehufco[0xF0], actbl->ehufsi[0xF0]))
404 return FALSE;
405 r -= 16;
406 }
408 temp2 = temp;
409 if (temp < 0) {
410 temp = -temp; /* temp is abs value of input */
411 /* This code assumes we are on a two's complement machine */
412 temp2--;
413 }
415 /* Find the number of bits needed for the magnitude of the coefficient */
416 nbits = 1; /* there must be at least one 1 bit */
417 while ((temp >>= 1))
418 nbits++;
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]))
426 return FALSE;
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))
431 return FALSE;
433 r = 0;
434 }
435 }
437 /* If the last coef(s) were zero, emit an end-of-block code */
438 if (r > 0)
439 if (! emit_bits(state, actbl->ehufco[0], actbl->ehufsi[0]))
440 return FALSE;
442 return TRUE;
443 }
446 /*
447 * Emit a restart marker & resynchronize predictions.
448 */
450 LOCAL(boolean)
451 emit_restart (working_state * state, int restart_num)
452 {
453 int ci;
455 if (! flush_bits(state))
456 return FALSE;
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. */
467 return TRUE;
468 }
471 /*
472 * Encode and output one MCU's worth of Huffman-compressed coefficients.
473 */
475 METHODDEF(boolean)
476 encode_mcu_huff (j_compress_ptr cinfo, JBLOCKROW *MCU_data)
477 {
478 huff_entropy_ptr entropy = (huff_entropy_ptr) cinfo->entropy;
479 working_state state;
480 int blkn, ci;
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);
487 state.cinfo = cinfo;
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))
493 return FALSE;
494 }
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]))
504 return FALSE;
505 /* Update last_dc_val */
506 state.cur.last_dc_val[ci] = MCU_data[blkn][0][0];
507 }
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;
520 }
521 entropy->restarts_to_go--;
522 }
524 return TRUE;
525 }
528 /*
529 * Finish up at the end of a Huffman-compressed scan.
530 */
532 METHODDEF(void)
533 finish_pass_huff (j_compress_ptr cinfo)
534 {
535 huff_entropy_ptr entropy = (huff_entropy_ptr) cinfo->entropy;
536 working_state state;
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);
542 state.cinfo = cinfo;
544 /* Flush out the last data */
545 if (! flush_bits(&state))
546 ERREXIT(cinfo, JERR_CANT_SUSPEND);
548 /* Update state */
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);
552 }
555 /*
556 * Huffman coding optimization.
557 *
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.
564 */
566 #ifdef ENTROPY_OPT_SUPPORTED
569 /* Process a single block's worth of coefficients */
571 LOCAL(void)
572 htest_one_block (j_compress_ptr cinfo, JCOEFPTR block, int last_dc_val,
573 long dc_counts[], long ac_counts[])
574 {
575 register int temp;
576 register int nbits;
577 register int k, r;
579 /* Encode the DC coefficient difference per section F.1.2.1 */
581 temp = block[0] - last_dc_val;
582 if (temp < 0)
583 temp = -temp;
585 /* Find the number of bits needed for the magnitude of the coefficient */
586 nbits = 0;
587 while (temp) {
588 nbits++;
589 temp >>= 1;
590 }
591 /* Check for out-of-range coefficient values.
592 * Since we're encoding a difference, the range limit is twice as much.
593 */
594 if (nbits > MAX_COEF_BITS+1)
595 ERREXIT(cinfo, JERR_BAD_DCT_COEF);
597 /* Count the Huffman symbol for the number of bits */
598 dc_counts[nbits]++;
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) {
606 r++;
607 } else {
608 /* if run length > 15, must emit special run-length-16 codes (0xF0) */
609 while (r > 15) {
610 ac_counts[0xF0]++;
611 r -= 16;
612 }
614 /* Find the number of bits needed for the magnitude of the coefficient */
615 if (temp < 0)
616 temp = -temp;
618 /* Find the number of bits needed for the magnitude of the coefficient */
619 nbits = 1; /* there must be at least one 1 bit */
620 while ((temp >>= 1))
621 nbits++;
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]++;
629 r = 0;
630 }
631 }
633 /* If the last coef(s) were zero, emit an end-of-block code */
634 if (r > 0)
635 ac_counts[0]++;
636 }
639 /*
640 * Trial-encode one MCU's worth of Huffman-compressed coefficients.
641 * No data is actually output, so no suspension return is possible.
642 */
644 METHODDEF(boolean)
645 encode_mcu_gather (j_compress_ptr cinfo, JBLOCKROW *MCU_data)
646 {
647 huff_entropy_ptr entropy = (huff_entropy_ptr) cinfo->entropy;
648 int blkn, ci;
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;
659 }
660 entropy->restarts_to_go--;
661 }
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];
670 }
672 return TRUE;
673 }
676 /*
677 * Generate the best Huffman code table for the given counts, fill htbl.
678 * Note this is also used by jcphuff.c.
679 *
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.
691 *
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.
702 */
704 GLOBAL(void)
705 jpeg_gen_optimal_table (j_compress_ptr cinfo, JHUFF_TBL * htbl, long freq[])
706 {
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 */
711 int c1, c2;
712 int p, i, j;
713 long v;
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.
726 */
728 /* Huffman's basic algorithm to assign optimal code lengths to symbols */
730 for (;;) {
731 /* Find the smallest nonzero frequency, set c1 = its symbol */
732 /* In case of ties, take the larger symbol number */
733 c1 = -1;
734 v = 1000000000L;
735 for (i = 0; i <= 256; i++) {
736 if (freq[i] && freq[i] <= v) {
737 v = freq[i];
738 c1 = i;
739 }
740 }
742 /* Find the next smallest nonzero frequency, set c2 = its symbol */
743 /* In case of ties, take the larger symbol number */
744 c2 = -1;
745 v = 1000000000L;
746 for (i = 0; i <= 256; i++) {
747 if (freq[i] && freq[i] <= v && i != c1) {
748 v = freq[i];
749 c2 = i;
750 }
751 }
753 /* Done if we've merged everything into one frequency */
754 if (c2 < 0)
755 break;
757 /* Else merge the two counts/trees */
758 freq[c1] += freq[c2];
759 freq[c2] = 0;
761 /* Increment the codesize of everything in c1's tree branch */
762 codesize[c1]++;
763 while (others[c1] >= 0) {
764 c1 = others[c1];
765 codesize[c1]++;
766 }
768 others[c1] = c2; /* chain c2 onto c1's tree branch */
770 /* Increment the codesize of everything in c2's tree branch */
771 codesize[c2]++;
772 while (others[c2] >= 0) {
773 c2 = others[c2];
774 codesize[c2]++;
775 }
776 }
778 /* Now count the number of symbols of each code length */
779 for (i = 0; i <= 256; i++) {
780 if (codesize[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);
786 bits[codesize[i]]++;
787 }
788 }
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
798 * one bit longer.
799 */
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 */
804 while (bits[j] == 0)
805 j--;
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 */
811 }
812 }
814 /* Remove the count for the pseudo-symbol 256 from the largest codelength */
815 while (bits[i] == 0) /* find largest codelength still in use */
816 i--;
817 bits[i]--;
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.
825 */
826 p = 0;
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;
831 p++;
832 }
833 }
834 }
836 /* Set sent_table FALSE so updated table will be written to JPEG file. */
837 htbl->sent_table = FALSE;
838 }
841 /*
842 * Finish up a statistics-gathering pass and create the new Huffman tables.
843 */
845 METHODDEF(void)
846 finish_pass_gather (j_compress_ptr cinfo)
847 {
848 huff_entropy_ptr entropy = (huff_entropy_ptr) cinfo->entropy;
849 int ci, dctbl, actbl;
850 jpeg_component_info * compptr;
851 JHUFF_TBL **htblptr;
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!
857 */
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;
871 }
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;
878 }
879 }
880 }
883 #endif /* ENTROPY_OPT_SUPPORTED */
886 /*
887 * Module initialization routine for Huffman entropy encoding.
888 */
890 GLOBAL(void)
891 jinit_huff_encoder (j_compress_ptr cinfo)
892 {
893 huff_entropy_ptr entropy;
894 int i;
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;
907 #endif
908 }
909 }