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 #ifdef _FX_MANAGED_CODE_ 23 #define savable_state savable_state_c 24 #endif 25 26 /* Expanded entropy encoder object for Huffman encoding. 27 * 28 * The savable_state subrecord contains fields that change within an MCU, 29 * but must not be updated permanently until we complete the MCU. 30 */ 31 32 typedef struct { 33 INT32 put_buffer; /* current bit-accumulation buffer */ 34 int put_bits; /* # of bits now in it */ 35 int last_dc_val[MAX_COMPS_IN_SCAN]; /* last DC coef for each component */ 36 } savable_state; 37 38 /* This macro is to work around compilers with missing or broken 39 * structure assignment. You'll need to fix this code if you have 40 * such a compiler and you change MAX_COMPS_IN_SCAN. 41 */ 42 43 #ifndef NO_STRUCT_ASSIGN 44 #define ASSIGN_STATE(dest,src) ((dest) = (src)) 45 #else 46 #if MAX_COMPS_IN_SCAN == 4 47 #define ASSIGN_STATE(dest,src) \ 48 ((dest).put_buffer = (src).put_buffer, \ 49 (dest).put_bits = (src).put_bits, \ 50 (dest).last_dc_val[0] = (src).last_dc_val[0], \ 51 (dest).last_dc_val[1] = (src).last_dc_val[1], \ 52 (dest).last_dc_val[2] = (src).last_dc_val[2], \ 53 (dest).last_dc_val[3] = (src).last_dc_val[3]) 54 #endif 55 #endif 56 57 58 typedef struct { 59 struct jpeg_entropy_encoder pub; /* public fields */ 60 61 savable_state saved; /* Bit buffer & DC state at start of MCU */ 62 63 /* These fields are NOT loaded into local working state. */ 64 unsigned int restarts_to_go; /* MCUs left in this restart interval */ 65 int next_restart_num; /* next restart number to write (0-7) */ 66 67 /* Pointers to derived tables (these workspaces have image lifespan) */ 68 c_derived_tbl * dc_derived_tbls[NUM_HUFF_TBLS]; 69 c_derived_tbl * ac_derived_tbls[NUM_HUFF_TBLS]; 70 71 #ifdef ENTROPY_OPT_SUPPORTED /* Statistics tables for optimization */ 72 long * dc_count_ptrs[NUM_HUFF_TBLS]; 73 long * ac_count_ptrs[NUM_HUFF_TBLS]; 74 #endif 75 } huff_entropy_encoder; 76 77 typedef huff_entropy_encoder * huff_entropy_ptr; 78 79 /* Working state while writing an MCU. 80 * This struct contains all the fields that are needed by subroutines. 81 */ 82 83 typedef struct { 84 JOCTET * next_output_byte; /* => next byte to write in buffer */ 85 size_t free_in_buffer; /* # of byte spaces remaining in buffer */ 86 savable_state cur; /* Current bit buffer & DC state */ 87 j_compress_ptr cinfo; /* dump_buffer needs access to this */ 88 } working_state; 89 90 91 /* Forward declarations */ 92 METHODDEF(boolean) encode_mcu_huff JPP((j_compress_ptr cinfo, 93 JBLOCKROW *MCU_data)); 94 METHODDEF(void) finish_pass_huff JPP((j_compress_ptr cinfo)); 95 #ifdef ENTROPY_OPT_SUPPORTED 96 METHODDEF(boolean) encode_mcu_gather JPP((j_compress_ptr cinfo, 97 JBLOCKROW *MCU_data)); 98 METHODDEF(void) finish_pass_gather JPP((j_compress_ptr cinfo)); 99 #endif 100 101 102 /* 103 * Initialize for a Huffman-compressed scan. 104 * If gather_statistics is TRUE, we do not output anything during the scan, 105 * just count the Huffman symbols used and generate Huffman code tables. 106 */ 107 108 METHODDEF(void) 109 start_pass_huff (j_compress_ptr cinfo, boolean gather_statistics) 110 { 111 huff_entropy_ptr entropy = (huff_entropy_ptr) cinfo->entropy; 112 int ci, dctbl, actbl; 113 jpeg_component_info * compptr; 114 115 if (gather_statistics) { 116 #ifdef ENTROPY_OPT_SUPPORTED 117 entropy->pub.encode_mcu = encode_mcu_gather; 118 entropy->pub.finish_pass = finish_pass_gather; 119 #else 120 ERREXIT(cinfo, JERR_NOT_COMPILED); 121 #endif 122 } else { 123 entropy->pub.encode_mcu = encode_mcu_huff; 124 entropy->pub.finish_pass = finish_pass_huff; 125 } 126 127 for (ci = 0; ci < cinfo->comps_in_scan; ci++) { 128 compptr = cinfo->cur_comp_info[ci]; 129 dctbl = compptr->dc_tbl_no; 130 actbl = compptr->ac_tbl_no; 131 if (gather_statistics) { 132 #ifdef ENTROPY_OPT_SUPPORTED 133 /* Check for invalid table indexes */ 134 /* (make_c_derived_tbl does this in the other path) */ 135 if (dctbl < 0 || dctbl >= NUM_HUFF_TBLS) 136 ERREXIT1(cinfo, JERR_NO_HUFF_TABLE, dctbl); 137 if (actbl < 0 || actbl >= NUM_HUFF_TBLS) 138 ERREXIT1(cinfo, JERR_NO_HUFF_TABLE, actbl); 139 /* Allocate and zero the statistics tables */ 140 /* Note that jpeg_gen_optimal_table expects 257 entries in each table! */ 141 if (entropy->dc_count_ptrs[dctbl] == NULL) 142 entropy->dc_count_ptrs[dctbl] = (long *) 143 (*cinfo->mem->alloc_small) ((j_common_ptr) cinfo, JPOOL_IMAGE, 144 257 * SIZEOF(long)); 145 MEMZERO(entropy->dc_count_ptrs[dctbl], 257 * SIZEOF(long)); 146 if (entropy->ac_count_ptrs[actbl] == NULL) 147 entropy->ac_count_ptrs[actbl] = (long *) 148 (*cinfo->mem->alloc_small) ((j_common_ptr) cinfo, JPOOL_IMAGE, 149 257 * SIZEOF(long)); 150 MEMZERO(entropy->ac_count_ptrs[actbl], 257 * SIZEOF(long)); 151 #endif 152 } else { 153 /* Compute derived values for Huffman tables */ 154 /* We may do this more than once for a table, but it's not expensive */ 155 jpeg_make_c_derived_tbl(cinfo, TRUE, dctbl, 156 & entropy->dc_derived_tbls[dctbl]); 157 jpeg_make_c_derived_tbl(cinfo, FALSE, actbl, 158 & entropy->ac_derived_tbls[actbl]); 159 } 160 /* Initialize DC predictions to 0 */ 161 entropy->saved.last_dc_val[ci] = 0; 162 } 163 164 /* Initialize bit buffer to empty */ 165 entropy->saved.put_buffer = 0; 166 entropy->saved.put_bits = 0; 167 168 /* Initialize restart stuff */ 169 entropy->restarts_to_go = cinfo->restart_interval; 170 entropy->next_restart_num = 0; 171 } 172 173 174 /* 175 * Compute the derived values for a Huffman table. 176 * This routine also performs some validation checks on the table. 177 * 178 * Note this is also used by jcphuff.c. 179 */ 180 181 GLOBAL(void) 182 jpeg_make_c_derived_tbl (j_compress_ptr cinfo, boolean isDC, int tblno, 183 c_derived_tbl ** pdtbl) 184 { 185 JHUFF_TBL *htbl; 186 c_derived_tbl *dtbl; 187 int p, i, l, lastp, _si, maxsymbol; 188 char huffsize[257]; 189 unsigned int huffcode[257]; 190 unsigned int code; 191 192 /* Note that huffsize[] and huffcode[] are filled in code-length order, 193 * paralleling the order of the symbols themselves in htbl->huffval[]. 194 */ 195 196 /* Find the input Huffman table */ 197 if (tblno < 0 || tblno >= NUM_HUFF_TBLS) 198 ERREXIT1(cinfo, JERR_NO_HUFF_TABLE, tblno); 199 htbl = 200 isDC ? cinfo->dc_huff_tbl_ptrs[tblno] : cinfo->ac_huff_tbl_ptrs[tblno]; 201 if (htbl == NULL) 202 ERREXIT1(cinfo, JERR_NO_HUFF_TABLE, tblno); 203 204 /* Allocate a workspace if we haven't already done so. */ 205 if (*pdtbl == NULL) 206 *pdtbl = (c_derived_tbl *) 207 (*cinfo->mem->alloc_small) ((j_common_ptr) cinfo, JPOOL_IMAGE, 208 SIZEOF(c_derived_tbl)); 209 dtbl = *pdtbl; 210 211 /* Figure C.1: make table of Huffman code length for each symbol */ 212 213 p = 0; 214 for (l = 1; l <= 16; l++) { 215 i = (int) htbl->bits[l]; 216 if (i < 0 || p + i > 256) /* protect against table overrun */ 217 ERREXIT(cinfo, JERR_BAD_HUFF_TABLE); 218 while (i--) 219 huffsize[p++] = (char) l; 220 } 221 huffsize[p] = 0; 222 lastp = p; 223 224 /* Figure C.2: generate the codes themselves */ 225 /* We also validate that the counts represent a legal Huffman code tree. */ 226 227 code = 0; 228 _si = huffsize[0]; 229 p = 0; 230 while (huffsize[p]) { 231 while (((int) huffsize[p]) == _si) { 232 huffcode[p++] = code; 233 code++; 234 } 235 /* code is now 1 more than the last code used for codelength si; but 236 * it must still fit in si bits, since no code is allowed to be all ones. 237 */ 238 if (((INT32) code) >= (((INT32) 1) << _si)) 239 ERREXIT(cinfo, JERR_BAD_HUFF_TABLE); 240 code <<= 1; 241 _si++; 242 } 243 244 /* Figure C.3: generate encoding tables */ 245 /* These are code and size indexed by symbol value */ 246 247 /* Set all codeless symbols to have code length 0; 248 * this lets us detect duplicate VAL entries here, and later 249 * allows emit_bits to detect any attempt to emit such symbols. 250 */ 251 MEMZERO(dtbl->ehufsi, SIZEOF(dtbl->ehufsi)); 252 253 /* This is also a convenient place to check for out-of-range 254 * and duplicated VAL entries. We allow 0..255 for AC symbols 255 * but only 0..15 for DC. (We could constrain them further 256 * based on data depth and mode, but this seems enough.) 257 */ 258 maxsymbol = isDC ? 15 : 255; 259 260 for (p = 0; p < lastp; p++) { 261 i = htbl->huffval[p]; 262 if (i < 0 || i > maxsymbol || dtbl->ehufsi[i]) 263 ERREXIT(cinfo, JERR_BAD_HUFF_TABLE); 264 dtbl->ehufco[i] = huffcode[p]; 265 dtbl->ehufsi[i] = huffsize[p]; 266 } 267 } 268 269 270 /* Outputting bytes to the file */ 271 272 /* Emit a byte, taking 'action' if must suspend. */ 273 #define emit_byte(state,val,action) \ 274 { *(state)->next_output_byte++ = (JOCTET) (val); \ 275 if (--(state)->free_in_buffer == 0) \ 276 if (! dump_buffer(state)) \ 277 { action; } } 278 279 280 LOCAL(boolean) 281 dump_buffer (working_state * state) 282 /* Empty the output buffer; return TRUE if successful, FALSE if must suspend */ 283 { 284 struct jpeg_destination_mgr * dest = state->cinfo->dest; 285 286 if (! (*dest->empty_output_buffer) (state->cinfo)) 287 return FALSE; 288 /* After a successful buffer dump, must reset buffer pointers */ 289 state->next_output_byte = dest->next_output_byte; 290 state->free_in_buffer = dest->free_in_buffer; 291 return TRUE; 292 } 293 294 295 /* Outputting bits to the file */ 296 297 /* Only the right 24 bits of put_buffer are used; the valid bits are 298 * left-justified in this part. At most 16 bits can be passed to emit_bits 299 * in one call, and we never retain more than 7 bits in put_buffer 300 * between calls, so 24 bits are sufficient. 301 */ 302 303 INLINE 304 LOCAL(boolean) 305 emit_bits (working_state * state, unsigned int code, int size) 306 /* Emit some bits; return TRUE if successful, FALSE if must suspend */ 307 { 308 /* This routine is heavily used, so it's worth coding tightly. */ 309 register INT32 put_buffer = (INT32) code; 310 register int put_bits = state->cur.put_bits; 311 312 /* if size is 0, caller used an invalid Huffman table entry */ 313 if (size == 0) 314 ERREXIT(state->cinfo, JERR_HUFF_MISSING_CODE); 315 316 put_buffer &= (((INT32) 1)<<size) - 1; /* mask off any extra bits in code */ 317 318 put_bits += size; /* new number of bits in buffer */ 319 320 put_buffer <<= 24 - put_bits; /* align incoming bits */ 321 322 put_buffer |= state->cur.put_buffer; /* and merge with old buffer contents */ 323 324 while (put_bits >= 8) { 325 int c = (int) ((put_buffer >> 16) & 0xFF); 326 327 emit_byte(state, c, return FALSE); 328 if (c == 0xFF) { /* need to stuff a zero byte? */ 329 emit_byte(state, 0, return FALSE); 330 } 331 put_buffer <<= 8; 332 put_bits -= 8; 333 } 334 335 state->cur.put_buffer = put_buffer; /* update state variables */ 336 state->cur.put_bits = put_bits; 337 338 return TRUE; 339 } 340 341 342 LOCAL(boolean) 343 flush_bits (working_state * state) 344 { 345 if (! emit_bits(state, 0x7F, 7)) /* fill any partial byte with ones */ 346 return FALSE; 347 state->cur.put_buffer = 0; /* and reset bit-buffer to empty */ 348 state->cur.put_bits = 0; 349 return TRUE; 350 } 351 352 353 /* Encode a single block's worth of coefficients */ 354 355 LOCAL(boolean) 356 encode_one_block (working_state * state, JCOEFPTR block, int last_dc_val, 357 c_derived_tbl *dctbl, c_derived_tbl *actbl) 358 { 359 register int temp, temp2; 360 register int nbits; 361 register int k, r, i; 362 363 /* Encode the DC coefficient difference per section F.1.2.1 */ 364 365 temp = temp2 = block[0] - last_dc_val; 366 367 if (temp < 0) { 368 temp = -temp; /* temp is abs value of input */ 369 /* For a negative input, want temp2 = bitwise complement of abs(input) */ 370 /* This code assumes we are on a two's complement machine */ 371 temp2--; 372 } 373 374 /* Find the number of bits needed for the magnitude of the coefficient */ 375 nbits = 0; 376 while (temp) { 377 nbits++; 378 temp >>= 1; 379 } 380 /* Check for out-of-range coefficient values. 381 * Since we're encoding a difference, the range limit is twice as much. 382 */ 383 if (nbits > MAX_COEF_BITS+1) 384 ERREXIT(state->cinfo, JERR_BAD_DCT_COEF); 385 386 /* Emit the Huffman-coded symbol for the number of bits */ 387 if (! emit_bits(state, dctbl->ehufco[nbits], dctbl->ehufsi[nbits])) 388 return FALSE; 389 390 /* Emit that number of bits of the value, if positive, */ 391 /* or the complement of its magnitude, if negative. */ 392 if (nbits) /* emit_bits rejects calls with size 0 */ 393 if (! emit_bits(state, (unsigned int) temp2, nbits)) 394 return FALSE; 395 396 /* Encode the AC coefficients per section F.1.2.2 */ 397 398 r = 0; /* r = run length of zeros */ 399 400 for (k = 1; k < DCTSIZE2; k++) { 401 if ((temp = block[jpeg_natural_order[k]]) == 0) { 402 r++; 403 } else { 404 /* if run length > 15, must emit special run-length-16 codes (0xF0) */ 405 while (r > 15) { 406 if (! emit_bits(state, actbl->ehufco[0xF0], actbl->ehufsi[0xF0])) 407 return FALSE; 408 r -= 16; 409 } 410 411 temp2 = temp; 412 if (temp < 0) { 413 temp = -temp; /* temp is abs value of input */ 414 /* This code assumes we are on a two's complement machine */ 415 temp2--; 416 } 417 418 /* Find the number of bits needed for the magnitude of the coefficient */ 419 nbits = 1; /* there must be at least one 1 bit */ 420 while ((temp >>= 1)) 421 nbits++; 422 /* Check for out-of-range coefficient values */ 423 if (nbits > MAX_COEF_BITS) 424 ERREXIT(state->cinfo, JERR_BAD_DCT_COEF); 425 426 /* Emit Huffman symbol for run length / number of bits */ 427 i = (r << 4) + nbits; 428 if (! emit_bits(state, actbl->ehufco[i], actbl->ehufsi[i])) 429 return FALSE; 430 431 /* Emit that number of bits of the value, if positive, */ 432 /* or the complement of its magnitude, if negative. */ 433 if (! emit_bits(state, (unsigned int) temp2, nbits)) 434 return FALSE; 435 436 r = 0; 437 } 438 } 439 440 /* If the last coef(s) were zero, emit an end-of-block code */ 441 if (r > 0) 442 if (! emit_bits(state, actbl->ehufco[0], actbl->ehufsi[0])) 443 return FALSE; 444 445 return TRUE; 446 } 447 448 449 /* 450 * Emit a restart marker & resynchronize predictions. 451 */ 452 453 LOCAL(boolean) 454 emit_restart (working_state * state, int restart_num) 455 { 456 int ci; 457 458 if (! flush_bits(state)) 459 return FALSE; 460 461 emit_byte(state, 0xFF, return FALSE); 462 emit_byte(state, JPEG_RST0 + restart_num, return FALSE); 463 464 /* Re-initialize DC predictions to 0 */ 465 for (ci = 0; ci < state->cinfo->comps_in_scan; ci++) 466 state->cur.last_dc_val[ci] = 0; 467 468 /* The restart counter is not updated until we successfully write the MCU. */ 469 470 return TRUE; 471 } 472 473 474 /* 475 * Encode and output one MCU's worth of Huffman-compressed coefficients. 476 */ 477 478 METHODDEF(boolean) 479 encode_mcu_huff (j_compress_ptr cinfo, JBLOCKROW *MCU_data) 480 { 481 huff_entropy_ptr entropy = (huff_entropy_ptr) cinfo->entropy; 482 working_state state; 483 int blkn, ci; 484 jpeg_component_info * compptr; 485 486 /* Load up working state */ 487 state.next_output_byte = cinfo->dest->next_output_byte; 488 state.free_in_buffer = cinfo->dest->free_in_buffer; 489 ASSIGN_STATE(state.cur, entropy->saved); 490 state.cinfo = cinfo; 491 492 /* Emit restart marker if needed */ 493 if (cinfo->restart_interval) { 494 if (entropy->restarts_to_go == 0) 495 if (! emit_restart(&state, entropy->next_restart_num)) 496 return FALSE; 497 } 498 499 /* Encode the MCU data blocks */ 500 for (blkn = 0; blkn < cinfo->blocks_in_MCU; blkn++) { 501 ci = cinfo->MCU_membership[blkn]; 502 compptr = cinfo->cur_comp_info[ci]; 503 if (! encode_one_block(&state, 504 MCU_data[blkn][0], state.cur.last_dc_val[ci], 505 entropy->dc_derived_tbls[compptr->dc_tbl_no], 506 entropy->ac_derived_tbls[compptr->ac_tbl_no])) 507 return FALSE; 508 /* Update last_dc_val */ 509 state.cur.last_dc_val[ci] = MCU_data[blkn][0][0]; 510 } 511 512 /* Completed MCU, so update state */ 513 cinfo->dest->next_output_byte = state.next_output_byte; 514 cinfo->dest->free_in_buffer = state.free_in_buffer; 515 ASSIGN_STATE(entropy->saved, state.cur); 516 517 /* Update restart-interval state too */ 518 if (cinfo->restart_interval) { 519 if (entropy->restarts_to_go == 0) { 520 entropy->restarts_to_go = cinfo->restart_interval; 521 entropy->next_restart_num++; 522 entropy->next_restart_num &= 7; 523 } 524 entropy->restarts_to_go--; 525 } 526 527 return TRUE; 528 } 529 530 531 /* 532 * Finish up at the end of a Huffman-compressed scan. 533 */ 534 535 METHODDEF(void) 536 finish_pass_huff (j_compress_ptr cinfo) 537 { 538 huff_entropy_ptr entropy = (huff_entropy_ptr) cinfo->entropy; 539 working_state state; 540 541 /* Load up working state ... flush_bits needs it */ 542 state.next_output_byte = cinfo->dest->next_output_byte; 543 state.free_in_buffer = cinfo->dest->free_in_buffer; 544 ASSIGN_STATE(state.cur, entropy->saved); 545 state.cinfo = cinfo; 546 547 /* Flush out the last data */ 548 if (! flush_bits(&state)) 549 ERREXIT(cinfo, JERR_CANT_SUSPEND); 550 551 /* Update state */ 552 cinfo->dest->next_output_byte = state.next_output_byte; 553 cinfo->dest->free_in_buffer = state.free_in_buffer; 554 ASSIGN_STATE(entropy->saved, state.cur); 555 } 556 557 558 /* 559 * Huffman coding optimization. 560 * 561 * We first scan the supplied data and count the number of uses of each symbol 562 * that is to be Huffman-coded. (This process MUST agree with the code above.) 563 * Then we build a Huffman coding tree for the observed counts. 564 * Symbols which are not needed at all for the particular image are not 565 * assigned any code, which saves space in the DHT marker as well as in 566 * the compressed data. 567 */ 568 569 #ifdef ENTROPY_OPT_SUPPORTED 570 571 572 /* Process a single block's worth of coefficients */ 573 574 LOCAL(void) 575 htest_one_block (j_compress_ptr cinfo, JCOEFPTR block, int last_dc_val, 576 long dc_counts[], long ac_counts[]) 577 { 578 register int temp; 579 register int nbits; 580 register int k, r; 581 582 /* Encode the DC coefficient difference per section F.1.2.1 */ 583 584 temp = block[0] - last_dc_val; 585 if (temp < 0) 586 temp = -temp; 587 588 /* Find the number of bits needed for the magnitude of the coefficient */ 589 nbits = 0; 590 while (temp) { 591 nbits++; 592 temp >>= 1; 593 } 594 /* Check for out-of-range coefficient values. 595 * Since we're encoding a difference, the range limit is twice as much. 596 */ 597 if (nbits > MAX_COEF_BITS+1) 598 ERREXIT(cinfo, JERR_BAD_DCT_COEF); 599 600 /* Count the Huffman symbol for the number of bits */ 601 dc_counts[nbits]++; 602 603 /* Encode the AC coefficients per section F.1.2.2 */ 604 605 r = 0; /* r = run length of zeros */ 606 607 for (k = 1; k < DCTSIZE2; k++) { 608 if ((temp = block[jpeg_natural_order[k]]) == 0) { 609 r++; 610 } else { 611 /* if run length > 15, must emit special run-length-16 codes (0xF0) */ 612 while (r > 15) { 613 ac_counts[0xF0]++; 614 r -= 16; 615 } 616 617 /* Find the number of bits needed for the magnitude of the coefficient */ 618 if (temp < 0) 619 temp = -temp; 620 621 /* Find the number of bits needed for the magnitude of the coefficient */ 622 nbits = 1; /* there must be at least one 1 bit */ 623 while ((temp >>= 1)) 624 nbits++; 625 /* Check for out-of-range coefficient values */ 626 if (nbits > MAX_COEF_BITS) 627 ERREXIT(cinfo, JERR_BAD_DCT_COEF); 628 629 /* Count Huffman symbol for run length / number of bits */ 630 ac_counts[(r << 4) + nbits]++; 631 632 r = 0; 633 } 634 } 635 636 /* If the last coef(s) were zero, emit an end-of-block code */ 637 if (r > 0) 638 ac_counts[0]++; 639 } 640 641 642 /* 643 * Trial-encode one MCU's worth of Huffman-compressed coefficients. 644 * No data is actually output, so no suspension return is possible. 645 */ 646 647 METHODDEF(boolean) 648 encode_mcu_gather (j_compress_ptr cinfo, JBLOCKROW *MCU_data) 649 { 650 huff_entropy_ptr entropy = (huff_entropy_ptr) cinfo->entropy; 651 int blkn, ci; 652 jpeg_component_info * compptr; 653 654 /* Take care of restart intervals if needed */ 655 if (cinfo->restart_interval) { 656 if (entropy->restarts_to_go == 0) { 657 /* Re-initialize DC predictions to 0 */ 658 for (ci = 0; ci < cinfo->comps_in_scan; ci++) 659 entropy->saved.last_dc_val[ci] = 0; 660 /* Update restart state */ 661 entropy->restarts_to_go = cinfo->restart_interval; 662 } 663 entropy->restarts_to_go--; 664 } 665 666 for (blkn = 0; blkn < cinfo->blocks_in_MCU; blkn++) { 667 ci = cinfo->MCU_membership[blkn]; 668 compptr = cinfo->cur_comp_info[ci]; 669 htest_one_block(cinfo, MCU_data[blkn][0], entropy->saved.last_dc_val[ci], 670 entropy->dc_count_ptrs[compptr->dc_tbl_no], 671 entropy->ac_count_ptrs[compptr->ac_tbl_no]); 672 entropy->saved.last_dc_val[ci] = MCU_data[blkn][0][0]; 673 } 674 675 return TRUE; 676 } 677 678 679 /* 680 * Generate the best Huffman code table for the given counts, fill htbl. 681 * Note this is also used by jcphuff.c. 682 * 683 * The JPEG standard requires that no symbol be assigned a codeword of all 684 * one bits (so that padding bits added at the end of a compressed segment 685 * can't look like a valid code). Because of the canonical ordering of 686 * codewords, this just means that there must be an unused slot in the 687 * longest codeword length category. Section K.2 of the JPEG spec suggests 688 * reserving such a slot by pretending that symbol 256 is a valid symbol 689 * with count 1. In theory that's not optimal; giving it count zero but 690 * including it in the symbol set anyway should give a better Huffman code. 691 * But the theoretically better code actually seems to come out worse in 692 * practice, because it produces more all-ones bytes (which incur stuffed 693 * zero bytes in the final file). In any case the difference is tiny. 694 * 695 * The JPEG standard requires Huffman codes to be no more than 16 bits long. 696 * If some symbols have a very small but nonzero probability, the Huffman tree 697 * must be adjusted to meet the code length restriction. We currently use 698 * the adjustment method suggested in JPEG section K.2. This method is *not* 699 * optimal; it may not choose the best possible limited-length code. But 700 * typically only very-low-frequency symbols will be given less-than-optimal 701 * lengths, so the code is almost optimal. Experimental comparisons against 702 * an optimal limited-length-code algorithm indicate that the difference is 703 * microscopic --- usually less than a hundredth of a percent of total size. 704 * So the extra complexity of an optimal algorithm doesn't seem worthwhile. 705 */ 706 707 GLOBAL(void) 708 jpeg_gen_optimal_table (j_compress_ptr cinfo, JHUFF_TBL * htbl, long freq[]) 709 { 710 #define MAX_CLEN 32 /* assumed maximum initial code length */ 711 UINT8 bits[MAX_CLEN+1]; /* bits[k] = # of symbols with code length k */ 712 int codesize[257]; /* codesize[k] = code length of symbol k */ 713 int others[257]; /* next symbol in current branch of tree */ 714 int c1, c2; 715 int p, i, j; 716 long v; 717 718 /* This algorithm is explained in section K.2 of the JPEG standard */ 719 720 MEMZERO(bits, SIZEOF(bits)); 721 MEMZERO(codesize, SIZEOF(codesize)); 722 for (i = 0; i < 257; i++) 723 others[i] = -1; /* init links to empty */ 724 725 freq[256] = 1; /* make sure 256 has a nonzero count */ 726 /* Including the pseudo-symbol 256 in the Huffman procedure guarantees 727 * that no real symbol is given code-value of all ones, because 256 728 * will be placed last in the largest codeword category. 729 */ 730 731 /* Huffman's basic algorithm to assign optimal code lengths to symbols */ 732 733 for (;;) { 734 /* Find the smallest nonzero frequency, set c1 = its symbol */ 735 /* In case of ties, take the larger symbol number */ 736 c1 = -1; 737 v = 1000000000L; 738 for (i = 0; i <= 256; i++) { 739 if (freq[i] && freq[i] <= v) { 740 v = freq[i]; 741 c1 = i; 742 } 743 } 744 745 /* Find the next smallest nonzero frequency, set c2 = its symbol */ 746 /* In case of ties, take the larger symbol number */ 747 c2 = -1; 748 v = 1000000000L; 749 for (i = 0; i <= 256; i++) { 750 if (freq[i] && freq[i] <= v && i != c1) { 751 v = freq[i]; 752 c2 = i; 753 } 754 } 755 756 /* Done if we've merged everything into one frequency */ 757 if (c2 < 0) 758 break; 759 760 /* Else merge the two counts/trees */ 761 freq[c1] += freq[c2]; 762 freq[c2] = 0; 763 764 /* Increment the codesize of everything in c1's tree branch */ 765 codesize[c1]++; 766 while (others[c1] >= 0) { 767 c1 = others[c1]; 768 codesize[c1]++; 769 } 770 771 others[c1] = c2; /* chain c2 onto c1's tree branch */ 772 773 /* Increment the codesize of everything in c2's tree branch */ 774 codesize[c2]++; 775 while (others[c2] >= 0) { 776 c2 = others[c2]; 777 codesize[c2]++; 778 } 779 } 780 781 /* Now count the number of symbols of each code length */ 782 for (i = 0; i <= 256; i++) { 783 if (codesize[i]) { 784 /* The JPEG standard seems to think that this can't happen, */ 785 /* but I'm paranoid... */ 786 if (codesize[i] > MAX_CLEN) 787 ERREXIT(cinfo, JERR_HUFF_CLEN_OVERFLOW); 788 789 bits[codesize[i]]++; 790 } 791 } 792 793 /* JPEG doesn't allow symbols with code lengths over 16 bits, so if the pure 794 * Huffman procedure assigned any such lengths, we must adjust the coding. 795 * Here is what the JPEG spec says about how this next bit works: 796 * Since symbols are paired for the longest Huffman code, the symbols are 797 * removed from this length category two at a time. The prefix for the pair 798 * (which is one bit shorter) is allocated to one of the pair; then, 799 * skipping the BITS entry for that prefix length, a code word from the next 800 * shortest nonzero BITS entry is converted into a prefix for two code words 801 * one bit longer. 802 */ 803 804 for (i = MAX_CLEN; i > 16; i--) { 805 while (bits[i] > 0) { 806 j = i - 2; /* find length of new prefix to be used */ 807 while (bits[j] == 0) 808 j--; 809 810 bits[i] -= 2; /* remove two symbols */ 811 bits[i-1]++; /* one goes in this length */ 812 bits[j+1] += 2; /* two new symbols in this length */ 813 bits[j]--; /* symbol of this length is now a prefix */ 814 } 815 } 816 817 /* Remove the count for the pseudo-symbol 256 from the largest codelength */ 818 while (bits[i] == 0) /* find largest codelength still in use */ 819 i--; 820 bits[i]--; 821 822 /* Return final symbol counts (only for lengths 0..16) */ 823 MEMCOPY(htbl->bits, bits, SIZEOF(htbl->bits)); 824 825 /* Return a list of the symbols sorted by code length */ 826 /* It's not real clear to me why we don't need to consider the codelength 827 * changes made above, but the JPEG spec seems to think this works. 828 */ 829 p = 0; 830 for (i = 1; i <= MAX_CLEN; i++) { 831 for (j = 0; j <= 255; j++) { 832 if (codesize[j] == i) { 833 htbl->huffval[p] = (UINT8) j; 834 p++; 835 } 836 } 837 } 838 839 /* Set sent_table FALSE so updated table will be written to JPEG file. */ 840 htbl->sent_table = FALSE; 841 } 842 843 844 /* 845 * Finish up a statistics-gathering pass and create the new Huffman tables. 846 */ 847 848 METHODDEF(void) 849 finish_pass_gather (j_compress_ptr cinfo) 850 { 851 huff_entropy_ptr entropy = (huff_entropy_ptr) cinfo->entropy; 852 int ci, dctbl, actbl; 853 jpeg_component_info * compptr; 854 JHUFF_TBL **htblptr; 855 boolean did_dc[NUM_HUFF_TBLS]; 856 boolean did_ac[NUM_HUFF_TBLS]; 857 858 /* It's important not to apply jpeg_gen_optimal_table more than once 859 * per table, because it clobbers the input frequency counts! 860 */ 861 MEMZERO(did_dc, SIZEOF(did_dc)); 862 MEMZERO(did_ac, SIZEOF(did_ac)); 863 864 for (ci = 0; ci < cinfo->comps_in_scan; ci++) { 865 compptr = cinfo->cur_comp_info[ci]; 866 dctbl = compptr->dc_tbl_no; 867 actbl = compptr->ac_tbl_no; 868 if (! did_dc[dctbl]) { 869 htblptr = & cinfo->dc_huff_tbl_ptrs[dctbl]; 870 if (*htblptr == NULL) 871 *htblptr = jpeg_alloc_huff_table((j_common_ptr) cinfo); 872 jpeg_gen_optimal_table(cinfo, *htblptr, entropy->dc_count_ptrs[dctbl]); 873 did_dc[dctbl] = TRUE; 874 } 875 if (! did_ac[actbl]) { 876 htblptr = & cinfo->ac_huff_tbl_ptrs[actbl]; 877 if (*htblptr == NULL) 878 *htblptr = jpeg_alloc_huff_table((j_common_ptr) cinfo); 879 jpeg_gen_optimal_table(cinfo, *htblptr, entropy->ac_count_ptrs[actbl]); 880 did_ac[actbl] = TRUE; 881 } 882 } 883 } 884 885 886 #endif /* ENTROPY_OPT_SUPPORTED */ 887 888 889 /* 890 * Module initialization routine for Huffman entropy encoding. 891 */ 892 893 GLOBAL(void) 894 jinit_huff_encoder (j_compress_ptr cinfo) 895 { 896 huff_entropy_ptr entropy; 897 int i; 898 899 entropy = (huff_entropy_ptr) 900 (*cinfo->mem->alloc_small) ((j_common_ptr) cinfo, JPOOL_IMAGE, 901 SIZEOF(huff_entropy_encoder)); 902 cinfo->entropy = (struct jpeg_entropy_encoder *) entropy; 903 entropy->pub.start_pass = start_pass_huff; 904 905 /* Mark tables unallocated */ 906 for (i = 0; i < NUM_HUFF_TBLS; i++) { 907 entropy->dc_derived_tbls[i] = entropy->ac_derived_tbls[i] = NULL; 908 #ifdef ENTROPY_OPT_SUPPORTED 909 entropy->dc_count_ptrs[i] = entropy->ac_count_ptrs[i] = NULL; 910 #endif 911 } 912 } 913
