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mozjpeg/jchuff.c

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32 KiB
C
Исходник Ответственный История

/*
* jchuff.c
*
* This file was part of the Independent JPEG Group's software:
* Copyright (C) 1991-1997, Thomas G. Lane.
* libjpeg-turbo Modifications:
* Copyright (C) 2009-2011, 2014-2015 D. R. Commander.
* For conditions of distribution and use, see the accompanying README.ijg
* 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 <limits.h>
/*
* NOTE: If USE_CLZ_INTRINSIC is defined, then clz/bsr instructions will be
* used for bit counting rather than the lookup table. This will reduce the
* memory footprint by 64k, which is important for some mobile applications
* that create many isolated instances of libjpeg-turbo (web browsers, for
* instance.) This may improve performance on some mobile platforms as well.
* This feature is enabled by default only on ARM processors, because some x86
* chips have a slow implementation of bsr, and the use of clz/bsr cannot be
* shown to have a significant performance impact even on the x86 chips that
* have a fast implementation of it. When building for ARMv6, you can
* explicitly disable the use of clz/bsr by adding -mthumb to the compiler
* flags (this defines __thumb__).
*/
/* NOTE: Both GCC and Clang define __GNUC__ */
#if defined __GNUC__ && (defined __arm__ || defined __aarch64__)
#if !defined __thumb__ || defined __thumb2__
#define USE_CLZ_INTRINSIC
#endif
#endif
#ifdef USE_CLZ_INTRINSIC
#define JPEG_NBITS_NONZERO(x) (32 - __builtin_clz(x))
#define JPEG_NBITS(x) (x ? JPEG_NBITS_NONZERO(x) : 0)
#else
#include "jpeg_nbits_table.h"
#define JPEG_NBITS(x) (jpeg_nbits_table[x])
#define JPEG_NBITS_NONZERO(x) JPEG_NBITS(x)
#endif
#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 (j_compress_ptr cinfo, JBLOCKROW *MCU_data);
METHODDEF(void) finish_pass_huff (j_compress_ptr cinfo);
#ifdef ENTROPY_OPT_SUPPORTED
METHODDEF(boolean) encode_mcu_gather (j_compress_ptr cinfo,
JBLOCKROW *MCU_data);
METHODDEF(void) finish_pass_gather (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 */
/* 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 !defined(_WIN32) && !defined(SIZEOF_SIZE_T)
#error Cannot determine word size
#endif
#if SIZEOF_SIZE_T==8 || defined(_WIN64)
#define EMIT_BITS(code, size) { \
CHECKBUF47() \
PUT_BITS(code, size) \
}
#define EMIT_CODE(code, size) { \
temp2 &= (((INT32) 1)<<nbits) - 1; \
CHECKBUF31() \
PUT_BITS(code, size) \
PUT_BITS(temp2, nbits) \
}
#else
#define EMIT_BITS(code, size) { \
PUT_BITS(code, size) \
CHECKBUF15() \
}
#define EMIT_CODE(code, size) { \
temp2 &= (((INT32) 1)<<nbits) - 1; \
PUT_BITS(code, size) \
CHECKBUF15() \
PUT_BITS(temp2, nbits) \
CHECKBUF15() \
}
#endif
/* Although it is exceedingly rare, it is possible for a Huffman-encoded
* coefficient block to be larger than the 128-byte unencoded block. For each
* of the 64 coefficients, PUT_BITS is invoked twice, and each invocation can
* theoretically store 16 bits (for a maximum of 2048 bits or 256 bytes per
* encoded block.) If, for instance, one artificially sets the AC
* coefficients to alternating values of 32767 and -32768 (using the JPEG
* scanning order-- 1, 8, 16, etc.), then this will produce an encoded block
* larger than 200 bytes.
*/
#define BUFSIZE (DCTSIZE2 * 4)
#define LOAD_BUFFER() { \
if (state->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(temp);
/* Emit the Huffman-coded symbol for the number of bits */
code = dctbl->ehufco[nbits];
size = dctbl->ehufsi[nbits];
EMIT_BITS(code, size)
/* Mask off any extra bits in code */
temp2 &= (((INT32) 1)<<nbits) - 1;
/* Emit that number of bits of the value, if positive, */
/* or the complement of its magnitude, if negative. */
EMIT_BITS(temp2, nbits)
/* Encode the AC coefficients per section F.1.2.2 */
r = 0; /* r = run length of zeros */
/* Manually unroll the k loop to eliminate the counter variable. This
* improves performance greatly on systems with a limited number of
* registers (such as x86.)
*/
#define kloop(jpeg_natural_order_of_k) { \
if ((temp = block[jpeg_natural_order_of_k]) == 0) { \
r++; \
} else { \
temp2 = temp; \
/* Branch-less absolute value, bitwise complement, etc., same as above */ \
temp3 = temp >> (CHAR_BIT * sizeof(int) - 1); \
temp ^= temp3; \
temp -= temp3; \
temp2 += temp3; \
nbits = JPEG_NBITS_NONZERO(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
}
}
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