jchuff.c
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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  */
16 
17 #define JPEG_INTERNALS
18 #include "jinclude.h"
19 #include "jpeglib.h"
20 #include "jchuff.h" /* Declarations shared with jcphuff.c */
21 
22 
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  */
28 
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 */
34 
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  */
39 
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
53 
54 
55 typedef struct {
56  struct jpeg_entropy_encoder pub; /* public fields */
57 
58  savable_state saved; /* Bit buffer & DC state at start of MCU */
59 
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) */
63 
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];
67 
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
73 
75 
76 /* Working state while writing an MCU.
77  * This struct contains all the fields that are needed by subroutines.
78  */
79 
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 */
86 
87 
88 /* Forward declarations */
90  JBLOCKROW *MCU_data));
92 #ifdef ENTROPY_OPT_SUPPORTED
94  JBLOCKROW *MCU_data));
96 #endif
97 
98 
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  */
104 
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;
111 
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  }
123 
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  }
160 
161  /* Initialize bit buffer to empty */
162  entropy->saved.put_buffer = 0;
163  entropy->saved.put_bits = 0;
164 
165  /* Initialize restart stuff */
166  entropy->restarts_to_go = cinfo->restart_interval;
167  entropy->next_restart_num = 0;
168 }
169 
170 
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  */
177 
178 GLOBAL(void)
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;
188 
189  /* Note that huffsize[] and huffcode[] are filled in code-length order,
190  * paralleling the order of the symbols themselves in htbl->huffval[].
191  */
192 
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);
200 
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,
206  dtbl = *pdtbl;
207 
208  /* Figure C.1: make table of Huffman code length for each symbol */
209 
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;
220 
221  /* Figure C.2: generate the codes themselves */
222  /* We also validate that the counts represent a legal Huffman code tree. */
223 
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  }
240 
241  /* Figure C.3: generate encoding tables */
242  /* These are code and size indexed by symbol value */
243 
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));
249 
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;
256 
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 }
265 
266 
267 /* Outputting bytes to the file */
268 
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; } }
275 
276 
277 LOCAL(boolean)
279 /* Empty the output buffer; return TRUE if successful, FALSE if must suspend */
280 {
281  struct jpeg_destination_mgr * dest = state->cinfo->dest;
282 
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 }
290 
291 
292 /* Outputting bits to the file */
293 
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  */
299 
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;
308 
309  /* if size is 0, caller used an invalid Huffman table entry */
310  if (size == 0)
311  ERREXIT(state->cinfo, JERR_HUFF_MISSING_CODE);
312 
313  put_buffer &= (((INT32) 1)<<size) - 1; /* mask off any extra bits in code */
314 
315  put_bits += size; /* new number of bits in buffer */
316 
317  put_buffer <<= 24 - put_bits; /* align incoming bits */
318 
319  put_buffer |= state->cur.put_buffer; /* and merge with old buffer contents */
320 
321  while (put_bits >= 8) {
322  int c = (int) ((put_buffer >> 16) & 0xFF);
323 
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  }
331 
332  state->cur.put_buffer = put_buffer; /* update state variables */
333  state->cur.put_bits = put_bits;
334 
335  return TRUE;
336 }
337 
338 
339 LOCAL(boolean)
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 }
348 
349 
350 /* Encode a single block's worth of coefficients */
351 
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;
359 
360  /* Encode the DC coefficient difference per section F.1.2.1 */
361 
362  temp = temp2 = block[0] - last_dc_val;
363 
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  }
370 
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);
382 
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;
386 
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;
392 
393  /* Encode the AC coefficients per section F.1.2.2 */
394 
395  r = 0; /* r = run length of zeros */
396 
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  }
407 
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  }
414 
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);
422 
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;
427 
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;
432 
433  r = 0;
434  }
435  }
436 
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;
441 
442  return TRUE;
443 }
444 
445 
446 /*
447  * Emit a restart marker & resynchronize predictions.
448  */
449 
450 LOCAL(boolean)
451 emit_restart (working_state * state, int restart_num)
452 {
453  int ci;
454 
455  if (! flush_bits(state))
456  return FALSE;
457 
458  emit_byte(state, 0xFF, return FALSE);
459  emit_byte(state, JPEG_RST0 + restart_num, return FALSE);
460 
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;
464 
465  /* The restart counter is not updated until we successfully write the MCU. */
466 
467  return TRUE;
468 }
469 
470 
471 /*
472  * Encode and output one MCU's worth of Huffman-compressed coefficients.
473  */
474 
475 METHODDEF(boolean)
477 {
478  huff_entropy_ptr entropy = (huff_entropy_ptr) cinfo->entropy;
479  working_state state;
480  int blkn, ci;
482 
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;
488 
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  }
495 
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  }
508 
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);
513 
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  }
523 
524  return TRUE;
525 }
526 
527 
528 /*
529  * Finish up at the end of a Huffman-compressed scan.
530  */
531 
532 METHODDEF(void)
534 {
535  huff_entropy_ptr entropy = (huff_entropy_ptr) cinfo->entropy;
536  working_state state;
537 
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;
543 
544  /* Flush out the last data */
545  if (! flush_bits(&state))
546  ERREXIT(cinfo, JERR_CANT_SUSPEND);
547 
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 }
553 
554 
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  */
565 
566 #ifdef ENTROPY_OPT_SUPPORTED
567 
568 
569 /* Process a single block's worth of coefficients */
570 
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;
578 
579  /* Encode the DC coefficient difference per section F.1.2.1 */
580 
581  temp = block[0] - last_dc_val;
582  if (temp < 0)
583  temp = -temp;
584 
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);
596 
597  /* Count the Huffman symbol for the number of bits */
598  dc_counts[nbits]++;
599 
600  /* Encode the AC coefficients per section F.1.2.2 */
601 
602  r = 0; /* r = run length of zeros */
603 
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  }
613 
614  /* Find the number of bits needed for the magnitude of the coefficient */
615  if (temp < 0)
616  temp = -temp;
617 
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);
625 
626  /* Count Huffman symbol for run length / number of bits */
627  ac_counts[(r << 4) + nbits]++;
628 
629  r = 0;
630  }
631  }
632 
633  /* If the last coef(s) were zero, emit an end-of-block code */
634  if (r > 0)
635  ac_counts[0]++;
636 }
637 
638 
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  */
643 
644 METHODDEF(boolean)
646 {
647  huff_entropy_ptr entropy = (huff_entropy_ptr) cinfo->entropy;
648  int blkn, ci;
650 
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  }
662 
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  }
671 
672  return TRUE;
673 }
674 
675 
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  */
703 
704 GLOBAL(void)
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;
714 
715  /* This algorithm is explained in section K.2 of the JPEG standard */
716 
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 */
721 
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  */
727 
728  /* Huffman's basic algorithm to assign optimal code lengths to symbols */
729 
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  }
741 
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  }
752 
753  /* Done if we've merged everything into one frequency */
754  if (c2 < 0)
755  break;
756 
757  /* Else merge the two counts/trees */
758  freq[c1] += freq[c2];
759  freq[c2] = 0;
760 
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  }
767 
768  others[c1] = c2; /* chain c2 onto c1's tree branch */
769 
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  }
777 
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);
785 
786  bits[codesize[i]]++;
787  }
788  }
789 
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  */
800 
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--;
806 
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  }
813 
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]--;
818 
819  /* Return final symbol counts (only for lengths 0..16) */
820  MEMCOPY(htbl->bits, bits, SIZEOF(htbl->bits));
821 
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  }
835 
836  /* Set sent_table FALSE so updated table will be written to JPEG file. */
837  htbl->sent_table = FALSE;
838 }
839 
840 
841 /*
842  * Finish up a statistics-gathering pass and create the new Huffman tables.
843  */
844 
845 METHODDEF(void)
847 {
848  huff_entropy_ptr entropy = (huff_entropy_ptr) cinfo->entropy;
849  int ci, dctbl, actbl;
851  JHUFF_TBL **htblptr;
852  boolean did_dc[NUM_HUFF_TBLS];
853  boolean did_ac[NUM_HUFF_TBLS];
854 
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));
860 
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 }
881 
882 
883 #endif /* ENTROPY_OPT_SUPPORTED */
884 
885 
886 /*
887  * Module initialization routine for Huffman entropy encoding.
888  */
889 
890 GLOBAL(void)
892 {
893  huff_entropy_ptr entropy;
894  int i;
895 
896  entropy = (huff_entropy_ptr)
897  (*cinfo->mem->alloc_small) ((j_common_ptr) cinfo, JPOOL_IMAGE,
899  cinfo->entropy = (struct jpeg_entropy_encoder *) entropy;
900  entropy->pub.start_pass = start_pass_huff;
901 
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 }
MAX_COMPS_IN_SCAN
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Definition: jpeglib.h:46
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openhrp3
Author(s): AIST, General Robotix Inc., Nakamura Lab of Dept. of Mechano Informatics at University of Tokyo
autogenerated on Wed Sep 7 2022 02:51:03