/* * jchuff.c * * This file was part of the Independent JPEG Group's software: * Copyright (C) 1991-1997, Thomas G. Lane. * Modifications: * Copyright (C) 2009-2011, D. R. Commander. * 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 */ #include static unsigned char jpeg_nbits_table[65536]; static int jpeg_nbits_table_init = 0; #ifndef min #define min(a,b) ((a)<(b)?(a):(b)) #endif /* 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 { size_t 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]; } if(!jpeg_nbits_table_init) { for(i = 0; i < 65536; i++) { int nbits = 0, temp = i; while (temp) {temp >>= 1; nbits++;} jpeg_nbits_table[i] = nbits; } jpeg_nbits_table_init = 1; } } /* 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 */ /* These macros perform the same task as the emit_bits() function in the * original libjpeg code. In addition to reducing overhead by explicitly * inlining the code, additional performance is achieved by taking into * account the size of the bit buffer and waiting until it is almost full * before emptying it. This mostly benefits 64-bit platforms, since 6 * bytes can be stored in a 64-bit bit buffer before it has to be emptied. */ #define EMIT_BYTE() { \ JOCTET c; \ put_bits -= 8; \ c = (JOCTET)GETJOCTET(put_buffer >> put_bits); \ *buffer++ = c; \ if (c == 0xFF) /* need to stuff a zero byte? */ \ *buffer++ = 0; \ } #define PUT_BITS(code, size) { \ put_bits += size; \ put_buffer = (put_buffer << size) | code; \ } #define CHECKBUF15() { \ if (put_bits > 15) { \ EMIT_BYTE() \ EMIT_BYTE() \ } \ } #define CHECKBUF31() { \ if (put_bits > 31) { \ EMIT_BYTE() \ EMIT_BYTE() \ EMIT_BYTE() \ EMIT_BYTE() \ } \ } #define CHECKBUF47() { \ if (put_bits > 47) { \ EMIT_BYTE() \ EMIT_BYTE() \ EMIT_BYTE() \ EMIT_BYTE() \ EMIT_BYTE() \ EMIT_BYTE() \ } \ } #if __WORDSIZE==64 || defined(_WIN64) #define EMIT_BITS(code, size) { \ CHECKBUF47() \ PUT_BITS(code, size) \ } #define EMIT_CODE(code, size) { \ temp2 &= (((INT32) 1)<free_in_buffer < BUFSIZE) { \ localbuf = 1; \ buffer = _buffer; \ } \ else buffer = state->next_output_byte; \ } #define STORE_BUFFER() { \ if (localbuf) { \ bytes = buffer - _buffer; \ buffer = _buffer; \ while (bytes > 0) { \ bytestocopy = min(bytes, state->free_in_buffer); \ MEMCOPY(state->next_output_byte, buffer, bytestocopy); \ state->next_output_byte += bytestocopy; \ buffer += bytestocopy; \ state->free_in_buffer -= bytestocopy; \ if (state->free_in_buffer == 0) \ if (! dump_buffer(state)) return FALSE; \ bytes -= bytestocopy; \ } \ } \ else { \ state->free_in_buffer -= (buffer - state->next_output_byte); \ state->next_output_byte = buffer; \ } \ } LOCAL(boolean) flush_bits (working_state * state) { JOCTET _buffer[BUFSIZE], *buffer; size_t put_buffer; int put_bits; size_t bytes, bytestocopy; int localbuf = 0; put_buffer = state->cur.put_buffer; put_bits = state->cur.put_bits; LOAD_BUFFER() /* fill any partial byte with ones */ PUT_BITS(0x7F, 7) while (put_bits >= 8) EMIT_BYTE() state->cur.put_buffer = 0; /* and reset bit-buffer to empty */ state->cur.put_bits = 0; STORE_BUFFER() 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) { int temp, temp2, temp3; int nbits; int r, code, size; JOCTET _buffer[BUFSIZE], *buffer; size_t put_buffer; int put_bits; int code_0xf0 = actbl->ehufco[0xf0], size_0xf0 = actbl->ehufsi[0xf0]; size_t bytes, bytestocopy; int localbuf = 0; put_buffer = state->cur.put_buffer; put_bits = state->cur.put_bits; LOAD_BUFFER() /* Encode the DC coefficient difference per section F.1.2.1 */ temp = temp2 = block[0] - last_dc_val; /* This is a well-known technique for obtaining the absolute value without a * branch. It is derived from an assembly language technique presented in * "How to Optimize for the Pentium Processors", Copyright (c) 1996, 1997 by * Agner Fog. */ temp3 = temp >> (CHAR_BIT * sizeof(int) - 1); temp ^= temp3; temp -= temp3; /* For a negative input, want temp2 = bitwise complement of abs(input) */ /* This code assumes we are on a two's complement machine */ temp2 += temp3; /* Find the number of bits needed for the magnitude of the coefficient */ nbits = jpeg_nbits_table[temp]; /* Emit the Huffman-coded symbol for the number of bits */ code = dctbl->ehufco[nbits]; size = dctbl->ehufsi[nbits]; PUT_BITS(code, size) CHECKBUF15() /* Mask off any extra bits in code */ temp2 &= (((INT32) 1)<> (CHAR_BIT * sizeof(int) - 1); \ temp ^= temp3; \ temp -= temp3; \ temp2 += temp3; \ nbits = jpeg_nbits_table[temp]; \ /* if run length > 15, must emit special run-length-16 codes (0xF0) */ \ while (r > 15) { \ EMIT_BITS(code_0xf0, size_0xf0) \ r -= 16; \ } \ /* Emit Huffman symbol for run length / number of bits */ \ temp3 = (r << 4) + nbits; \ code = actbl->ehufco[temp3]; \ size = actbl->ehufsi[temp3]; \ EMIT_CODE(code, size) \ r = 0; \ } \ } /* One iteration for each value in jpeg_natural_order[] */ kloop(1); kloop(8); kloop(16); kloop(9); kloop(2); kloop(3); kloop(10); kloop(17); kloop(24); kloop(32); kloop(25); kloop(18); kloop(11); kloop(4); kloop(5); kloop(12); kloop(19); kloop(26); kloop(33); kloop(40); kloop(48); kloop(41); kloop(34); kloop(27); kloop(20); kloop(13); kloop(6); kloop(7); kloop(14); kloop(21); kloop(28); kloop(35); kloop(42); kloop(49); kloop(56); kloop(57); kloop(50); kloop(43); kloop(36); kloop(29); kloop(22); kloop(15); kloop(23); kloop(30); kloop(37); kloop(44); kloop(51); kloop(58); kloop(59); kloop(52); kloop(45); kloop(38); kloop(31); kloop(39); kloop(46); kloop(53); kloop(60); kloop(61); kloop(54); kloop(47); kloop(55); kloop(62); kloop(63); /* If the last coef(s) were zero, emit an end-of-block code */ if (r > 0) { code = actbl->ehufco[0]; size = actbl->ehufsi[0]; EMIT_BITS(code, size) } state->cur.put_buffer = put_buffer; state->cur.put_bits = put_bits; STORE_BUFFER() 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 } }