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Diffstat (limited to 'code/jpeg-6b/jchuff.c')
| -rw-r--r-- | code/jpeg-6b/jchuff.c | 909 |
1 files changed, 909 insertions, 0 deletions
diff --git a/code/jpeg-6b/jchuff.c b/code/jpeg-6b/jchuff.c new file mode 100644 index 0000000..f235250 --- /dev/null +++ b/code/jpeg-6b/jchuff.c @@ -0,0 +1,909 @@ +/* + * jchuff.c + * + * Copyright (C) 1991-1997, Thomas G. Lane. + * This file is part of the Independent JPEG Group's software. + * For conditions of distribution and use, see the accompanying README file. + * + * This file contains Huffman entropy encoding routines. + * + * Much of the complexity here has to do with supporting output suspension. + * If the data destination module demands suspension, we want to be able to + * back up to the start of the current MCU. To do this, we copy state + * variables into local working storage, and update them back to the + * permanent JPEG objects only upon successful completion of an MCU. + */ + +#define JPEG_INTERNALS +#include "jinclude.h" +#include "jpeglib.h" +#include "jchuff.h" /* Declarations shared with jcphuff.c */ + + +/* Expanded entropy encoder object for Huffman encoding. + * + * The savable_state subrecord contains fields that change within an MCU, + * but must not be updated permanently until we complete the MCU. + */ + +typedef struct { + INT32 put_buffer; /* current bit-accumulation buffer */ + int put_bits; /* # of bits now in it */ + int last_dc_val[MAX_COMPS_IN_SCAN]; /* last DC coef for each component */ +} savable_state; + +/* This macro is to work around compilers with missing or broken + * structure assignment. You'll need to fix this code if you have + * such a compiler and you change MAX_COMPS_IN_SCAN. + */ + +#ifndef NO_STRUCT_ASSIGN +#define ASSIGN_STATE(dest,src) ((dest) = (src)) +#else +#if MAX_COMPS_IN_SCAN == 4 +#define ASSIGN_STATE(dest,src) \ + ((dest).put_buffer = (src).put_buffer, \ + (dest).put_bits = (src).put_bits, \ + (dest).last_dc_val[0] = (src).last_dc_val[0], \ + (dest).last_dc_val[1] = (src).last_dc_val[1], \ + (dest).last_dc_val[2] = (src).last_dc_val[2], \ + (dest).last_dc_val[3] = (src).last_dc_val[3]) +#endif +#endif + + +typedef struct { + struct jpeg_entropy_encoder pub; /* public fields */ + + savable_state saved; /* Bit buffer & DC state at start of MCU */ + + /* These fields are NOT loaded into local working state. */ + unsigned int restarts_to_go; /* MCUs left in this restart interval */ + int next_restart_num; /* next restart number to write (0-7) */ + + /* Pointers to derived tables (these workspaces have image lifespan) */ + c_derived_tbl * dc_derived_tbls[NUM_HUFF_TBLS]; + c_derived_tbl * ac_derived_tbls[NUM_HUFF_TBLS]; + +#ifdef ENTROPY_OPT_SUPPORTED /* Statistics tables for optimization */ + long * dc_count_ptrs[NUM_HUFF_TBLS]; + long * ac_count_ptrs[NUM_HUFF_TBLS]; +#endif +} huff_entropy_encoder; + +typedef huff_entropy_encoder * huff_entropy_ptr; + +/* Working state while writing an MCU. + * This struct contains all the fields that are needed by subroutines. + */ + +typedef struct { + JOCTET * next_output_byte; /* => next byte to write in buffer */ + size_t free_in_buffer; /* # of byte spaces remaining in buffer */ + savable_state cur; /* Current bit buffer & DC state */ + j_compress_ptr cinfo; /* dump_buffer needs access to this */ +} working_state; + + +/* Forward declarations */ +METHODDEF(boolean) encode_mcu_huff JPP((j_compress_ptr cinfo, + JBLOCKROW *MCU_data)); +METHODDEF(void) finish_pass_huff JPP((j_compress_ptr cinfo)); +#ifdef ENTROPY_OPT_SUPPORTED +METHODDEF(boolean) encode_mcu_gather JPP((j_compress_ptr cinfo, + JBLOCKROW *MCU_data)); +METHODDEF(void) finish_pass_gather JPP((j_compress_ptr cinfo)); +#endif + + +/* + * Initialize for a Huffman-compressed scan. + * If gather_statistics is TRUE, we do not output anything during the scan, + * just count the Huffman symbols used and generate Huffman code tables. + */ + +METHODDEF(void) +start_pass_huff (j_compress_ptr cinfo, boolean gather_statistics) +{ + huff_entropy_ptr entropy = (huff_entropy_ptr) cinfo->entropy; + int ci, dctbl, actbl; + jpeg_component_info * compptr; + + if (gather_statistics) { +#ifdef ENTROPY_OPT_SUPPORTED + entropy->pub.encode_mcu = encode_mcu_gather; + entropy->pub.finish_pass = finish_pass_gather; +#else + ERREXIT(cinfo, JERR_NOT_COMPILED); +#endif + } else { + entropy->pub.encode_mcu = encode_mcu_huff; + entropy->pub.finish_pass = finish_pass_huff; + } + + for (ci = 0; ci < cinfo->comps_in_scan; ci++) { + compptr = cinfo->cur_comp_info[ci]; + dctbl = compptr->dc_tbl_no; + actbl = compptr->ac_tbl_no; + if (gather_statistics) { +#ifdef ENTROPY_OPT_SUPPORTED + /* Check for invalid table indexes */ + /* (make_c_derived_tbl does this in the other path) */ + if (dctbl < 0 || dctbl >= NUM_HUFF_TBLS) + ERREXIT1(cinfo, JERR_NO_HUFF_TABLE, dctbl); + if (actbl < 0 || actbl >= NUM_HUFF_TBLS) + ERREXIT1(cinfo, JERR_NO_HUFF_TABLE, actbl); + /* Allocate and zero the statistics tables */ + /* Note that jpeg_gen_optimal_table expects 257 entries in each table! */ + if (entropy->dc_count_ptrs[dctbl] == NULL) + entropy->dc_count_ptrs[dctbl] = (long *) + (*cinfo->mem->alloc_small) ((j_common_ptr) cinfo, JPOOL_IMAGE, + 257 * SIZEOF(long)); + MEMZERO(entropy->dc_count_ptrs[dctbl], 257 * SIZEOF(long)); + if (entropy->ac_count_ptrs[actbl] == NULL) + entropy->ac_count_ptrs[actbl] = (long *) + (*cinfo->mem->alloc_small) ((j_common_ptr) cinfo, JPOOL_IMAGE, + 257 * SIZEOF(long)); + MEMZERO(entropy->ac_count_ptrs[actbl], 257 * SIZEOF(long)); +#endif + } else { + /* Compute derived values for Huffman tables */ + /* We may do this more than once for a table, but it's not expensive */ + jpeg_make_c_derived_tbl(cinfo, TRUE, dctbl, + & entropy->dc_derived_tbls[dctbl]); + jpeg_make_c_derived_tbl(cinfo, FALSE, actbl, + & entropy->ac_derived_tbls[actbl]); + } + /* Initialize DC predictions to 0 */ + entropy->saved.last_dc_val[ci] = 0; + } + + /* Initialize bit buffer to empty */ + entropy->saved.put_buffer = 0; + entropy->saved.put_bits = 0; + + /* Initialize restart stuff */ + entropy->restarts_to_go = cinfo->restart_interval; + entropy->next_restart_num = 0; +} + + +/* + * Compute the derived values for a Huffman table. + * This routine also performs some validation checks on the table. + * + * Note this is also used by jcphuff.c. + */ + +GLOBAL(void) +jpeg_make_c_derived_tbl (j_compress_ptr cinfo, boolean isDC, int tblno, + c_derived_tbl ** pdtbl) +{ + JHUFF_TBL *htbl; + c_derived_tbl *dtbl; + int p, i, l, lastp, si, maxsymbol; + char huffsize[257]; + unsigned int huffcode[257]; + unsigned int code; + + /* Note that huffsize[] and huffcode[] are filled in code-length order, + * paralleling the order of the symbols themselves in htbl->huffval[]. + */ + + /* Find the input Huffman table */ + if (tblno < 0 || tblno >= NUM_HUFF_TBLS) + ERREXIT1(cinfo, JERR_NO_HUFF_TABLE, tblno); + htbl = + isDC ? cinfo->dc_huff_tbl_ptrs[tblno] : cinfo->ac_huff_tbl_ptrs[tblno]; + if (htbl == NULL) + ERREXIT1(cinfo, JERR_NO_HUFF_TABLE, tblno); + + /* Allocate a workspace if we haven't already done so. */ + if (*pdtbl == NULL) + *pdtbl = (c_derived_tbl *) + (*cinfo->mem->alloc_small) ((j_common_ptr) cinfo, JPOOL_IMAGE, + SIZEOF(c_derived_tbl)); + dtbl = *pdtbl; + + /* Figure C.1: make table of Huffman code length for each symbol */ + + p = 0; + for (l = 1; l <= 16; l++) { + i = (int) htbl->bits[l]; + if (i < 0 || p + i > 256) /* protect against table overrun */ + ERREXIT(cinfo, JERR_BAD_HUFF_TABLE); + while (i--) + huffsize[p++] = (char) l; + } + huffsize[p] = 0; + lastp = p; + + /* Figure C.2: generate the codes themselves */ + /* We also validate that the counts represent a legal Huffman code tree. */ + + code = 0; + si = huffsize[0]; + p = 0; + while (huffsize[p]) { + while (((int) huffsize[p]) == si) { + huffcode[p++] = code; + code++; + } + /* code is now 1 more than the last code used for codelength si; but + * it must still fit in si bits, since no code is allowed to be all ones. + */ + if (((INT32) code) >= (((INT32) 1) << si)) + ERREXIT(cinfo, JERR_BAD_HUFF_TABLE); + code <<= 1; + si++; + } + + /* Figure C.3: generate encoding tables */ + /* These are code and size indexed by symbol value */ + + /* Set all codeless symbols to have code length 0; + * this lets us detect duplicate VAL entries here, and later + * allows emit_bits to detect any attempt to emit such symbols. + */ + MEMZERO(dtbl->ehufsi, SIZEOF(dtbl->ehufsi)); + + /* This is also a convenient place to check for out-of-range + * and duplicated VAL entries. We allow 0..255 for AC symbols + * but only 0..15 for DC. (We could constrain them further + * based on data depth and mode, but this seems enough.) + */ + maxsymbol = isDC ? 15 : 255; + + for (p = 0; p < lastp; p++) { + i = htbl->huffval[p]; + if (i < 0 || i > maxsymbol || dtbl->ehufsi[i]) + ERREXIT(cinfo, JERR_BAD_HUFF_TABLE); + dtbl->ehufco[i] = huffcode[p]; + dtbl->ehufsi[i] = huffsize[p]; + } +} + + +/* Outputting bytes to the file */ + +/* Emit a byte, taking 'action' if must suspend. */ +#define emit_byte(state,val,action) \ + { *(state)->next_output_byte++ = (JOCTET) (val); \ + if (--(state)->free_in_buffer == 0) \ + if (! dump_buffer(state)) \ + { action; } } + + +LOCAL(boolean) +dump_buffer (working_state * state) +/* Empty the output buffer; return TRUE if successful, FALSE if must suspend */ +{ + struct jpeg_destination_mgr * dest = state->cinfo->dest; + + if (! (*dest->empty_output_buffer) (state->cinfo)) + return FALSE; + /* After a successful buffer dump, must reset buffer pointers */ + state->next_output_byte = dest->next_output_byte; + state->free_in_buffer = dest->free_in_buffer; + return TRUE; +} + + +/* Outputting bits to the file */ + +/* Only the right 24 bits of put_buffer are used; the valid bits are + * left-justified in this part. At most 16 bits can be passed to emit_bits + * in one call, and we never retain more than 7 bits in put_buffer + * between calls, so 24 bits are sufficient. + */ + +INLINE +LOCAL(boolean) +emit_bits (working_state * state, unsigned int code, int size) +/* Emit some bits; return TRUE if successful, FALSE if must suspend */ +{ + /* This routine is heavily used, so it's worth coding tightly. */ + register INT32 put_buffer = (INT32) code; + register int put_bits = state->cur.put_bits; + + /* if size is 0, caller used an invalid Huffman table entry */ + if (size == 0) + ERREXIT(state->cinfo, JERR_HUFF_MISSING_CODE); + + put_buffer &= (((INT32) 1)<<size) - 1; /* mask off any extra bits in code */ + + put_bits += size; /* new number of bits in buffer */ + + put_buffer <<= 24 - put_bits; /* align incoming bits */ + + put_buffer |= state->cur.put_buffer; /* and merge with old buffer contents */ + + while (put_bits >= 8) { + int c = (int) ((put_buffer >> 16) & 0xFF); + + emit_byte(state, c, return FALSE); + if (c == 0xFF) { /* need to stuff a zero byte? */ + emit_byte(state, 0, return FALSE); + } + put_buffer <<= 8; + put_bits -= 8; + } + + state->cur.put_buffer = put_buffer; /* update state variables */ + state->cur.put_bits = put_bits; + + return TRUE; +} + + +LOCAL(boolean) +flush_bits (working_state * state) +{ + if (! emit_bits(state, 0x7F, 7)) /* fill any partial byte with ones */ + return FALSE; + state->cur.put_buffer = 0; /* and reset bit-buffer to empty */ + state->cur.put_bits = 0; + return TRUE; +} + + +/* Encode a single block's worth of coefficients */ + +LOCAL(boolean) +encode_one_block (working_state * state, JCOEFPTR block, int last_dc_val, + c_derived_tbl *dctbl, c_derived_tbl *actbl) +{ + register int temp, temp2; + register int nbits; + register int k, r, i; + + /* Encode the DC coefficient difference per section F.1.2.1 */ + + temp = temp2 = block[0] - last_dc_val; + + if (temp < 0) { + temp = -temp; /* temp is abs value of input */ + /* For a negative input, want temp2 = bitwise complement of abs(input) */ + /* This code assumes we are on a two's complement machine */ + temp2--; + } + + /* Find the number of bits needed for the magnitude of the coefficient */ + nbits = 0; + while (temp) { + nbits++; + temp >>= 1; + } + /* Check for out-of-range coefficient values. + * Since we're encoding a difference, the range limit is twice as much. + */ + if (nbits > MAX_COEF_BITS+1) + ERREXIT(state->cinfo, JERR_BAD_DCT_COEF); + + /* Emit the Huffman-coded symbol for the number of bits */ + if (! emit_bits(state, dctbl->ehufco[nbits], dctbl->ehufsi[nbits])) + return FALSE; + + /* Emit that number of bits of the value, if positive, */ + /* or the complement of its magnitude, if negative. */ + if (nbits) /* emit_bits rejects calls with size 0 */ + if (! emit_bits(state, (unsigned int) temp2, nbits)) + return FALSE; + + /* Encode the AC coefficients per section F.1.2.2 */ + + r = 0; /* r = run length of zeros */ + + for (k = 1; k < DCTSIZE2; k++) { + if ((temp = block[jpeg_natural_order[k]]) == 0) { + r++; + } else { + /* if run length > 15, must emit special run-length-16 codes (0xF0) */ + while (r > 15) { + if (! emit_bits(state, actbl->ehufco[0xF0], actbl->ehufsi[0xF0])) + return FALSE; + r -= 16; + } + + temp2 = temp; + if (temp < 0) { + temp = -temp; /* temp is abs value of input */ + /* This code assumes we are on a two's complement machine */ + temp2--; + } + + /* Find the number of bits needed for the magnitude of the coefficient */ + nbits = 1; /* there must be at least one 1 bit */ + while ((temp >>= 1)) + nbits++; + /* Check for out-of-range coefficient values */ + if (nbits > MAX_COEF_BITS) + ERREXIT(state->cinfo, JERR_BAD_DCT_COEF); + + /* Emit Huffman symbol for run length / number of bits */ + i = (r << 4) + nbits; + if (! emit_bits(state, actbl->ehufco[i], actbl->ehufsi[i])) + return FALSE; + + /* Emit that number of bits of the value, if positive, */ + /* or the complement of its magnitude, if negative. */ + if (! emit_bits(state, (unsigned int) temp2, nbits)) + return FALSE; + + r = 0; + } + } + + /* If the last coef(s) were zero, emit an end-of-block code */ + if (r > 0) + if (! emit_bits(state, actbl->ehufco[0], actbl->ehufsi[0])) + return FALSE; + + return TRUE; +} + + +/* + * Emit a restart marker & resynchronize predictions. + */ + +LOCAL(boolean) +emit_restart (working_state * state, int restart_num) +{ + int ci; + + if (! flush_bits(state)) + return FALSE; + + emit_byte(state, 0xFF, return FALSE); + emit_byte(state, JPEG_RST0 + restart_num, return FALSE); + + /* Re-initialize DC predictions to 0 */ + for (ci = 0; ci < state->cinfo->comps_in_scan; ci++) + state->cur.last_dc_val[ci] = 0; + + /* The restart counter is not updated until we successfully write the MCU. */ + + return TRUE; +} + + +/* + * Encode and output one MCU's worth of Huffman-compressed coefficients. + */ + +METHODDEF(boolean) +encode_mcu_huff (j_compress_ptr cinfo, JBLOCKROW *MCU_data) +{ + huff_entropy_ptr entropy = (huff_entropy_ptr) cinfo->entropy; + working_state state; + int blkn, ci; + jpeg_component_info * compptr; + + /* Load up working state */ + state.next_output_byte = cinfo->dest->next_output_byte; + state.free_in_buffer = cinfo->dest->free_in_buffer; + ASSIGN_STATE(state.cur, entropy->saved); + state.cinfo = cinfo; + + /* Emit restart marker if needed */ + if (cinfo->restart_interval) { + if (entropy->restarts_to_go == 0) + if (! emit_restart(&state, entropy->next_restart_num)) + return FALSE; + } + + /* Encode the MCU data blocks */ + for (blkn = 0; blkn < cinfo->blocks_in_MCU; blkn++) { + ci = cinfo->MCU_membership[blkn]; + compptr = cinfo->cur_comp_info[ci]; + if (! encode_one_block(&state, + MCU_data[blkn][0], state.cur.last_dc_val[ci], + entropy->dc_derived_tbls[compptr->dc_tbl_no], + entropy->ac_derived_tbls[compptr->ac_tbl_no])) + return FALSE; + /* Update last_dc_val */ + state.cur.last_dc_val[ci] = MCU_data[blkn][0][0]; + } + + /* Completed MCU, so update state */ + cinfo->dest->next_output_byte = state.next_output_byte; + cinfo->dest->free_in_buffer = state.free_in_buffer; + ASSIGN_STATE(entropy->saved, state.cur); + + /* Update restart-interval state too */ + if (cinfo->restart_interval) { + if (entropy->restarts_to_go == 0) { + entropy->restarts_to_go = cinfo->restart_interval; + entropy->next_restart_num++; + entropy->next_restart_num &= 7; + } + entropy->restarts_to_go--; + } + + return TRUE; +} + + +/* + * Finish up at the end of a Huffman-compressed scan. + */ + +METHODDEF(void) +finish_pass_huff (j_compress_ptr cinfo) +{ + huff_entropy_ptr entropy = (huff_entropy_ptr) cinfo->entropy; + working_state state; + + /* Load up working state ... flush_bits needs it */ + state.next_output_byte = cinfo->dest->next_output_byte; + state.free_in_buffer = cinfo->dest->free_in_buffer; + ASSIGN_STATE(state.cur, entropy->saved); + state.cinfo = cinfo; + + /* Flush out the last data */ + if (! flush_bits(&state)) + ERREXIT(cinfo, JERR_CANT_SUSPEND); + + /* Update state */ + cinfo->dest->next_output_byte = state.next_output_byte; + cinfo->dest->free_in_buffer = state.free_in_buffer; + ASSIGN_STATE(entropy->saved, state.cur); +} + + +/* + * Huffman coding optimization. + * + * We first scan the supplied data and count the number of uses of each symbol + * that is to be Huffman-coded. (This process MUST agree with the code above.) + * Then we build a Huffman coding tree for the observed counts. + * Symbols which are not needed at all for the particular image are not + * assigned any code, which saves space in the DHT marker as well as in + * the compressed data. + */ + +#ifdef ENTROPY_OPT_SUPPORTED + + +/* Process a single block's worth of coefficients */ + +LOCAL(void) +htest_one_block (j_compress_ptr cinfo, JCOEFPTR block, int last_dc_val, + long dc_counts[], long ac_counts[]) +{ + register int temp; + register int nbits; + register int k, r; + + /* Encode the DC coefficient difference per section F.1.2.1 */ + + temp = block[0] - last_dc_val; + if (temp < 0) + temp = -temp; + + /* Find the number of bits needed for the magnitude of the coefficient */ + nbits = 0; + while (temp) { + nbits++; + temp >>= 1; + } + /* Check for out-of-range coefficient values. + * Since we're encoding a difference, the range limit is twice as much. + */ + if (nbits > MAX_COEF_BITS+1) + ERREXIT(cinfo, JERR_BAD_DCT_COEF); + + /* Count the Huffman symbol for the number of bits */ + dc_counts[nbits]++; + + /* Encode the AC coefficients per section F.1.2.2 */ + + r = 0; /* r = run length of zeros */ + + for (k = 1; k < DCTSIZE2; k++) { + if ((temp = block[jpeg_natural_order[k]]) == 0) { + r++; + } else { + /* if run length > 15, must emit special run-length-16 codes (0xF0) */ + while (r > 15) { + ac_counts[0xF0]++; + r -= 16; + } + + /* Find the number of bits needed for the magnitude of the coefficient */ + if (temp < 0) + temp = -temp; + + /* Find the number of bits needed for the magnitude of the coefficient */ + nbits = 1; /* there must be at least one 1 bit */ + while ((temp >>= 1)) + nbits++; + /* Check for out-of-range coefficient values */ + if (nbits > MAX_COEF_BITS) + ERREXIT(cinfo, JERR_BAD_DCT_COEF); + + /* Count Huffman symbol for run length / number of bits */ + ac_counts[(r << 4) + nbits]++; + + r = 0; + } + } + + /* If the last coef(s) were zero, emit an end-of-block code */ + if (r > 0) + ac_counts[0]++; +} + + +/* + * Trial-encode one MCU's worth of Huffman-compressed coefficients. + * No data is actually output, so no suspension return is possible. + */ + +METHODDEF(boolean) +encode_mcu_gather (j_compress_ptr cinfo, JBLOCKROW *MCU_data) +{ + huff_entropy_ptr entropy = (huff_entropy_ptr) cinfo->entropy; + int blkn, ci; + jpeg_component_info * compptr; + + /* Take care of restart intervals if needed */ + if (cinfo->restart_interval) { + if (entropy->restarts_to_go == 0) { + /* Re-initialize DC predictions to 0 */ + for (ci = 0; ci < cinfo->comps_in_scan; ci++) + entropy->saved.last_dc_val[ci] = 0; + /* Update restart state */ + entropy->restarts_to_go = cinfo->restart_interval; + } + entropy->restarts_to_go--; + } + + for (blkn = 0; blkn < cinfo->blocks_in_MCU; blkn++) { + ci = cinfo->MCU_membership[blkn]; + compptr = cinfo->cur_comp_info[ci]; + htest_one_block(cinfo, MCU_data[blkn][0], entropy->saved.last_dc_val[ci], + entropy->dc_count_ptrs[compptr->dc_tbl_no], + entropy->ac_count_ptrs[compptr->ac_tbl_no]); + entropy->saved.last_dc_val[ci] = MCU_data[blkn][0][0]; + } + + return TRUE; +} + + +/* + * Generate the best Huffman code table for the given counts, fill htbl. + * Note this is also used by jcphuff.c. + * + * The JPEG standard requires that no symbol be assigned a codeword of all + * one bits (so that padding bits added at the end of a compressed segment + * can't look like a valid code). Because of the canonical ordering of + * codewords, this just means that there must be an unused slot in the + * longest codeword length category. Section K.2 of the JPEG spec suggests + * reserving such a slot by pretending that symbol 256 is a valid symbol + * with count 1. In theory that's not optimal; giving it count zero but + * including it in the symbol set anyway should give a better Huffman code. + * But the theoretically better code actually seems to come out worse in + * practice, because it produces more all-ones bytes (which incur stuffed + * zero bytes in the final file). In any case the difference is tiny. + * + * The JPEG standard requires Huffman codes to be no more than 16 bits long. + * If some symbols have a very small but nonzero probability, the Huffman tree + * must be adjusted to meet the code length restriction. We currently use + * the adjustment method suggested in JPEG section K.2. This method is *not* + * optimal; it may not choose the best possible limited-length code. But + * typically only very-low-frequency symbols will be given less-than-optimal + * lengths, so the code is almost optimal. Experimental comparisons against + * an optimal limited-length-code algorithm indicate that the difference is + * microscopic --- usually less than a hundredth of a percent of total size. + * So the extra complexity of an optimal algorithm doesn't seem worthwhile. + */ + +GLOBAL(void) +jpeg_gen_optimal_table (j_compress_ptr cinfo, JHUFF_TBL * htbl, long freq[]) +{ +#define MAX_CLEN 32 /* assumed maximum initial code length */ + UINT8 bits[MAX_CLEN+1]; /* bits[k] = # of symbols with code length k */ + int codesize[257]; /* codesize[k] = code length of symbol k */ + int others[257]; /* next symbol in current branch of tree */ + int c1, c2; + int p, i, j; + long v; + + /* This algorithm is explained in section K.2 of the JPEG standard */ + + MEMZERO(bits, SIZEOF(bits)); + MEMZERO(codesize, SIZEOF(codesize)); + for (i = 0; i < 257; i++) + others[i] = -1; /* init links to empty */ + + freq[256] = 1; /* make sure 256 has a nonzero count */ + /* Including the pseudo-symbol 256 in the Huffman procedure guarantees + * that no real symbol is given code-value of all ones, because 256 + * will be placed last in the largest codeword category. + */ + + /* Huffman's basic algorithm to assign optimal code lengths to symbols */ + + for (;;) { + /* Find the smallest nonzero frequency, set c1 = its symbol */ + /* In case of ties, take the larger symbol number */ + c1 = -1; + v = 1000000000L; + for (i = 0; i <= 256; i++) { + if (freq[i] && freq[i] <= v) { + v = freq[i]; + c1 = i; + } + } + + /* Find the next smallest nonzero frequency, set c2 = its symbol */ + /* In case of ties, take the larger symbol number */ + c2 = -1; + v = 1000000000L; + for (i = 0; i <= 256; i++) { + if (freq[i] && freq[i] <= v && i != c1) { + v = freq[i]; + c2 = i; + } + } + + /* Done if we've merged everything into one frequency */ + if (c2 < 0) + break; + + /* Else merge the two counts/trees */ + freq[c1] += freq[c2]; + freq[c2] = 0; + + /* Increment the codesize of everything in c1's tree branch */ + codesize[c1]++; + while (others[c1] >= 0) { + c1 = others[c1]; + codesize[c1]++; + } + + others[c1] = c2; /* chain c2 onto c1's tree branch */ + + /* Increment the codesize of everything in c2's tree branch */ + codesize[c2]++; + while (others[c2] >= 0) { + c2 = others[c2]; + codesize[c2]++; + } + } + + /* Now count the number of symbols of each code length */ + for (i = 0; i <= 256; i++) { + if (codesize[i]) { + /* The JPEG standard seems to think that this can't happen, */ + /* but I'm paranoid... */ + if (codesize[i] > MAX_CLEN) + ERREXIT(cinfo, JERR_HUFF_CLEN_OVERFLOW); + + bits[codesize[i]]++; + } + } + + /* JPEG doesn't allow symbols with code lengths over 16 bits, so if the pure + * Huffman procedure assigned any such lengths, we must adjust the coding. + * Here is what the JPEG spec says about how this next bit works: + * Since symbols are paired for the longest Huffman code, the symbols are + * removed from this length category two at a time. The prefix for the pair + * (which is one bit shorter) is allocated to one of the pair; then, + * skipping the BITS entry for that prefix length, a code word from the next + * shortest nonzero BITS entry is converted into a prefix for two code words + * one bit longer. + */ + + for (i = MAX_CLEN; i > 16; i--) { + while (bits[i] > 0) { + j = i - 2; /* find length of new prefix to be used */ + while (bits[j] == 0) + j--; + + bits[i] -= 2; /* remove two symbols */ + bits[i-1]++; /* one goes in this length */ + bits[j+1] += 2; /* two new symbols in this length */ + bits[j]--; /* symbol of this length is now a prefix */ + } + } + + /* Remove the count for the pseudo-symbol 256 from the largest codelength */ + while (bits[i] == 0) /* find largest codelength still in use */ + i--; + bits[i]--; + + /* Return final symbol counts (only for lengths 0..16) */ + MEMCOPY(htbl->bits, bits, SIZEOF(htbl->bits)); + + /* Return a list of the symbols sorted by code length */ + /* It's not real clear to me why we don't need to consider the codelength + * changes made above, but the JPEG spec seems to think this works. + */ + p = 0; + for (i = 1; i <= MAX_CLEN; i++) { + for (j = 0; j <= 255; j++) { + if (codesize[j] == i) { + htbl->huffval[p] = (UINT8) j; + p++; + } + } + } + + /* Set sent_table FALSE so updated table will be written to JPEG file. */ + htbl->sent_table = FALSE; +} + + +/* + * Finish up a statistics-gathering pass and create the new Huffman tables. + */ + +METHODDEF(void) +finish_pass_gather (j_compress_ptr cinfo) +{ + huff_entropy_ptr entropy = (huff_entropy_ptr) cinfo->entropy; + int ci, dctbl, actbl; + jpeg_component_info * compptr; + JHUFF_TBL **htblptr; + boolean did_dc[NUM_HUFF_TBLS]; + boolean did_ac[NUM_HUFF_TBLS]; + + /* It's important not to apply jpeg_gen_optimal_table more than once + * per table, because it clobbers the input frequency counts! + */ + MEMZERO(did_dc, SIZEOF(did_dc)); + MEMZERO(did_ac, SIZEOF(did_ac)); + + for (ci = 0; ci < cinfo->comps_in_scan; ci++) { + compptr = cinfo->cur_comp_info[ci]; + dctbl = compptr->dc_tbl_no; + actbl = compptr->ac_tbl_no; + if (! did_dc[dctbl]) { + htblptr = & cinfo->dc_huff_tbl_ptrs[dctbl]; + if (*htblptr == NULL) + *htblptr = jpeg_alloc_huff_table((j_common_ptr) cinfo); + jpeg_gen_optimal_table(cinfo, *htblptr, entropy->dc_count_ptrs[dctbl]); + did_dc[dctbl] = TRUE; + } + if (! did_ac[actbl]) { + htblptr = & cinfo->ac_huff_tbl_ptrs[actbl]; + if (*htblptr == NULL) + *htblptr = jpeg_alloc_huff_table((j_common_ptr) cinfo); + jpeg_gen_optimal_table(cinfo, *htblptr, entropy->ac_count_ptrs[actbl]); + did_ac[actbl] = TRUE; + } + } +} + + +#endif /* ENTROPY_OPT_SUPPORTED */ + + +/* + * Module initialization routine for Huffman entropy encoding. + */ + +GLOBAL(void) +jinit_huff_encoder (j_compress_ptr cinfo) +{ + huff_entropy_ptr entropy; + int i; + + entropy = (huff_entropy_ptr) + (*cinfo->mem->alloc_small) ((j_common_ptr) cinfo, JPOOL_IMAGE, + SIZEOF(huff_entropy_encoder)); + cinfo->entropy = (struct jpeg_entropy_encoder *) entropy; + entropy->pub.start_pass = start_pass_huff; + + /* Mark tables unallocated */ + for (i = 0; i < NUM_HUFF_TBLS; i++) { + entropy->dc_derived_tbls[i] = entropy->ac_derived_tbls[i] = NULL; +#ifdef ENTROPY_OPT_SUPPORTED + entropy->dc_count_ptrs[i] = entropy->ac_count_ptrs[i] = NULL; +#endif + } +} |