From e912bd55bee7b4b34a7b93f31a51723383768896 Mon Sep 17 00:00:00 2001
From: Vikas Arora <vikasa@google.com>
Date: Thu, 23 Oct 2014 09:01:46 -0700
Subject: [PATCH] Fix bug in VP8LCalculateEstimateForCacheSize.

The method VP8LCalculateEstimateForCacheSize is not evaluating the all possible
range for cache_bits.
Also added a small penality for choosing the larger cache-size. This is done to
strike a balance between additional memory/CPU cost (with larger cache-size) and
byte savings from smaller WebP lossless files.

This change saves about 0.07% bytes and speeds up compression by 8% (default
settings). There's small speedup at Q=50 along with byte savings as well.
Compression at Quality=25 is not effected by this change.

Change-Id: Id8f87dee6b5bccb2baa6dbdee479ee9cda8f4f77
---
 src/enc/backward_references.c | 19 +++++++++++++++----
 src/enc/vp8l.c                | 14 ++++++--------
 2 files changed, 21 insertions(+), 12 deletions(-)

diff --git a/src/enc/backward_references.c b/src/enc/backward_references.c
index 4bbc8c07..337b58d1 100644
--- a/src/enc/backward_references.c
+++ b/src/enc/backward_references.c
@@ -927,33 +927,44 @@ int VP8LCalculateEstimateForCacheSize(const uint32_t* const argb,
   int eval_high = 1;
   double entropy_low = MAX_ENTROPY;
   double entropy_high = MAX_ENTROPY;
+  const double cost_mul = 5e-4;
   int cache_bits_low = 0;
-  int cache_bits_high = MAX_COLOR_CACHE_BITS;
+  int cache_bits_high = *best_cache_bits;
+
+  assert(cache_bits_high <= MAX_COLOR_CACHE_BITS);
+
+  if (cache_bits_high == 0) {
+    // Local color cache is disabled.
+    return 1;
+  }
 
   if (!BackwardReferencesHashChain(xsize, ysize, argb, 0, quality, hash_chain,
                                    refs)) {
     return 0;
   }
   // Do a binary search to find the optimal entropy for cache_bits.
-  while (cache_bits_high - cache_bits_low > 1) {
+  while (eval_low || eval_high) {
     if (eval_low) {
       entropy_low =
           ComputeCacheEntropy(argb, xsize, ysize, refs, cache_bits_low);
+      entropy_low += entropy_low * cache_bits_low * cost_mul;
       eval_low = 0;
     }
     if (eval_high) {
       entropy_high =
           ComputeCacheEntropy(argb, xsize, ysize, refs, cache_bits_high);
+      entropy_high += entropy_high * cache_bits_high * cost_mul;
       eval_high = 0;
     }
     if (entropy_high < entropy_low) {
+      const int prev_cache_bits_low = cache_bits_low;
       *best_cache_bits = cache_bits_high;
       cache_bits_low = (cache_bits_low + cache_bits_high) / 2;
-      eval_low = 1;
+      if (cache_bits_low != prev_cache_bits_low) eval_low = 1;
     } else {
       *best_cache_bits = cache_bits_low;
       cache_bits_high = (cache_bits_low + cache_bits_high) / 2;
-      eval_high = 1;
+      if (cache_bits_high != cache_bits_low) eval_high = 1;
     }
   }
   return 1;
diff --git a/src/enc/vp8l.c b/src/enc/vp8l.c
index 8b999ad5..4f5a745d 100644
--- a/src/enc/vp8l.c
+++ b/src/enc/vp8l.c
@@ -284,7 +284,7 @@ static int GetTransformBits(int method, int histo_bits) {
 static int GetCacheBits(int use_palette, float quality) {
   // Color cache is disabled for compression at lower quality or when a palette
   // is used.
-  return (use_palette || (quality <= 25.f)) ? 0 : 7;
+  return (use_palette || (quality <= 25.f)) ? 0 : MAX_COLOR_CACHE_BITS;
 }
 
 static int EvalSubtractGreenForPalette(int palette_size, float quality) {
@@ -1309,13 +1309,11 @@ WebPEncodingError VP8LEncodeStream(const WebPConfig* const config,
   // ---------------------------------------------------------------------------
   // Estimate the color cache size.
 
-  if (enc->cache_bits_ > 0) {
-    if (!VP8LCalculateEstimateForCacheSize(enc->argb_, enc->current_width_,
-                                           height, quality, &enc->hash_chain_,
-                                           &enc->refs_[0], &enc->cache_bits_)) {
-      err = VP8_ENC_ERROR_OUT_OF_MEMORY;
-      goto Error;
-    }
+  if (!VP8LCalculateEstimateForCacheSize(enc->argb_, enc->current_width_,
+                                         height, quality, &enc->hash_chain_,
+                                         &enc->refs_[0], &enc->cache_bits_)) {
+    err = VP8_ENC_ERROR_OUT_OF_MEMORY;
+    goto Error;
   }
 
   // ---------------------------------------------------------------------------