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 */ 33 } savable_state; 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 72 } huff_entropy_encoder; 73 74 typedef huff_entropy_encoder * huff_entropy_ptr; 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 */ 85 } working_state; 86 87 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 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; 110 jpeg_component_info * compptr; 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) 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; 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, 205 SIZEOF(c_derived_tbl)); 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) 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; 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) 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 } 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) 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; 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) 533 finish_pass_huff (j_compress_ptr cinfo) 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) 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; 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) 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; 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) 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]; 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) 891 jinit_huff_encoder (j_compress_ptr cinfo) 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, 898 SIZEOF(huff_entropy_encoder)); 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 } 910
