Source code
Revision control
Copy as Markdown
Other Tools
// Copyright 2012 Google Inc. All Rights Reserved.
//
// Use of this source code is governed by a BSD-style license
// that can be found in the COPYING file in the root of the source
// tree. An additional intellectual property rights grant can be found
// in the file PATENTS. All contributing project authors may
// be found in the AUTHORS file in the root of the source tree.
// -----------------------------------------------------------------------------
//
// Author: Jyrki Alakuijala (jyrki@google.com)
//
#ifdef HAVE_CONFIG_H
#include "src/webp/config.h"
#endif
#include <assert.h>
#include <stdlib.h>
#include <string.h>
#include "src/dsp/lossless.h"
#include "src/dsp/lossless_common.h"
#include "src/enc/backward_references_enc.h"
#include "src/enc/histogram_enc.h"
#include "src/enc/vp8i_enc.h"
#include "src/utils/utils.h"
#include "src/webp/encode.h"
#include "src/webp/format_constants.h"
#include "src/webp/types.h"
// Number of partitions for the three dominant (literal, red and blue) symbol
// costs.
#define NUM_PARTITIONS 4
// The size of the bin-hash corresponding to the three dominant costs.
#define BIN_SIZE (NUM_PARTITIONS * NUM_PARTITIONS * NUM_PARTITIONS)
// Maximum number of histograms allowed in greedy combining algorithm.
#define MAX_HISTO_GREEDY 100
// Enum to meaningfully access the elements of the Histogram arrays.
typedef enum {
LITERAL = 0,
RED,
BLUE,
ALPHA,
DISTANCE
} HistogramIndex;
// Return the size of the histogram for a given cache_bits.
static int GetHistogramSize(int cache_bits) {
const int literal_size = VP8LHistogramNumCodes(cache_bits);
const size_t total_size = sizeof(VP8LHistogram) + sizeof(int) * literal_size;
assert(total_size <= (size_t)0x7fffffff);
return (int)total_size;
}
static void HistogramStatsClear(VP8LHistogram* const h) {
int i;
for (i = 0; i < 5; ++i) {
h->trivial_symbol[i] = VP8L_NON_TRIVIAL_SYM;
// By default, the histogram is assumed to be used.
h->is_used[i] = 1;
}
h->bit_cost = 0;
memset(h->costs, 0, sizeof(h->costs));
}
static void HistogramClear(VP8LHistogram* const h) {
uint32_t* const literal = h->literal;
const int cache_bits = h->palette_code_bits;
const int histo_size = GetHistogramSize(cache_bits);
memset(h, 0, histo_size);
h->palette_code_bits = cache_bits;
h->literal = literal;
HistogramStatsClear(h);
}
// Swap two histogram pointers.
static void HistogramSwap(VP8LHistogram** const h1, VP8LHistogram** const h2) {
VP8LHistogram* const tmp = *h1;
*h1 = *h2;
*h2 = tmp;
}
static void HistogramCopy(const VP8LHistogram* const src,
VP8LHistogram* const dst) {
uint32_t* const dst_literal = dst->literal;
const int dst_cache_bits = dst->palette_code_bits;
const int literal_size = VP8LHistogramNumCodes(dst_cache_bits);
const int histo_size = GetHistogramSize(dst_cache_bits);
assert(src->palette_code_bits == dst_cache_bits);
memcpy(dst, src, histo_size);
dst->literal = dst_literal;
memcpy(dst->literal, src->literal, literal_size * sizeof(*dst->literal));
}
void VP8LFreeHistogram(VP8LHistogram* const h) { WebPSafeFree(h); }
void VP8LFreeHistogramSet(VP8LHistogramSet* const histograms) {
WebPSafeFree(histograms);
}
void VP8LHistogramCreate(VP8LHistogram* const h,
const VP8LBackwardRefs* const refs,
int palette_code_bits) {
if (palette_code_bits >= 0) {
h->palette_code_bits = palette_code_bits;
}
HistogramClear(h);
VP8LHistogramStoreRefs(refs, /*distance_modifier=*/NULL,
/*distance_modifier_arg0=*/0, h);
}
void VP8LHistogramInit(VP8LHistogram* const h, int palette_code_bits,
int init_arrays) {
h->palette_code_bits = palette_code_bits;
if (init_arrays) {
HistogramClear(h);
} else {
HistogramStatsClear(h);
}
}
VP8LHistogram* VP8LAllocateHistogram(int cache_bits) {
VP8LHistogram* histo = NULL;
const int total_size = GetHistogramSize(cache_bits);
uint8_t* const memory = (uint8_t*)WebPSafeMalloc(total_size, sizeof(*memory));
if (memory == NULL) return NULL;
histo = (VP8LHistogram*)memory;
// 'literal' won't necessary be aligned.
histo->literal = (uint32_t*)(memory + sizeof(VP8LHistogram));
VP8LHistogramInit(histo, cache_bits, /*init_arrays=*/ 0);
return histo;
}
// Resets the pointers of the histograms to point to the bit buffer in the set.
static void HistogramSetResetPointers(VP8LHistogramSet* const set,
int cache_bits) {
int i;
const int histo_size = GetHistogramSize(cache_bits);
uint8_t* memory = (uint8_t*) (set->histograms);
memory += set->max_size * sizeof(*set->histograms);
for (i = 0; i < set->max_size; ++i) {
memory = (uint8_t*) WEBP_ALIGN(memory);
set->histograms[i] = (VP8LHistogram*) memory;
// 'literal' won't necessary be aligned.
set->histograms[i]->literal = (uint32_t*)(memory + sizeof(VP8LHistogram));
memory += histo_size;
}
}
// Returns the total size of the VP8LHistogramSet.
static size_t HistogramSetTotalSize(int size, int cache_bits) {
const int histo_size = GetHistogramSize(cache_bits);
return (sizeof(VP8LHistogramSet) + size * (sizeof(VP8LHistogram*) +
histo_size + WEBP_ALIGN_CST));
}
VP8LHistogramSet* VP8LAllocateHistogramSet(int size, int cache_bits) {
int i;
VP8LHistogramSet* set;
const size_t total_size = HistogramSetTotalSize(size, cache_bits);
uint8_t* memory = (uint8_t*)WebPSafeMalloc(total_size, sizeof(*memory));
if (memory == NULL) return NULL;
set = (VP8LHistogramSet*)memory;
memory += sizeof(*set);
set->histograms = (VP8LHistogram**)memory;
set->max_size = size;
set->size = size;
HistogramSetResetPointers(set, cache_bits);
for (i = 0; i < size; ++i) {
VP8LHistogramInit(set->histograms[i], cache_bits, /*init_arrays=*/ 0);
}
return set;
}
void VP8LHistogramSetClear(VP8LHistogramSet* const set) {
int i;
const int cache_bits = set->histograms[0]->palette_code_bits;
const int size = set->max_size;
const size_t total_size = HistogramSetTotalSize(size, cache_bits);
uint8_t* memory = (uint8_t*)set;
memset(memory, 0, total_size);
memory += sizeof(*set);
set->histograms = (VP8LHistogram**)memory;
set->max_size = size;
set->size = size;
HistogramSetResetPointers(set, cache_bits);
for (i = 0; i < size; ++i) {
set->histograms[i]->palette_code_bits = cache_bits;
}
}
// Removes the histogram 'i' from 'set'.
static void HistogramSetRemoveHistogram(VP8LHistogramSet* const set, int i) {
set->histograms[i] = set->histograms[set->size - 1];
--set->size;
assert(set->size > 0);
}
// -----------------------------------------------------------------------------
static void HistogramAddSinglePixOrCopy(
VP8LHistogram* const histo, const PixOrCopy* const v,
int (*const distance_modifier)(int, int), int distance_modifier_arg0) {
if (PixOrCopyIsLiteral(v)) {
++histo->alpha[PixOrCopyLiteral(v, 3)];
++histo->red[PixOrCopyLiteral(v, 2)];
++histo->literal[PixOrCopyLiteral(v, 1)];
++histo->blue[PixOrCopyLiteral(v, 0)];
} else if (PixOrCopyIsCacheIdx(v)) {
const int literal_ix =
NUM_LITERAL_CODES + NUM_LENGTH_CODES + PixOrCopyCacheIdx(v);
assert(histo->palette_code_bits != 0);
++histo->literal[literal_ix];
} else {
int code, extra_bits;
VP8LPrefixEncodeBits(PixOrCopyLength(v), &code, &extra_bits);
++histo->literal[NUM_LITERAL_CODES + code];
if (distance_modifier == NULL) {
VP8LPrefixEncodeBits(PixOrCopyDistance(v), &code, &extra_bits);
} else {
VP8LPrefixEncodeBits(
distance_modifier(distance_modifier_arg0, PixOrCopyDistance(v)),
&code, &extra_bits);
}
++histo->distance[code];
}
}
void VP8LHistogramStoreRefs(const VP8LBackwardRefs* const refs,
int (*const distance_modifier)(int, int),
int distance_modifier_arg0,
VP8LHistogram* const histo) {
VP8LRefsCursor c = VP8LRefsCursorInit(refs);
while (VP8LRefsCursorOk(&c)) {
HistogramAddSinglePixOrCopy(histo, c.cur_pos, distance_modifier,
distance_modifier_arg0);
VP8LRefsCursorNext(&c);
}
}
// -----------------------------------------------------------------------------
// Entropy-related functions.
static WEBP_INLINE uint64_t BitsEntropyRefine(const VP8LBitEntropy* entropy) {
uint64_t mix;
if (entropy->nonzeros < 5) {
if (entropy->nonzeros <= 1) {
return 0;
}
// Two symbols, they will be 0 and 1 in a Huffman code.
// Let's mix in a bit of entropy to favor good clustering when
// distributions of these are combined.
if (entropy->nonzeros == 2) {
return DivRound(99 * ((uint64_t)entropy->sum << LOG_2_PRECISION_BITS) +
entropy->entropy,
100);
}
// No matter what the entropy says, we cannot be better than min_limit
// with Huffman coding. I am mixing a bit of entropy into the
// min_limit since it produces much better (~0.5 %) compression results
// perhaps because of better entropy clustering.
if (entropy->nonzeros == 3) {
mix = 950;
} else {
mix = 700; // nonzeros == 4.
}
} else {
mix = 627;
}
{
uint64_t min_limit = (uint64_t)(2 * entropy->sum - entropy->max_val)
<< LOG_2_PRECISION_BITS;
min_limit =
DivRound(mix * min_limit + (1000 - mix) * entropy->entropy, 1000);
return (entropy->entropy < min_limit) ? min_limit : entropy->entropy;
}
}
uint64_t VP8LBitsEntropy(const uint32_t* const array, int n) {
VP8LBitEntropy entropy;
VP8LBitsEntropyUnrefined(array, n, &entropy);
return BitsEntropyRefine(&entropy);
}
static uint64_t InitialHuffmanCost(void) {
// Small bias because Huffman code length is typically not stored in
// full length.
static const uint64_t kHuffmanCodeOfHuffmanCodeSize = CODE_LENGTH_CODES * 3;
// Subtract a bias of 9.1.
return (kHuffmanCodeOfHuffmanCodeSize << LOG_2_PRECISION_BITS) -
DivRound(91ll << LOG_2_PRECISION_BITS, 10);
}
// Finalize the Huffman cost based on streak numbers and length type (<3 or >=3)
static uint64_t FinalHuffmanCost(const VP8LStreaks* const stats) {
// The constants in this function are empirical and got rounded from
// their original values in 1/8 when switched to 1/1024.
uint64_t retval = InitialHuffmanCost();
// Second coefficient: Many zeros in the histogram are covered efficiently
// by a run-length encode. Originally 2/8.
uint32_t retval_extra = stats->counts[0] * 1600 + 240 * stats->streaks[0][1];
// Second coefficient: Constant values are encoded less efficiently, but still
// RLE'ed. Originally 6/8.
retval_extra += stats->counts[1] * 2640 + 720 * stats->streaks[1][1];
// 0s are usually encoded more efficiently than non-0s.
// Originally 15/8.
retval_extra += 1840 * stats->streaks[0][0];
// Originally 26/8.
retval_extra += 3360 * stats->streaks[1][0];
return retval + ((uint64_t)retval_extra << (LOG_2_PRECISION_BITS - 10));
}
// Get the symbol entropy for the distribution 'population'.
// Set 'trivial_sym', if there's only one symbol present in the distribution.
static uint64_t PopulationCost(const uint32_t* const population, int length,
uint16_t* const trivial_sym,
uint8_t* const is_used) {
VP8LBitEntropy bit_entropy;
VP8LStreaks stats;
VP8LGetEntropyUnrefined(population, length, &bit_entropy, &stats);
if (trivial_sym != NULL) {
*trivial_sym = (bit_entropy.nonzeros == 1) ? bit_entropy.nonzero_code
: VP8L_NON_TRIVIAL_SYM;
}
if (is_used != NULL) {
// The histogram is used if there is at least one non-zero streak.
*is_used = (stats.streaks[1][0] != 0 || stats.streaks[1][1] != 0);
}
return BitsEntropyRefine(&bit_entropy) + FinalHuffmanCost(&stats);
}
static WEBP_INLINE void GetPopulationInfo(const VP8LHistogram* const histo,
HistogramIndex index,
const uint32_t** population,
int* length) {
switch (index) {
case LITERAL:
*population = histo->literal;
*length = VP8LHistogramNumCodes(histo->palette_code_bits);
break;
case RED:
*population = histo->red;
*length = NUM_LITERAL_CODES;
break;
case BLUE:
*population = histo->blue;
*length = NUM_LITERAL_CODES;
break;
case ALPHA:
*population = histo->alpha;
*length = NUM_LITERAL_CODES;
break;
case DISTANCE:
*population = histo->distance;
*length = NUM_DISTANCE_CODES;
break;
}
}
// trivial_at_end is 1 if the two histograms only have one element that is
// non-zero: both the zero-th one, or both the last one.
// 'index' is the index of the symbol in the histogram (literal, red, blue,
// alpha, distance).
static WEBP_INLINE uint64_t GetCombinedEntropy(const VP8LHistogram* const h1,
const VP8LHistogram* const h2,
HistogramIndex index) {
const uint32_t* X;
const uint32_t* Y;
int length;
VP8LStreaks stats;
VP8LBitEntropy bit_entropy;
const int is_h1_used = h1->is_used[index];
const int is_h2_used = h2->is_used[index];
const int is_trivial = h1->trivial_symbol[index] != VP8L_NON_TRIVIAL_SYM &&
h1->trivial_symbol[index] == h2->trivial_symbol[index];
if (is_trivial || !is_h1_used || !is_h2_used) {
if (is_h1_used) return h1->costs[index];
return h2->costs[index];
}
assert(is_h1_used && is_h2_used);
GetPopulationInfo(h1, index, &X, &length);
GetPopulationInfo(h2, index, &Y, &length);
VP8LGetCombinedEntropyUnrefined(X, Y, length, &bit_entropy, &stats);
return BitsEntropyRefine(&bit_entropy) + FinalHuffmanCost(&stats);
}
// Estimates the Entropy + Huffman + other block overhead size cost.
uint64_t VP8LHistogramEstimateBits(const VP8LHistogram* const h) {
int i;
uint64_t cost = 0;
for (i = 0; i < 5; ++i) {
int length;
const uint32_t* population;
GetPopulationInfo(h, (HistogramIndex)i, &population, &length);
cost += PopulationCost(population, length, /*trivial_sym=*/NULL,
/*is_used=*/NULL);
}
cost += ((uint64_t)(VP8LExtraCost(h->literal + NUM_LITERAL_CODES,
NUM_LENGTH_CODES) +
VP8LExtraCost(h->distance, NUM_DISTANCE_CODES))
<< LOG_2_PRECISION_BITS);
return cost;
}
// -----------------------------------------------------------------------------
// Various histogram combine/cost-eval functions
// Set a + b in b, saturating at WEBP_INT64_MAX.
static WEBP_INLINE void SaturateAdd(uint64_t a, int64_t* b) {
if (*b < 0 || (int64_t)a <= WEBP_INT64_MAX - *b) {
*b += (int64_t)a;
} else {
*b = WEBP_INT64_MAX;
}
}
// Returns 1 if the cost of the combined histogram is less than the threshold.
// Otherwise returns 0 and the cost is invalid due to early bail-out.
WEBP_NODISCARD static int GetCombinedHistogramEntropy(
const VP8LHistogram* const a, const VP8LHistogram* const b,
int64_t cost_threshold_in, uint64_t* cost, uint64_t costs[5]) {
int i;
const uint64_t cost_threshold = (uint64_t)cost_threshold_in;
assert(a->palette_code_bits == b->palette_code_bits);
if (cost_threshold_in <= 0) return 0;
*cost = 0;
// No need to add the extra cost for length and distance as it is a constant
// that does not influence the histograms.
for (i = 0; i < 5; ++i) {
costs[i] = GetCombinedEntropy(a, b, (HistogramIndex)i);
*cost += costs[i];
if (*cost >= cost_threshold) return 0;
}
return 1;
}
static WEBP_INLINE void HistogramAdd(const VP8LHistogram* const h1,
const VP8LHistogram* const h2,
VP8LHistogram* const hout) {
int i;
assert(h1->palette_code_bits == h2->palette_code_bits);
for (i = 0; i < 5; ++i) {
int length;
const uint32_t *p1, *p2, *pout_const;
uint32_t* pout;
GetPopulationInfo(h1, (HistogramIndex)i, &p1, &length);
GetPopulationInfo(h2, (HistogramIndex)i, &p2, &length);
GetPopulationInfo(hout, (HistogramIndex)i, &pout_const, &length);
pout = (uint32_t*)pout_const;
if (h2 == hout) {
if (h1->is_used[i]) {
if (hout->is_used[i]) {
VP8LAddVectorEq(p1, pout, length);
} else {
memcpy(pout, p1, length * sizeof(pout[0]));
}
}
} else {
if (h1->is_used[i]) {
if (h2->is_used[i]) {
VP8LAddVector(p1, p2, pout, length);
} else {
memcpy(pout, p1, length * sizeof(pout[0]));
}
} else if (h2->is_used[i]) {
memcpy(pout, p2, length * sizeof(pout[0]));
} else {
memset(pout, 0, length * sizeof(pout[0]));
}
}
}
for (i = 0; i < 5; ++i) {
hout->trivial_symbol[i] = h1->trivial_symbol[i] == h2->trivial_symbol[i]
? h1->trivial_symbol[i]
: VP8L_NON_TRIVIAL_SYM;
hout->is_used[i] = h1->is_used[i] || h2->is_used[i];
}
}
static void UpdateHistogramCost(uint64_t bit_cost, uint64_t costs[5],
VP8LHistogram* const h) {
int i;
h->bit_cost = bit_cost;
for (i = 0; i < 5; ++i) {
h->costs[i] = costs[i];
}
}
// Performs out = a + b, computing the cost C(a+b) - C(a) - C(b) while comparing
// to the threshold value 'cost_threshold'. The score returned is
// Score = C(a+b) - C(a) - C(b), where C(a) + C(b) is known and fixed.
// Since the previous score passed is 'cost_threshold', we only need to compare
// the partial cost against 'cost_threshold + C(a) + C(b)' to possibly bail-out
// early.
// Returns 1 if the cost is less than the threshold.
// Otherwise returns 0 and the cost is invalid due to early bail-out.
WEBP_NODISCARD static int HistogramAddEval(const VP8LHistogram* const a,
const VP8LHistogram* const b,
VP8LHistogram* const out,
int64_t cost_threshold) {
const uint64_t sum_cost = a->bit_cost + b->bit_cost;
uint64_t bit_cost, costs[5];
SaturateAdd(sum_cost, &cost_threshold);
if (!GetCombinedHistogramEntropy(a, b, cost_threshold, &bit_cost, costs)) {
return 0;
}
HistogramAdd(a, b, out);
UpdateHistogramCost(bit_cost, costs, out);
return 1;
}
// Same as HistogramAddEval(), except that the resulting histogram
// is not stored. Only the cost C(a+b) - C(a) is evaluated. We omit
// the term C(b) which is constant over all the evaluations.
// Returns 1 if the cost is less than the threshold.
// Otherwise returns 0 and the cost is invalid due to early bail-out.
WEBP_NODISCARD static int HistogramAddThresh(const VP8LHistogram* const a,
const VP8LHistogram* const b,
int64_t cost_threshold,
int64_t* cost_out) {
uint64_t cost, costs[5];
assert(a != NULL && b != NULL);
SaturateAdd(a->bit_cost, &cost_threshold);
if (!GetCombinedHistogramEntropy(a, b, cost_threshold, &cost, costs)) {
return 0;
}
*cost_out = (int64_t)cost - (int64_t)a->bit_cost;
return 1;
}
// -----------------------------------------------------------------------------
// The structure to keep track of cost range for the three dominant entropy
// symbols.
typedef struct {
uint64_t literal_max;
uint64_t literal_min;
uint64_t red_max;
uint64_t red_min;
uint64_t blue_max;
uint64_t blue_min;
} DominantCostRange;
static void DominantCostRangeInit(DominantCostRange* const c) {
c->literal_max = 0;
c->literal_min = WEBP_UINT64_MAX;
c->red_max = 0;
c->red_min = WEBP_UINT64_MAX;
c->blue_max = 0;
c->blue_min = WEBP_UINT64_MAX;
}
static void UpdateDominantCostRange(
const VP8LHistogram* const h, DominantCostRange* const c) {
if (c->literal_max < h->costs[LITERAL]) c->literal_max = h->costs[LITERAL];
if (c->literal_min > h->costs[LITERAL]) c->literal_min = h->costs[LITERAL];
if (c->red_max < h->costs[RED]) c->red_max = h->costs[RED];
if (c->red_min > h->costs[RED]) c->red_min = h->costs[RED];
if (c->blue_max < h->costs[BLUE]) c->blue_max = h->costs[BLUE];
if (c->blue_min > h->costs[BLUE]) c->blue_min = h->costs[BLUE];
}
static void ComputeHistogramCost(VP8LHistogram* const h) {
int i;
// No need to add the extra cost for length and distance as it is a constant
// that does not influence the histograms.
for (i = 0; i < 5; ++i) {
const uint32_t* population;
int length;
GetPopulationInfo(h, i, &population, &length);
h->costs[i] = PopulationCost(population, length, &h->trivial_symbol[i],
&h->is_used[i]);
}
h->bit_cost = h->costs[LITERAL] + h->costs[RED] + h->costs[BLUE] +
h->costs[ALPHA] + h->costs[DISTANCE];
}
static int GetBinIdForEntropy(uint64_t min, uint64_t max, uint64_t val) {
const uint64_t range = max - min;
if (range > 0) {
const uint64_t delta = val - min;
return (int)((NUM_PARTITIONS - 1e-6) * delta / range);
} else {
return 0;
}
}
static int GetHistoBinIndex(const VP8LHistogram* const h,
const DominantCostRange* const c, int low_effort) {
int bin_id =
GetBinIdForEntropy(c->literal_min, c->literal_max, h->costs[LITERAL]);
assert(bin_id < NUM_PARTITIONS);
if (!low_effort) {
bin_id = bin_id * NUM_PARTITIONS +
GetBinIdForEntropy(c->red_min, c->red_max, h->costs[RED]);
bin_id = bin_id * NUM_PARTITIONS +
GetBinIdForEntropy(c->blue_min, c->blue_max, h->costs[BLUE]);
assert(bin_id < BIN_SIZE);
}
return bin_id;
}
// Construct the histograms from backward references.
static void HistogramBuild(
int xsize, int histo_bits, const VP8LBackwardRefs* const backward_refs,
VP8LHistogramSet* const image_histo) {
int x = 0, y = 0;
const int histo_xsize = VP8LSubSampleSize(xsize, histo_bits);
VP8LHistogram** const histograms = image_histo->histograms;
VP8LRefsCursor c = VP8LRefsCursorInit(backward_refs);
assert(histo_bits > 0);
VP8LHistogramSetClear(image_histo);
while (VP8LRefsCursorOk(&c)) {
const PixOrCopy* const v = c.cur_pos;
const int ix = (y >> histo_bits) * histo_xsize + (x >> histo_bits);
HistogramAddSinglePixOrCopy(histograms[ix], v, NULL, 0);
x += PixOrCopyLength(v);
while (x >= xsize) {
x -= xsize;
++y;
}
VP8LRefsCursorNext(&c);
}
}
// Copies the histograms and computes its bit_cost.
static void HistogramCopyAndAnalyze(VP8LHistogramSet* const orig_histo,
VP8LHistogramSet* const image_histo) {
int i;
VP8LHistogram** const orig_histograms = orig_histo->histograms;
VP8LHistogram** const histograms = image_histo->histograms;
assert(image_histo->max_size == orig_histo->max_size);
image_histo->size = 0;
for (i = 0; i < orig_histo->max_size; ++i) {
VP8LHistogram* const histo = orig_histograms[i];
ComputeHistogramCost(histo);
// Skip the histogram if it is completely empty, which can happen for tiles
// with no information (when they are skipped because of LZ77).
if (!histo->is_used[LITERAL] && !histo->is_used[RED] &&
!histo->is_used[BLUE] && !histo->is_used[ALPHA] &&
!histo->is_used[DISTANCE]) {
// The first histogram is always used.
assert(i > 0);
orig_histograms[i] = NULL;
} else {
// Copy histograms from orig_histo[] to image_histo[].
HistogramCopy(histo, histograms[image_histo->size]);
++image_histo->size;
}
}
}
// Partition histograms to different entropy bins for three dominant (literal,
// red and blue) symbol costs and compute the histogram aggregate bit_cost.
static void HistogramAnalyzeEntropyBin(VP8LHistogramSet* const image_histo,
int low_effort) {
int i;
VP8LHistogram** const histograms = image_histo->histograms;
const int histo_size = image_histo->size;
DominantCostRange cost_range;
DominantCostRangeInit(&cost_range);
// Analyze the dominant (literal, red and blue) entropy costs.
for (i = 0; i < histo_size; ++i) {
UpdateDominantCostRange(histograms[i], &cost_range);
}
// bin-hash histograms on three of the dominant (literal, red and blue)
// symbol costs and store the resulting bin_id for each histogram.
for (i = 0; i < histo_size; ++i) {
histograms[i]->bin_id =
GetHistoBinIndex(histograms[i], &cost_range, low_effort);
}
}
// Merges some histograms with same bin_id together if it's advantageous.
// Sets the remaining histograms to NULL.
// 'combine_cost_factor' has to be divided by 100.
static void HistogramCombineEntropyBin(VP8LHistogramSet* const image_histo,
VP8LHistogram* cur_combo, int num_bins,
int32_t combine_cost_factor,
int low_effort) {
VP8LHistogram** const histograms = image_histo->histograms;
int idx;
struct {
int16_t first; // position of the histogram that accumulates all
// histograms with the same bin_id
uint16_t num_combine_failures; // number of combine failures per bin_id
} bin_info[BIN_SIZE];
assert(num_bins <= BIN_SIZE);
for (idx = 0; idx < num_bins; ++idx) {
bin_info[idx].first = -1;
bin_info[idx].num_combine_failures = 0;
}
for (idx = 0; idx < image_histo->size;) {
const int bin_id = histograms[idx]->bin_id;
const int first = bin_info[bin_id].first;
if (first == -1) {
bin_info[bin_id].first = idx;
++idx;
} else if (low_effort) {
HistogramAdd(histograms[idx], histograms[first], histograms[first]);
HistogramSetRemoveHistogram(image_histo, idx);
} else {
// try to merge #idx into #first (both share the same bin_id)
const uint64_t bit_cost = histograms[idx]->bit_cost;
const int64_t bit_cost_thresh =
-DivRound((int64_t)bit_cost * combine_cost_factor, 100);
if (HistogramAddEval(histograms[first], histograms[idx], cur_combo,
bit_cost_thresh)) {
const int max_combine_failures = 32;
// Try to merge two histograms only if the combo is a trivial one or
// the two candidate histograms are already non-trivial.
// For some images, 'try_combine' turns out to be false for a lot of
// histogram pairs. In that case, we fallback to combining
// histograms as usual to avoid increasing the header size.
int try_combine =
cur_combo->trivial_symbol[RED] != VP8L_NON_TRIVIAL_SYM &&
cur_combo->trivial_symbol[BLUE] != VP8L_NON_TRIVIAL_SYM &&
cur_combo->trivial_symbol[ALPHA] != VP8L_NON_TRIVIAL_SYM;
if (!try_combine) {
try_combine =
histograms[idx]->trivial_symbol[RED] == VP8L_NON_TRIVIAL_SYM ||
histograms[idx]->trivial_symbol[BLUE] == VP8L_NON_TRIVIAL_SYM ||
histograms[idx]->trivial_symbol[ALPHA] == VP8L_NON_TRIVIAL_SYM;
try_combine &=
histograms[first]->trivial_symbol[RED] == VP8L_NON_TRIVIAL_SYM ||
histograms[first]->trivial_symbol[BLUE] == VP8L_NON_TRIVIAL_SYM ||
histograms[first]->trivial_symbol[ALPHA] == VP8L_NON_TRIVIAL_SYM;
}
if (try_combine ||
bin_info[bin_id].num_combine_failures >= max_combine_failures) {
// move the (better) merged histogram to its final slot
HistogramSwap(&cur_combo, &histograms[first]);
HistogramSetRemoveHistogram(image_histo, idx);
} else {
++bin_info[bin_id].num_combine_failures;
++idx;
}
} else {
++idx;
}
}
}
if (low_effort) {
// for low_effort case, update the final cost when everything is merged
for (idx = 0; idx < image_histo->size; ++idx) {
ComputeHistogramCost(histograms[idx]);
}
}
}
// Implement a Lehmer random number generator with a multiplicative constant of
// 48271 and a modulo constant of 2^31 - 1.
static uint32_t MyRand(uint32_t* const seed) {
*seed = (uint32_t)(((uint64_t)(*seed) * 48271u) % 2147483647u);
assert(*seed > 0);
return *seed;
}
// -----------------------------------------------------------------------------
// Histogram pairs priority queue
// Pair of histograms. Negative idx1 value means that pair is out-of-date.
typedef struct {
int idx1;
int idx2;
int64_t cost_diff;
uint64_t cost_combo;
uint64_t costs[5];
} HistogramPair;
typedef struct {
HistogramPair* queue;
int size;
int max_size;
} HistoQueue;
static int HistoQueueInit(HistoQueue* const histo_queue, const int max_size) {
histo_queue->size = 0;
histo_queue->max_size = max_size;
// We allocate max_size + 1 because the last element at index "size" is
// used as temporary data (and it could be up to max_size).
histo_queue->queue = (HistogramPair*)WebPSafeMalloc(
histo_queue->max_size + 1, sizeof(*histo_queue->queue));
return histo_queue->queue != NULL;
}
static void HistoQueueClear(HistoQueue* const histo_queue) {
assert(histo_queue != NULL);
WebPSafeFree(histo_queue->queue);
histo_queue->size = 0;
histo_queue->max_size = 0;
}
// Pop a specific pair in the queue by replacing it with the last one
// and shrinking the queue.
static void HistoQueuePopPair(HistoQueue* const histo_queue,
HistogramPair* const pair) {
assert(pair >= histo_queue->queue &&
pair < (histo_queue->queue + histo_queue->size));
assert(histo_queue->size > 0);
*pair = histo_queue->queue[histo_queue->size - 1];
--histo_queue->size;
}
// Check whether a pair in the queue should be updated as head or not.
static void HistoQueueUpdateHead(HistoQueue* const histo_queue,
HistogramPair* const pair) {
assert(pair->cost_diff < 0);
assert(pair >= histo_queue->queue &&
pair < (histo_queue->queue + histo_queue->size));
assert(histo_queue->size > 0);
if (pair->cost_diff < histo_queue->queue[0].cost_diff) {
// Replace the best pair.
const HistogramPair tmp = histo_queue->queue[0];
histo_queue->queue[0] = *pair;
*pair = tmp;
}
}
// Replaces the bad_id with good_id in the pair.
static void HistoQueueFixPair(int bad_id, int good_id,
HistogramPair* const pair) {
if (pair->idx1 == bad_id) pair->idx1 = good_id;
if (pair->idx2 == bad_id) pair->idx2 = good_id;
if (pair->idx1 > pair->idx2) {
const int tmp = pair->idx1;
pair->idx1 = pair->idx2;
pair->idx2 = tmp;
}
}
// Update the cost diff and combo of a pair of histograms. This needs to be
// called when the histograms have been merged with a third one.
// Returns 1 if the cost diff is less than the threshold.
// Otherwise returns 0 and the cost is invalid due to early bail-out.
WEBP_NODISCARD static int HistoQueueUpdatePair(const VP8LHistogram* const h1,
const VP8LHistogram* const h2,
int64_t cost_threshold,
HistogramPair* const pair) {
const int64_t sum_cost = h1->bit_cost + h2->bit_cost;
SaturateAdd(sum_cost, &cost_threshold);
if (!GetCombinedHistogramEntropy(h1, h2, cost_threshold, &pair->cost_combo,
pair->costs)) {
return 0;
}
pair->cost_diff = (int64_t)pair->cost_combo - sum_cost;
return 1;
}
// Create a pair from indices "idx1" and "idx2" provided its cost
// is inferior to "threshold", a negative entropy.
// It returns the cost of the pair, or 0 if it superior to threshold.
static int64_t HistoQueuePush(HistoQueue* const histo_queue,
VP8LHistogram** const histograms, int idx1,
int idx2, int64_t threshold) {
const VP8LHistogram* h1;
const VP8LHistogram* h2;
HistogramPair pair;
// Stop here if the queue is full.
if (histo_queue->size == histo_queue->max_size) return 0;
assert(threshold <= 0);
if (idx1 > idx2) {
const int tmp = idx2;
idx2 = idx1;
idx1 = tmp;
}
pair.idx1 = idx1;
pair.idx2 = idx2;
h1 = histograms[idx1];
h2 = histograms[idx2];
// Do not even consider the pair if it does not improve the entropy.
if (!HistoQueueUpdatePair(h1, h2, threshold, &pair)) return 0;
histo_queue->queue[histo_queue->size++] = pair;
HistoQueueUpdateHead(histo_queue, &histo_queue->queue[histo_queue->size - 1]);
return pair.cost_diff;
}
// -----------------------------------------------------------------------------
// Combines histograms by continuously choosing the one with the highest cost
// reduction.
static int HistogramCombineGreedy(VP8LHistogramSet* const image_histo) {
int ok = 0;
const int image_histo_size = image_histo->size;
int i, j;
VP8LHistogram** const histograms = image_histo->histograms;
// Priority queue of histogram pairs.
HistoQueue histo_queue;
// image_histo_size^2 for the queue size is safe. If you look at
// HistogramCombineGreedy, and imagine that UpdateQueueFront always pushes
// data to the queue, you insert at most:
// - image_histo_size*(image_histo_size-1)/2 (the first two for loops)
// - image_histo_size - 1 in the last for loop at the first iteration of
// the while loop, image_histo_size - 2 at the second iteration ...
// therefore image_histo_size*(image_histo_size-1)/2 overall too
if (!HistoQueueInit(&histo_queue, image_histo_size * image_histo_size)) {
goto End;
}
// Initialize the queue.
for (i = 0; i < image_histo_size; ++i) {
for (j = i + 1; j < image_histo_size; ++j) {
HistoQueuePush(&histo_queue, histograms, i, j, 0);
}
}
while (histo_queue.size > 0) {
const int idx1 = histo_queue.queue[0].idx1;
const int idx2 = histo_queue.queue[0].idx2;
HistogramAdd(histograms[idx2], histograms[idx1], histograms[idx1]);
UpdateHistogramCost(histo_queue.queue[0].cost_combo,
histo_queue.queue[0].costs, histograms[idx1]);
// Remove merged histogram.
HistogramSetRemoveHistogram(image_histo, idx2);
// Remove pairs intersecting the just combined best pair.
for (i = 0; i < histo_queue.size;) {
HistogramPair* const p = histo_queue.queue + i;
if (p->idx1 == idx1 || p->idx2 == idx1 ||
p->idx1 == idx2 || p->idx2 == idx2) {
HistoQueuePopPair(&histo_queue, p);
} else {
HistoQueueFixPair(image_histo->size, idx2, p);
HistoQueueUpdateHead(&histo_queue, p);
++i;
}
}
// Push new pairs formed with combined histogram to the queue.
for (i = 0; i < image_histo->size; ++i) {
if (i == idx1) continue;
HistoQueuePush(&histo_queue, image_histo->histograms, idx1, i, 0);
}
}
ok = 1;
End:
HistoQueueClear(&histo_queue);
return ok;
}
// Perform histogram aggregation using a stochastic approach.
// 'do_greedy' is set to 1 if a greedy approach needs to be performed
// afterwards, 0 otherwise.
static int HistogramCombineStochastic(VP8LHistogramSet* const image_histo,
int min_cluster_size,
int* const do_greedy) {
int j, iter;
uint32_t seed = 1;
int tries_with_no_success = 0;
const int outer_iters = image_histo->size;
const int num_tries_no_success = outer_iters / 2;
VP8LHistogram** const histograms = image_histo->histograms;
// Priority queue of histogram pairs. Its size of 'kHistoQueueSize'
// impacts the quality of the compression and the speed: the smaller the
// faster but the worse for the compression.
HistoQueue histo_queue;
const int kHistoQueueSize = 9;
int ok = 0;
if (image_histo->size < min_cluster_size) {
*do_greedy = 1;
return 1;
}
if (!HistoQueueInit(&histo_queue, kHistoQueueSize)) goto End;
// Collapse similar histograms in 'image_histo'.
for (iter = 0; iter < outer_iters && image_histo->size >= min_cluster_size &&
++tries_with_no_success < num_tries_no_success;
++iter) {
int64_t best_cost =
(histo_queue.size == 0) ? 0 : histo_queue.queue[0].cost_diff;
int best_idx1 = -1, best_idx2 = 1;
const uint32_t rand_range = (image_histo->size - 1) * (image_histo->size);
// (image_histo->size) / 2 was chosen empirically. Less means faster but
// worse compression.
const int num_tries = (image_histo->size) / 2;
// Pick random samples.
for (j = 0; image_histo->size >= 2 && j < num_tries; ++j) {
int64_t curr_cost;
// Choose two different histograms at random and try to combine them.
const uint32_t tmp = MyRand(&seed) % rand_range;
uint32_t idx1 = tmp / (image_histo->size - 1);
uint32_t idx2 = tmp % (image_histo->size - 1);
if (idx2 >= idx1) ++idx2;
// Calculate cost reduction on combination.
curr_cost =
HistoQueuePush(&histo_queue, histograms, idx1, idx2, best_cost);
if (curr_cost < 0) { // found a better pair?
best_cost = curr_cost;
// Empty the queue if we reached full capacity.
if (histo_queue.size == histo_queue.max_size) break;
}
}
if (histo_queue.size == 0) continue;
// Get the best histograms.
best_idx1 = histo_queue.queue[0].idx1;
best_idx2 = histo_queue.queue[0].idx2;
assert(best_idx1 < best_idx2);
// Merge the histograms and remove best_idx2 from the queue.
HistogramAdd(histograms[best_idx2], histograms[best_idx1],
histograms[best_idx1]);
UpdateHistogramCost(histo_queue.queue[0].cost_combo,
histo_queue.queue[0].costs, histograms[best_idx1]);
HistogramSetRemoveHistogram(image_histo, best_idx2);
// Parse the queue and update each pair that deals with best_idx1,
// best_idx2 or image_histo_size.
for (j = 0; j < histo_queue.size;) {
HistogramPair* const p = histo_queue.queue + j;
const int is_idx1_best = p->idx1 == best_idx1 || p->idx1 == best_idx2;
const int is_idx2_best = p->idx2 == best_idx1 || p->idx2 == best_idx2;
// The front pair could have been duplicated by a random pick so
// check for it all the time nevertheless.
if (is_idx1_best && is_idx2_best) {
HistoQueuePopPair(&histo_queue, p);
continue;
}
// Any pair containing one of the two best indices should only refer to
// best_idx1. Its cost should also be updated.
if (is_idx1_best || is_idx2_best) {
HistoQueueFixPair(best_idx2, best_idx1, p);
// Re-evaluate the cost of an updated pair.
if (!HistoQueueUpdatePair(histograms[p->idx1], histograms[p->idx2], 0,
p)) {
HistoQueuePopPair(&histo_queue, p);
continue;
}
}
HistoQueueFixPair(image_histo->size, best_idx2, p);
HistoQueueUpdateHead(&histo_queue, p);
++j;
}
tries_with_no_success = 0;
}
*do_greedy = (image_histo->size <= min_cluster_size);
ok = 1;
End:
HistoQueueClear(&histo_queue);
return ok;
}
// -----------------------------------------------------------------------------
// Histogram refinement
// Find the best 'out' histogram for each of the 'in' histograms.
// At call-time, 'out' contains the histograms of the clusters.
// Note: we assume that out[]->bit_cost is already up-to-date.
static void HistogramRemap(const VP8LHistogramSet* const in,
VP8LHistogramSet* const out,
uint32_t* const symbols) {
int i;
VP8LHistogram** const in_histo = in->histograms;
VP8LHistogram** const out_histo = out->histograms;
const int in_size = out->max_size;
const int out_size = out->size;
if (out_size > 1) {
for (i = 0; i < in_size; ++i) {
int best_out = 0;
int64_t best_bits = WEBP_INT64_MAX;
int k;
if (in_histo[i] == NULL) {
// Arbitrarily set to the previous value if unused to help future LZ77.
symbols[i] = symbols[i - 1];
continue;
}
for (k = 0; k < out_size; ++k) {
int64_t cur_bits;
if (HistogramAddThresh(out_histo[k], in_histo[i], best_bits,
&cur_bits)) {
best_bits = cur_bits;
best_out = k;
}
}
symbols[i] = best_out;
}
} else {
assert(out_size == 1);
for (i = 0; i < in_size; ++i) {
symbols[i] = 0;
}
}
// Recompute each out based on raw and symbols.
VP8LHistogramSetClear(out);
out->size = out_size;
for (i = 0; i < in_size; ++i) {
int idx;
if (in_histo[i] == NULL) continue;
idx = symbols[i];
HistogramAdd(in_histo[i], out_histo[idx], out_histo[idx]);
}
}
static int32_t GetCombineCostFactor(int histo_size, int quality) {
int32_t combine_cost_factor = 16;
if (quality < 90) {
if (histo_size > 256) combine_cost_factor /= 2;
if (histo_size > 512) combine_cost_factor /= 2;
if (histo_size > 1024) combine_cost_factor /= 2;
if (quality <= 50) combine_cost_factor /= 2;
}
return combine_cost_factor;
}
int VP8LGetHistoImageSymbols(int xsize, int ysize,
const VP8LBackwardRefs* const refs, int quality,
int low_effort, int histogram_bits, int cache_bits,
VP8LHistogramSet* const image_histo,
VP8LHistogram* const tmp_histo,
uint32_t* const histogram_symbols,
const WebPPicture* const pic, int percent_range,
int* const percent) {
const int histo_xsize =
histogram_bits ? VP8LSubSampleSize(xsize, histogram_bits) : 1;
const int histo_ysize =
histogram_bits ? VP8LSubSampleSize(ysize, histogram_bits) : 1;
const int image_histo_raw_size = histo_xsize * histo_ysize;
VP8LHistogramSet* const orig_histo =
VP8LAllocateHistogramSet(image_histo_raw_size, cache_bits);
// Don't attempt linear bin-partition heuristic for
// histograms of small sizes (as bin_map will be very sparse) and
// maximum quality q==100 (to preserve the compression gains at that level).
const int entropy_combine_num_bins = low_effort ? NUM_PARTITIONS : BIN_SIZE;
int entropy_combine;
if (orig_histo == NULL) {
WebPEncodingSetError(pic, VP8_ENC_ERROR_OUT_OF_MEMORY);
goto Error;
}
// Construct the histograms from backward references.
HistogramBuild(xsize, histogram_bits, refs, orig_histo);
HistogramCopyAndAnalyze(orig_histo, image_histo);
entropy_combine =
(image_histo->size > entropy_combine_num_bins * 2) && (quality < 100);
if (entropy_combine) {
const int32_t combine_cost_factor =
GetCombineCostFactor(image_histo_raw_size, quality);
HistogramAnalyzeEntropyBin(image_histo, low_effort);
// Collapse histograms with similar entropy.
HistogramCombineEntropyBin(image_histo, tmp_histo, entropy_combine_num_bins,
combine_cost_factor, low_effort);
}
// Don't combine the histograms using stochastic and greedy heuristics for
// low-effort compression mode.
if (!low_effort || !entropy_combine) {
// cubic ramp between 1 and MAX_HISTO_GREEDY:
const int threshold_size =
(int)(1 + DivRound(quality * quality * quality * (MAX_HISTO_GREEDY - 1),
100 * 100 * 100));
int do_greedy;
if (!HistogramCombineStochastic(image_histo, threshold_size, &do_greedy)) {
WebPEncodingSetError(pic, VP8_ENC_ERROR_OUT_OF_MEMORY);
goto Error;
}
if (do_greedy) {
if (!HistogramCombineGreedy(image_histo)) {
WebPEncodingSetError(pic, VP8_ENC_ERROR_OUT_OF_MEMORY);
goto Error;
}
}
}
// Find the optimal map from original histograms to the final ones.
HistogramRemap(orig_histo, image_histo, histogram_symbols);
if (!WebPReportProgress(pic, *percent + percent_range, percent)) {
goto Error;
}
Error:
VP8LFreeHistogramSet(orig_histo);
return (pic->error_code == VP8_ENC_OK);
}