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 /* Expanded entropy encoder object for Huffman encoding. 23 * 24 * The savable_state subrecord contains fields that change within an MCU, 25 * but must not be updated permanently until we complete the MCU. 26 */ 27 28 typedef struct { 29 INT32 put_buffer; /* current bit-accumulation buffer */ 30 int put_bits; /* # of bits now in it */ 31 int last_dc_val[MAX_COMPS_IN_SCAN]; /* last DC coef for each component */ 32 } savable_state; 33 34 /* This macro is to work around compilers with missing or broken 35 * structure assignment. You'll need to fix this code if you have 36 * such a compiler and you change MAX_COMPS_IN_SCAN. 37 */ 38 39 #ifndef NO_STRUCT_ASSIGN 40 #define ASSIGN_STATE(dest,src) ((dest) = (src)) 41 #else 42 #if MAX_COMPS_IN_SCAN == 4 43 #define ASSIGN_STATE(dest,src) \ 44 ((dest).put_buffer = (src).put_buffer, \ 45 (dest).put_bits = (src).put_bits, \ 46 (dest).last_dc_val[0] = (src).last_dc_val[0], \ 47 (dest).last_dc_val[1] = (src).last_dc_val[1], \ 48 (dest).last_dc_val[2] = (src).last_dc_val[2], \ 49 (dest).last_dc_val[3] = (src).last_dc_val[3]) 50 #endif 51 #endif 52 53 54 typedef struct { 55 struct jpeg_entropy_encoder pub; /* public fields */ 56 57 savable_state saved; /* Bit buffer & DC state at start of MCU */ 58 59 /* These fields are NOT loaded into local working state. */ 60 unsigned int restarts_to_go; /* MCUs left in this restart interval */ 61 int next_restart_num; /* next restart number to write (0-7) */ 62 63 /* Pointers to derived tables (these workspaces have image lifespan) */ 64 c_derived_tbl * dc_derived_tbls[NUM_HUFF_TBLS]; 65 c_derived_tbl * ac_derived_tbls[NUM_HUFF_TBLS]; 66 67 #ifdef ENTROPY_OPT_SUPPORTED /* Statistics tables for optimization */ 68 long * dc_count_ptrs[NUM_HUFF_TBLS]; 69 long * ac_count_ptrs[NUM_HUFF_TBLS]; 70 #endif 71 } huff_entropy_encoder; 72 73 typedef huff_entropy_encoder * huff_entropy_ptr; 74 75 /* Working state while writing an MCU. 76 * This struct contains all the fields that are needed by subroutines. 77 */ 78 79 typedef struct { 80 JOCTET * next_output_byte; /* => next byte to write in buffer */ 81 size_t free_in_buffer; /* # of byte spaces remaining in buffer */ 82 savable_state cur; /* Current bit buffer & DC state */ 83 j_compress_ptr cinfo; /* dump_buffer needs access to this */ 84 } working_state; 85 86 87 /* Forward declarations */ 88 METHODDEF(boolean) encode_mcu_huff JPP((j_compress_ptr cinfo, 89 JBLOCKROW *MCU_data)); 90 METHODDEF(void) finish_pass_huff JPP((j_compress_ptr cinfo)); 91 #ifdef ENTROPY_OPT_SUPPORTED 92 METHODDEF(boolean) encode_mcu_gather JPP((j_compress_ptr cinfo, 93 JBLOCKROW *MCU_data)); 94 METHODDEF(void) finish_pass_gather JPP((j_compress_ptr cinfo)); 95 #endif 96 97 98 /* 99 * Initialize for a Huffman-compressed scan. 100 * If gather_statistics is TRUE, we do not output anything during the scan, 101 * just count the Huffman symbols used and generate Huffman code tables. 102 */ 103 104 METHODDEF(void) 105 start_pass_huff (j_compress_ptr cinfo, boolean gather_statistics) 106 { 107 huff_entropy_ptr entropy = (huff_entropy_ptr) cinfo->entropy; 108 int ci, dctbl, actbl; 109 jpeg_component_info * compptr; 110 111 if (gather_statistics) { 112 #ifdef ENTROPY_OPT_SUPPORTED 113 entropy->pub.encode_mcu = encode_mcu_gather; 114 entropy->pub.finish_pass = finish_pass_gather; 115 #else 116 ERREXIT(cinfo, JERR_NOT_COMPILED); 117 #endif 118 } else { 119 entropy->pub.encode_mcu = encode_mcu_huff; 120 entropy->pub.finish_pass = finish_pass_huff; 121 } 122 123 for (ci = 0; ci < cinfo->comps_in_scan; ci++) { 124 compptr = cinfo->cur_comp_info[ci]; 125 dctbl = compptr->dc_tbl_no; 126 actbl = compptr->ac_tbl_no; 127 if (gather_statistics) { 128 #ifdef ENTROPY_OPT_SUPPORTED 129 /* Check for invalid table indexes */ 130 /* (make_c_derived_tbl does this in the other path) */ 131 if (dctbl < 0 || dctbl >= NUM_HUFF_TBLS) 132 ERREXIT1(cinfo, JERR_NO_HUFF_TABLE, dctbl); 133 if (actbl < 0 || actbl >= NUM_HUFF_TBLS) 134 ERREXIT1(cinfo, JERR_NO_HUFF_TABLE, actbl); 135 /* Allocate and zero the statistics tables */ 136 /* Note that jpeg_gen_optimal_table expects 257 entries in each table! */ 137 if (entropy->dc_count_ptrs[dctbl] == NULL) 138 entropy->dc_count_ptrs[dctbl] = (long *) 139 (*cinfo->mem->alloc_small) ((j_common_ptr) cinfo, JPOOL_IMAGE, 140 257 * SIZEOF(long)); 141 MEMZERO(entropy->dc_count_ptrs[dctbl], 257 * SIZEOF(long)); 142 if (entropy->ac_count_ptrs[actbl] == NULL) 143 entropy->ac_count_ptrs[actbl] = (long *) 144 (*cinfo->mem->alloc_small) ((j_common_ptr) cinfo, JPOOL_IMAGE, 145 257 * SIZEOF(long)); 146 MEMZERO(entropy->ac_count_ptrs[actbl], 257 * SIZEOF(long)); 147 #endif 148 } else { 149 /* Compute derived values for Huffman tables */ 150 /* We may do this more than once for a table, but it's not expensive */ 151 jpeg_make_c_derived_tbl(cinfo, TRUE, dctbl, 152 & entropy->dc_derived_tbls[dctbl]); 153 jpeg_make_c_derived_tbl(cinfo, FALSE, actbl, 154 & entropy->ac_derived_tbls[actbl]); 155 } 156 /* Initialize DC predictions to 0 */ 157 entropy->saved.last_dc_val[ci] = 0; 158 } 159 160 /* Initialize bit buffer to empty */ 161 entropy->saved.put_buffer = 0; 162 entropy->saved.put_bits = 0; 163 164 /* Initialize restart stuff */ 165 entropy->restarts_to_go = cinfo->restart_interval; 166 entropy->next_restart_num = 0; 167 } 168 169 170 /* 171 * Compute the derived values for a Huffman table. 172 * This routine also performs some validation checks on the table. 173 * 174 * Note this is also used by jcphuff.c. 175 */ 176 177 GLOBAL(void) 178 jpeg_make_c_derived_tbl (j_compress_ptr cinfo, boolean isDC, int tblno, 179 c_derived_tbl ** pdtbl) 180 { 181 JHUFF_TBL *htbl; 182 c_derived_tbl *dtbl; 183 int p, i, l, lastp, _si, maxsymbol; 184 char huffsize[257]; 185 unsigned int huffcode[257]; 186 unsigned int code; 187 188 /* Note that huffsize[] and huffcode[] are filled in code-length order, 189 * paralleling the order of the symbols themselves in htbl->huffval[]. 190 */ 191 192 /* Find the input Huffman table */ 193 if (tblno < 0 || tblno >= NUM_HUFF_TBLS) 194 ERREXIT1(cinfo, JERR_NO_HUFF_TABLE, tblno); 195 htbl = 196 isDC ? cinfo->dc_huff_tbl_ptrs[tblno] : cinfo->ac_huff_tbl_ptrs[tblno]; 197 if (htbl == NULL) 198 ERREXIT1(cinfo, JERR_NO_HUFF_TABLE, tblno); 199 200 /* Allocate a workspace if we haven't already done so. */ 201 if (*pdtbl == NULL) 202 *pdtbl = (c_derived_tbl *) 203 (*cinfo->mem->alloc_small) ((j_common_ptr) cinfo, JPOOL_IMAGE, 204 SIZEOF(c_derived_tbl)); 205 dtbl = *pdtbl; 206 207 /* Figure C.1: make table of Huffman code length for each symbol */ 208 209 p = 0; 210 for (l = 1; l <= 16; l++) { 211 i = (int) htbl->bits[l]; 212 if (i < 0 || p + i > 256) /* protect against table overrun */ 213 ERREXIT(cinfo, JERR_BAD_HUFF_TABLE); 214 while (i--) 215 huffsize[p++] = (char) l; 216 } 217 huffsize[p] = 0; 218 lastp = p; 219 220 /* Figure C.2: generate the codes themselves */ 221 /* We also validate that the counts represent a legal Huffman code tree. */ 222 223 code = 0; 224 _si = huffsize[0]; 225 p = 0; 226 while (huffsize[p]) { 227 while (((int) huffsize[p]) == _si) { 228 huffcode[p++] = code; 229 code++; 230 } 231 /* code is now 1 more than the last code used for codelength si; but 232 * it must still fit in si bits, since no code is allowed to be all ones. 233 */ 234 if (((INT32) code) >= (((INT32) 1) << _si)) 235 ERREXIT(cinfo, JERR_BAD_HUFF_TABLE); 236 code <<= 1; 237 _si++; 238 } 239 240 /* Figure C.3: generate encoding tables */ 241 /* These are code and size indexed by symbol value */ 242 243 /* Set all codeless symbols to have code length 0; 244 * this lets us detect duplicate VAL entries here, and later 245 * allows emit_bits to detect any attempt to emit such symbols. 246 */ 247 MEMZERO(dtbl->ehufsi, SIZEOF(dtbl->ehufsi)); 248 249 /* This is also a convenient place to check for out-of-range 250 * and duplicated VAL entries. We allow 0..255 for AC symbols 251 * but only 0..15 for DC. (We could constrain them further 252 * based on data depth and mode, but this seems enough.) 253 */ 254 maxsymbol = isDC ? 15 : 255; 255 256 for (p = 0; p < lastp; p++) { 257 i = htbl->huffval[p]; 258 if (i < 0 || i > maxsymbol || dtbl->ehufsi[i]) 259 ERREXIT(cinfo, JERR_BAD_HUFF_TABLE); 260 dtbl->ehufco[i] = huffcode[p]; 261 dtbl->ehufsi[i] = huffsize[p]; 262 } 263 } 264 265 266 /* Outputting bytes to the file */ 267 268 /* Emit a byte, taking 'action' if must suspend. */ 269 #define emit_byte(state,val,action) \ 270 { *(state)->next_output_byte++ = (JOCTET) (val); \ 271 if (--(state)->free_in_buffer == 0) \ 272 if (! dump_buffer(state)) \ 273 { action; } } 274 275 276 LOCAL(boolean) 277 dump_buffer (working_state * state) 278 /* Empty the output buffer; return TRUE if successful, FALSE if must suspend */ 279 { 280 struct jpeg_destination_mgr * dest = state->cinfo->dest; 281 282 if (! (*dest->empty_output_buffer) (state->cinfo)) 283 return FALSE; 284 /* After a successful buffer dump, must reset buffer pointers */ 285 state->next_output_byte = dest->next_output_byte; 286 state->free_in_buffer = dest->free_in_buffer; 287 return TRUE; 288 } 289 290 291 /* Outputting bits to the file */ 292 293 /* Only the right 24 bits of put_buffer are used; the valid bits are 294 * left-justified in this part. At most 16 bits can be passed to emit_bits 295 * in one call, and we never retain more than 7 bits in put_buffer 296 * between calls, so 24 bits are sufficient. 297 */ 298 299 INLINE 300 LOCAL(boolean) 301 emit_bits (working_state * state, unsigned int code, int size) 302 /* Emit some bits; return TRUE if successful, FALSE if must suspend */ 303 { 304 /* This routine is heavily used, so it's worth coding tightly. */ 305 register INT32 put_buffer = (INT32) code; 306 register int put_bits = state->cur.put_bits; 307 308 /* if size is 0, caller used an invalid Huffman table entry */ 309 if (size == 0) 310 ERREXIT(state->cinfo, JERR_HUFF_MISSING_CODE); 311 312 put_buffer &= (((INT32) 1)<<size) - 1; /* mask off any extra bits in code */ 313 314 put_bits += size; /* new number of bits in buffer */ 315 316 put_buffer <<= 24 - put_bits; /* align incoming bits */ 317 318 put_buffer |= state->cur.put_buffer; /* and merge with old buffer contents */ 319 320 while (put_bits >= 8) { 321 int c = (int) ((put_buffer >> 16) & 0xFF); 322 323 emit_byte(state, c, return FALSE); 324 if (c == 0xFF) { /* need to stuff a zero byte? */ 325 emit_byte(state, 0, return FALSE); 326 } 327 put_buffer <<= 8; 328 put_bits -= 8; 329 } 330 331 state->cur.put_buffer = put_buffer; /* update state variables */ 332 state->cur.put_bits = put_bits; 333 334 return TRUE; 335 } 336 337 338 LOCAL(boolean) 339 flush_bits (working_state * state) 340 { 341 if (! emit_bits(state, 0x7F, 7)) /* fill any partial byte with ones */ 342 return FALSE; 343 state->cur.put_buffer = 0; /* and reset bit-buffer to empty */ 344 state->cur.put_bits = 0; 345 return TRUE; 346 } 347 348 349 /* Encode a single block's worth of coefficients */ 350 351 LOCAL(boolean) 352 encode_one_block (working_state * state, JCOEFPTR block, int last_dc_val, 353 c_derived_tbl *dctbl, c_derived_tbl *actbl) 354 { 355 register int temp, temp2; 356 register int nbits; 357 register int k, r, i; 358 359 /* Encode the DC coefficient difference per section F.1.2.1 */ 360 361 temp = temp2 = block[0] - last_dc_val; 362 363 if (temp < 0) { 364 temp = -temp; /* temp is abs value of input */ 365 /* For a negative input, want temp2 = bitwise complement of abs(input) */ 366 /* This code assumes we are on a two's complement machine */ 367 temp2--; 368 } 369 370 /* Find the number of bits needed for the magnitude of the coefficient */ 371 nbits = 0; 372 while (temp) { 373 nbits++; 374 temp >>= 1; 375 } 376 /* Check for out-of-range coefficient values. 377 * Since we're encoding a difference, the range limit is twice as much. 378 */ 379 if (nbits > MAX_COEF_BITS+1) 380 ERREXIT(state->cinfo, JERR_BAD_DCT_COEF); 381 382 /* Emit the Huffman-coded symbol for the number of bits */ 383 if (! emit_bits(state, dctbl->ehufco[nbits], dctbl->ehufsi[nbits])) 384 return FALSE; 385 386 /* Emit that number of bits of the value, if positive, */ 387 /* or the complement of its magnitude, if negative. */ 388 if (nbits) /* emit_bits rejects calls with size 0 */ 389 if (! emit_bits(state, (unsigned int) temp2, nbits)) 390 return FALSE; 391 392 /* Encode the AC coefficients per section F.1.2.2 */ 393 394 r = 0; /* r = run length of zeros */ 395 396 for (k = 1; k < DCTSIZE2; k++) { 397 if ((temp = block[jpeg_natural_order[k]]) == 0) { 398 r++; 399 } else { 400 /* if run length > 15, must emit special run-length-16 codes (0xF0) */ 401 while (r > 15) { 402 if (! emit_bits(state, actbl->ehufco[0xF0], actbl->ehufsi[0xF0])) 403 return FALSE; 404 r -= 16; 405 } 406 407 temp2 = temp; 408 if (temp < 0) { 409 temp = -temp; /* temp is abs value of input */ 410 /* This code assumes we are on a two's complement machine */ 411 temp2--; 412 } 413 414 /* Find the number of bits needed for the magnitude of the coefficient */ 415 nbits = 1; /* there must be at least one 1 bit */ 416 while ((temp >>= 1)) 417 nbits++; 418 /* Check for out-of-range coefficient values */ 419 if (nbits > MAX_COEF_BITS) 420 ERREXIT(state->cinfo, JERR_BAD_DCT_COEF); 421 422 /* Emit Huffman symbol for run length / number of bits */ 423 i = (r << 4) + nbits; 424 if (! emit_bits(state, actbl->ehufco[i], actbl->ehufsi[i])) 425 return FALSE; 426 427 /* Emit that number of bits of the value, if positive, */ 428 /* or the complement of its magnitude, if negative. */ 429 if (! emit_bits(state, (unsigned int) temp2, nbits)) 430 return FALSE; 431 432 r = 0; 433 } 434 } 435 436 /* If the last coef(s) were zero, emit an end-of-block code */ 437 if (r > 0) 438 if (! emit_bits(state, actbl->ehufco[0], actbl->ehufsi[0])) 439 return FALSE; 440 441 return TRUE; 442 } 443 444 445 /* 446 * Emit a restart marker & resynchronize predictions. 447 */ 448 449 LOCAL(boolean) 450 emit_restart (working_state * state, int restart_num) 451 { 452 int ci; 453 454 if (! flush_bits(state)) 455 return FALSE; 456 457 emit_byte(state, 0xFF, return FALSE); 458 emit_byte(state, JPEG_RST0 + restart_num, return FALSE); 459 460 /* Re-initialize DC predictions to 0 */ 461 for (ci = 0; ci < state->cinfo->comps_in_scan; ci++) 462 state->cur.last_dc_val[ci] = 0; 463 464 /* The restart counter is not updated until we successfully write the MCU. */ 465 466 return TRUE; 467 } 468 469 470 /* 471 * Encode and output one MCU's worth of Huffman-compressed coefficients. 472 */ 473 474 METHODDEF(boolean) 475 encode_mcu_huff (j_compress_ptr cinfo, JBLOCKROW *MCU_data) 476 { 477 huff_entropy_ptr entropy = (huff_entropy_ptr) cinfo->entropy; 478 working_state state; 479 int blkn, ci; 480 jpeg_component_info * compptr; 481 482 /* Load up working state */ 483 state.next_output_byte = cinfo->dest->next_output_byte; 484 state.free_in_buffer = cinfo->dest->free_in_buffer; 485 ASSIGN_STATE(state.cur, entropy->saved); 486 state.cinfo = cinfo; 487 488 /* Emit restart marker if needed */ 489 if (cinfo->restart_interval) { 490 if (entropy->restarts_to_go == 0) 491 if (! emit_restart(&state, entropy->next_restart_num)) 492 return FALSE; 493 } 494 495 /* Encode the MCU data blocks */ 496 for (blkn = 0; blkn < cinfo->blocks_in_MCU; blkn++) { 497 ci = cinfo->MCU_membership[blkn]; 498 compptr = cinfo->cur_comp_info[ci]; 499 if (! encode_one_block(&state, 500 MCU_data[blkn][0], state.cur.last_dc_val[ci], 501 entropy->dc_derived_tbls[compptr->dc_tbl_no], 502 entropy->ac_derived_tbls[compptr->ac_tbl_no])) 503 return FALSE; 504 /* Update last_dc_val */ 505 state.cur.last_dc_val[ci] = MCU_data[blkn][0][0]; 506 } 507 508 /* Completed MCU, so update state */ 509 cinfo->dest->next_output_byte = state.next_output_byte; 510 cinfo->dest->free_in_buffer = state.free_in_buffer; 511 ASSIGN_STATE(entropy->saved, state.cur); 512 513 /* Update restart-interval state too */ 514 if (cinfo->restart_interval) { 515 if (entropy->restarts_to_go == 0) { 516 entropy->restarts_to_go = cinfo->restart_interval; 517 entropy->next_restart_num++; 518 entropy->next_restart_num &= 7; 519 } 520 entropy->restarts_to_go--; 521 } 522 523 return TRUE; 524 } 525 526 527 /* 528 * Finish up at the end of a Huffman-compressed scan. 529 */ 530 531 METHODDEF(void) 532 finish_pass_huff (j_compress_ptr cinfo) 533 { 534 huff_entropy_ptr entropy = (huff_entropy_ptr) cinfo->entropy; 535 working_state state; 536 537 /* Load up working state ... flush_bits needs it */ 538 state.next_output_byte = cinfo->dest->next_output_byte; 539 state.free_in_buffer = cinfo->dest->free_in_buffer; 540 ASSIGN_STATE(state.cur, entropy->saved); 541 state.cinfo = cinfo; 542 543 /* Flush out the last data */ 544 if (! flush_bits(&state)) 545 ERREXIT(cinfo, JERR_CANT_SUSPEND); 546 547 /* Update state */ 548 cinfo->dest->next_output_byte = state.next_output_byte; 549 cinfo->dest->free_in_buffer = state.free_in_buffer; 550 ASSIGN_STATE(entropy->saved, state.cur); 551 } 552 553 554 /* 555 * Huffman coding optimization. 556 * 557 * We first scan the supplied data and count the number of uses of each symbol 558 * that is to be Huffman-coded. (This process MUST agree with the code above.) 559 * Then we build a Huffman coding tree for the observed counts. 560 * Symbols which are not needed at all for the particular image are not 561 * assigned any code, which saves space in the DHT marker as well as in 562 * the compressed data. 563 */ 564 565 #ifdef ENTROPY_OPT_SUPPORTED 566 567 568 /* Process a single block's worth of coefficients */ 569 570 LOCAL(void) 571 htest_one_block (j_compress_ptr cinfo, JCOEFPTR block, int last_dc_val, 572 long dc_counts[], long ac_counts[]) 573 { 574 register int temp; 575 register int nbits; 576 register int k, r; 577 578 /* Encode the DC coefficient difference per section F.1.2.1 */ 579 580 temp = block[0] - last_dc_val; 581 if (temp < 0) 582 temp = -temp; 583 584 /* Find the number of bits needed for the magnitude of the coefficient */ 585 nbits = 0; 586 while (temp) { 587 nbits++; 588 temp >>= 1; 589 } 590 /* Check for out-of-range coefficient values. 591 * Since we're encoding a difference, the range limit is twice as much. 592 */ 593 if (nbits > MAX_COEF_BITS+1) 594 ERREXIT(cinfo, JERR_BAD_DCT_COEF); 595 596 /* Count the Huffman symbol for the number of bits */ 597 dc_counts[nbits]++; 598 599 /* Encode the AC coefficients per section F.1.2.2 */ 600 601 r = 0; /* r = run length of zeros */ 602 603 for (k = 1; k < DCTSIZE2; k++) { 604 if ((temp = block[jpeg_natural_order[k]]) == 0) { 605 r++; 606 } else { 607 /* if run length > 15, must emit special run-length-16 codes (0xF0) */ 608 while (r > 15) { 609 ac_counts[0xF0]++; 610 r -= 16; 611 } 612 613 /* Find the number of bits needed for the magnitude of the coefficient */ 614 if (temp < 0) 615 temp = -temp; 616 617 /* Find the number of bits needed for the magnitude of the coefficient */ 618 nbits = 1; /* there must be at least one 1 bit */ 619 while ((temp >>= 1)) 620 nbits++; 621 /* Check for out-of-range coefficient values */ 622 if (nbits > MAX_COEF_BITS) 623 ERREXIT(cinfo, JERR_BAD_DCT_COEF); 624 625 /* Count Huffman symbol for run length / number of bits */ 626 ac_counts[(r << 4) + nbits]++; 627 628 r = 0; 629 } 630 } 631 632 /* If the last coef(s) were zero, emit an end-of-block code */ 633 if (r > 0) 634 ac_counts[0]++; 635 } 636 637 638 /* 639 * Trial-encode one MCU's worth of Huffman-compressed coefficients. 640 * No data is actually output, so no suspension return is possible. 641 */ 642 643 METHODDEF(boolean) 644 encode_mcu_gather (j_compress_ptr cinfo, JBLOCKROW *MCU_data) 645 { 646 huff_entropy_ptr entropy = (huff_entropy_ptr) cinfo->entropy; 647 int blkn, ci; 648 jpeg_component_info * compptr; 649 650 /* Take care of restart intervals if needed */ 651 if (cinfo->restart_interval) { 652 if (entropy->restarts_to_go == 0) { 653 /* Re-initialize DC predictions to 0 */ 654 for (ci = 0; ci < cinfo->comps_in_scan; ci++) 655 entropy->saved.last_dc_val[ci] = 0; 656 /* Update restart state */ 657 entropy->restarts_to_go = cinfo->restart_interval; 658 } 659 entropy->restarts_to_go--; 660 } 661 662 for (blkn = 0; blkn < cinfo->blocks_in_MCU; blkn++) { 663 ci = cinfo->MCU_membership[blkn]; 664 compptr = cinfo->cur_comp_info[ci]; 665 htest_one_block(cinfo, MCU_data[blkn][0], entropy->saved.last_dc_val[ci], 666 entropy->dc_count_ptrs[compptr->dc_tbl_no], 667 entropy->ac_count_ptrs[compptr->ac_tbl_no]); 668 entropy->saved.last_dc_val[ci] = MCU_data[blkn][0][0]; 669 } 670 671 return TRUE; 672 } 673 674 675 /* 676 * Generate the best Huffman code table for the given counts, fill htbl. 677 * Note this is also used by jcphuff.c. 678 * 679 * The JPEG standard requires that no symbol be assigned a codeword of all 680 * one bits (so that padding bits added at the end of a compressed segment 681 * can't look like a valid code). Because of the canonical ordering of 682 * codewords, this just means that there must be an unused slot in the 683 * longest codeword length category. Section K.2 of the JPEG spec suggests 684 * reserving such a slot by pretending that symbol 256 is a valid symbol 685 * with count 1. In theory that's not optimal; giving it count zero but 686 * including it in the symbol set anyway should give a better Huffman code. 687 * But the theoretically better code actually seems to come out worse in 688 * practice, because it produces more all-ones bytes (which incur stuffed 689 * zero bytes in the final file). In any case the difference is tiny. 690 * 691 * The JPEG standard requires Huffman codes to be no more than 16 bits long. 692 * If some symbols have a very small but nonzero probability, the Huffman tree 693 * must be adjusted to meet the code length restriction. We currently use 694 * the adjustment method suggested in JPEG section K.2. This method is *not* 695 * optimal; it may not choose the best possible limited-length code. But 696 * typically only very-low-frequency symbols will be given less-than-optimal 697 * lengths, so the code is almost optimal. Experimental comparisons against 698 * an optimal limited-length-code algorithm indicate that the difference is 699 * microscopic --- usually less than a hundredth of a percent of total size. 700 * So the extra complexity of an optimal algorithm doesn't seem worthwhile. 701 */ 702 703 GLOBAL(void) 704 jpeg_gen_optimal_table (j_compress_ptr cinfo, JHUFF_TBL * htbl, long freq[]) 705 { 706 #define MAX_CLEN 32 /* assumed maximum initial code length */ 707 UINT8 bits[MAX_CLEN+1]; /* bits[k] = # of symbols with code length k */ 708 int codesize[257]; /* codesize[k] = code length of symbol k */ 709 int others[257]; /* next symbol in current branch of tree */ 710 int c1, c2; 711 int p, i, j; 712 long v; 713 714 /* This algorithm is explained in section K.2 of the JPEG standard */ 715 716 MEMZERO(bits, SIZEOF(bits)); 717 MEMZERO(codesize, SIZEOF(codesize)); 718 for (i = 0; i < 257; i++) 719 others[i] = -1; /* init links to empty */ 720 721 freq[256] = 1; /* make sure 256 has a nonzero count */ 722 /* Including the pseudo-symbol 256 in the Huffman procedure guarantees 723 * that no real symbol is given code-value of all ones, because 256 724 * will be placed last in the largest codeword category. 725 */ 726 727 /* Huffman's basic algorithm to assign optimal code lengths to symbols */ 728 729 for (;;) { 730 /* Find the smallest nonzero frequency, set c1 = its symbol */ 731 /* In case of ties, take the larger symbol number */ 732 c1 = -1; 733 v = 1000000000L; 734 for (i = 0; i <= 256; i++) { 735 if (freq[i] && freq[i] <= v) { 736 v = freq[i]; 737 c1 = i; 738 } 739 } 740 741 /* Find the next smallest nonzero frequency, set c2 = its symbol */ 742 /* In case of ties, take the larger symbol number */ 743 c2 = -1; 744 v = 1000000000L; 745 for (i = 0; i <= 256; i++) { 746 if (freq[i] && freq[i] <= v && i != c1) { 747 v = freq[i]; 748 c2 = i; 749 } 750 } 751 752 /* Done if we've merged everything into one frequency */ 753 if (c2 < 0) 754 break; 755 756 /* Else merge the two counts/trees */ 757 freq[c1] += freq[c2]; 758 freq[c2] = 0; 759 760 /* Increment the codesize of everything in c1's tree branch */ 761 codesize[c1]++; 762 while (others[c1] >= 0) { 763 c1 = others[c1]; 764 codesize[c1]++; 765 } 766 767 others[c1] = c2; /* chain c2 onto c1's tree branch */ 768 769 /* Increment the codesize of everything in c2's tree branch */ 770 codesize[c2]++; 771 while (others[c2] >= 0) { 772 c2 = others[c2]; 773 codesize[c2]++; 774 } 775 } 776 777 /* Now count the number of symbols of each code length */ 778 for (i = 0; i <= 256; i++) { 779 if (codesize[i]) { 780 /* The JPEG standard seems to think that this can't happen, */ 781 /* but I'm paranoid... */ 782 if (codesize[i] > MAX_CLEN) 783 ERREXIT(cinfo, JERR_HUFF_CLEN_OVERFLOW); 784 785 bits[codesize[i]]++; 786 } 787 } 788 789 /* JPEG doesn't allow symbols with code lengths over 16 bits, so if the pure 790 * Huffman procedure assigned any such lengths, we must adjust the coding. 791 * Here is what the JPEG spec says about how this next bit works: 792 * Since symbols are paired for the longest Huffman code, the symbols are 793 * removed from this length category two at a time. The prefix for the pair 794 * (which is one bit shorter) is allocated to one of the pair; then, 795 * skipping the BITS entry for that prefix length, a code word from the next 796 * shortest nonzero BITS entry is converted into a prefix for two code words 797 * one bit longer. 798 */ 799 800 for (i = MAX_CLEN; i > 16; i--) { 801 while (bits[i] > 0) { 802 j = i - 2; /* find length of new prefix to be used */ 803 while (bits[j] == 0) 804 j--; 805 806 bits[i] -= 2; /* remove two symbols */ 807 bits[i-1]++; /* one goes in this length */ 808 bits[j+1] += 2; /* two new symbols in this length */ 809 bits[j]--; /* symbol of this length is now a prefix */ 810 } 811 } 812 813 /* Remove the count for the pseudo-symbol 256 from the largest codelength */ 814 while (bits[i] == 0) /* find largest codelength still in use */ 815 i--; 816 bits[i]--; 817 818 /* Return final symbol counts (only for lengths 0..16) */ 819 MEMCOPY(htbl->bits, bits, SIZEOF(htbl->bits)); 820 821 /* Return a list of the symbols sorted by code length */ 822 /* It's not real clear to me why we don't need to consider the codelength 823 * changes made above, but the JPEG spec seems to think this works. 824 */ 825 p = 0; 826 for (i = 1; i <= MAX_CLEN; i++) { 827 for (j = 0; j <= 255; j++) { 828 if (codesize[j] == i) { 829 htbl->huffval[p] = (UINT8) j; 830 p++; 831 } 832 } 833 } 834 835 /* Set sent_table FALSE so updated table will be written to JPEG file. */ 836 htbl->sent_table = FALSE; 837 } 838 839 840 /* 841 * Finish up a statistics-gathering pass and create the new Huffman tables. 842 */ 843 844 METHODDEF(void) 845 finish_pass_gather (j_compress_ptr cinfo) 846 { 847 huff_entropy_ptr entropy = (huff_entropy_ptr) cinfo->entropy; 848 int ci, dctbl, actbl; 849 jpeg_component_info * compptr; 850 JHUFF_TBL **htblptr; 851 boolean did_dc[NUM_HUFF_TBLS]; 852 boolean did_ac[NUM_HUFF_TBLS]; 853 854 /* It's important not to apply jpeg_gen_optimal_table more than once 855 * per table, because it clobbers the input frequency counts! 856 */ 857 MEMZERO(did_dc, SIZEOF(did_dc)); 858 MEMZERO(did_ac, SIZEOF(did_ac)); 859 860 for (ci = 0; ci < cinfo->comps_in_scan; ci++) { 861 compptr = cinfo->cur_comp_info[ci]; 862 dctbl = compptr->dc_tbl_no; 863 actbl = compptr->ac_tbl_no; 864 if (! did_dc[dctbl]) { 865 htblptr = & cinfo->dc_huff_tbl_ptrs[dctbl]; 866 if (*htblptr == NULL) 867 *htblptr = jpeg_alloc_huff_table((j_common_ptr) cinfo); 868 jpeg_gen_optimal_table(cinfo, *htblptr, entropy->dc_count_ptrs[dctbl]); 869 did_dc[dctbl] = TRUE; 870 } 871 if (! did_ac[actbl]) { 872 htblptr = & cinfo->ac_huff_tbl_ptrs[actbl]; 873 if (*htblptr == NULL) 874 *htblptr = jpeg_alloc_huff_table((j_common_ptr) cinfo); 875 jpeg_gen_optimal_table(cinfo, *htblptr, entropy->ac_count_ptrs[actbl]); 876 did_ac[actbl] = TRUE; 877 } 878 } 879 } 880 881 882 #endif /* ENTROPY_OPT_SUPPORTED */ 883 884 885 /* 886 * Module initialization routine for Huffman entropy encoding. 887 */ 888 889 GLOBAL(void) 890 jinit_huff_encoder (j_compress_ptr cinfo) 891 { 892 huff_entropy_ptr entropy; 893 int i; 894 895 entropy = (huff_entropy_ptr) 896 (*cinfo->mem->alloc_small) ((j_common_ptr) cinfo, JPOOL_IMAGE, 897 SIZEOF(huff_entropy_encoder)); 898 cinfo->entropy = (struct jpeg_entropy_encoder *) entropy; 899 entropy->pub.start_pass = start_pass_huff; 900 901 /* Mark tables unallocated */ 902 for (i = 0; i < NUM_HUFF_TBLS; i++) { 903 entropy->dc_derived_tbls[i] = entropy->ac_derived_tbls[i] = NULL; 904 #ifdef ENTROPY_OPT_SUPPORTED 905 entropy->dc_count_ptrs[i] = entropy->ac_count_ptrs[i] = NULL; 906 #endif 907 } 908 } 909
