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Remove the computation of ExtraCost when comparing histograms

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Entropy clustering merges symbol histograms to reduce the overall
entropy. The cost of 2 added histograms is compared to the 2 costs
of the individual histograms and if it is smaller, a merge is done.

Except for some symbols (distance and length), the computed cost is
 the real final cost based on the histogram, and some constant cost
(independent from the probabilities of the symbols and hence the
merge) because the symbol is encode as Golomb.

This constant cost is useless and can be removed.

Change-Id: I6271e8c0e4111cdeff544cbdb7dec3c67be5309c
This commit is contained in:
Vincent Rabaud 2025-03-28 15:00:41 +01:00
parent f2b3f52733
commit a4183d94c7
1 changed files with 8 additions and 14 deletions
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22
src/enc/histogram_enc.c
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@ -401,10 +401,8 @@ WEBP_NODISCARD static int GetCombinedHistogramEntropy(
*cost = GetCombinedEntropy(a->literal_, b->literal_,
VP8LHistogramNumCodes(palette_code_bits),
a->is_used_[0], b->is_used_[0], 0);
*cost += (uint64_t)VP8LExtraCostCombined(a->literal_ + NUM_LITERAL_CODES,
b->literal_ + NUM_LITERAL_CODES,
NUM_LENGTH_CODES)
<< LOG_2_PRECISION_BITS;
// No need to add the extra cost as it is a constant that does not influence
// the histograms.
if (*cost >= cost_threshold) return 0;
if (a->trivial_symbol_ != VP8L_NON_TRIVIAL_SYM &&
@ -434,9 +432,8 @@ WEBP_NODISCARD static int GetCombinedHistogramEntropy(
*cost += GetCombinedEntropy(a->distance_, b->distance_, NUM_DISTANCE_CODES,
a->is_used_[4], b->is_used_[4], 0);
*cost += (uint64_t)VP8LExtraCostCombined(a->distance_, b->distance_,
NUM_DISTANCE_CODES)
<< LOG_2_PRECISION_BITS;
// No need to add the extra cost as it is a constant that does not influence
// the histograms.
if (*cost >= cost_threshold) return 0;
return 1;
@ -528,16 +525,13 @@ static void UpdateHistogramCost(VP8LHistogram* const h) {
uint32_t alpha_sym, red_sym, blue_sym;
const uint64_t alpha_cost =
PopulationCost(h->alpha_, NUM_LITERAL_CODES, &alpha_sym, &h->is_used_[3]);
// No need to add the extra cost as it is a constant that does not influence
// the histograms.
const uint64_t distance_cost =
PopulationCost(h->distance_, NUM_DISTANCE_CODES, NULL, &h->is_used_[4]) +
((uint64_t)VP8LExtraCost(h->distance_, NUM_DISTANCE_CODES)
<< LOG_2_PRECISION_BITS);
PopulationCost(h->distance_, NUM_DISTANCE_CODES, NULL, &h->is_used_[4]);
const int num_codes = VP8LHistogramNumCodes(h->palette_code_bits_);
h->literal_cost_ =
PopulationCost(h->literal_, num_codes, NULL, &h->is_used_[0]) +
((uint64_t)VP8LExtraCost(h->literal_ + NUM_LITERAL_CODES,
NUM_LENGTH_CODES)
<< LOG_2_PRECISION_BITS);
PopulationCost(h->literal_, num_codes, NULL, &h->is_used_[0]);
h->red_cost_ =
PopulationCost(h->red_, NUM_LITERAL_CODES, &red_sym, &h->is_used_[1]);
h->blue_cost_ =