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Do a binary search to get the optimum cache bits.

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This speeds up the lossless encoder by a bit (1-2%), without impacting the
compression density.

Change-Id: Ied6fb38fab58eef9ded078697e0463fe7c560b26
This commit is contained in:
Vikas Arora 2014-03-13 10:29:50 -07:00
parent 24ca3678f9
commit 5f0cfa80ff
1 changed files with 30 additions and 18 deletions
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48
src/enc/backward_references.c
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@ -12,7 +12,6 @@
#include <assert.h> #include <assert.h>
#include <math.h> #include <math.h>
#include <stdio.h>
#include "./backward_references.h" #include "./backward_references.h"
#include "./histogram.h" #include "./histogram.h"
@ -26,6 +25,8 @@
#define HASH_SIZE (1 << HASH_BITS) #define HASH_SIZE (1 << HASH_BITS)
#define HASH_MULTIPLIER (0xc6a4a7935bd1e995ULL) #define HASH_MULTIPLIER (0xc6a4a7935bd1e995ULL)
#define MAX_ENTROPY (1e30f)
// 1M window (4M bytes) minus 120 special codes for short distances. // 1M window (4M bytes) minus 120 special codes for short distances.
#define WINDOW_SIZE ((1 << 20) - 120) #define WINDOW_SIZE ((1 << 20) - 120)
@ -833,7 +834,7 @@ static double ComputeCacheEntropy(const uint32_t* const argb,
if (use_color_cache) { if (use_color_cache) {
cc_init = VP8LColorCacheInit(&hashers, cache_bits); cc_init = VP8LColorCacheInit(&hashers, cache_bits);
if (!cc_init) return 1e99; if (!cc_init) return MAX_ENTROPY;
} }
VP8LHistogramInit(&histo, cache_bits); VP8LHistogramInit(&histo, cache_bits);
@ -873,27 +874,38 @@ int VP8LCalculateEstimateForCacheSize(const uint32_t* const argb,
int xsize, int ysize, int quality, int xsize, int ysize, int quality,
int* const best_cache_bits) { int* const best_cache_bits) {
int ok = 0; int ok = 0;
int cache_bits; int eval_low = 1;
// Number of tries to get optimal cache-bits after finding a local minima. int eval_high = 1;
const int max_tries_after_min = 3 + (quality >> 4); double entropy_low = MAX_ENTROPY;
int num_tries_after_min = 0; double entropy_high = MAX_ENTROPY;
double lowest_entropy = 1e99; int cache_bits_low = 0;
int cache_bits_high = MAX_COLOR_CACHE_BITS;
VP8LBackwardRefs refs; VP8LBackwardRefs refs;
if (!VP8LBackwardRefsAlloc(&refs, xsize * ysize) || if (!VP8LBackwardRefsAlloc(&refs, xsize * ysize) ||
!BackwardReferencesHashChain(xsize, ysize, argb, 0, quality, &refs)) { !BackwardReferencesHashChain(xsize, ysize, argb, 0, quality, &refs)) {
goto Error; goto Error;
} }
// The entropy is generally lower for higher cache_bits. Start searching from // Do a binary search to find the optimal entropy for cache_bits.
// the highest value and settle for a local minima. while (cache_bits_high - cache_bits_low > 1) {
for (cache_bits = MAX_COLOR_CACHE_BITS; if (eval_low) {
cache_bits >= 0 && num_tries_after_min < max_tries_after_min; entropy_low =
--cache_bits, ++num_tries_after_min) { ComputeCacheEntropy(argb, xsize, ysize, &refs, cache_bits_low);
const double cur_entropy = ComputeCacheEntropy(argb, xsize, ysize, eval_low = 0;
&refs, cache_bits); }
if (cur_entropy < lowest_entropy) { if (eval_high) {
*best_cache_bits = cache_bits; entropy_high =
lowest_entropy = cur_entropy; ComputeCacheEntropy(argb, xsize, ysize, &refs, cache_bits_high);
num_tries_after_min = 0; eval_high = 0;
}
if (entropy_high < entropy_low) {
*best_cache_bits = cache_bits_high;
cache_bits_low = (cache_bits_low + cache_bits_high) / 2;
eval_low = 1;
} else {
*best_cache_bits = cache_bits_low;
cache_bits_high = (cache_bits_low + cache_bits_high) / 2;
eval_high = 1;
} }
} }
ok = 1; ok = 1;