Do not use a palette for one color images.

1 color images now always take 30 bytes.

Change-Id: Ifa86bc5320362c659672b3836160353d63576467

src/enc/backward_references_enc.c

@@ -550,6 +550,25 @@ static int BackwardReferencesLz77(int xsize, int ysize,
   return ok;
 }
 
+static int BackwardReferencesNone(int xsize, int ysize,
+                                  const uint32_t* const argb, int cache_bits,
+                                  VP8LBackwardRefs* const refs) {
+  const int pix_count = xsize * ysize;
+  int i;
+  const int use_color_cache = (cache_bits > 0);
+  VP8LColorCache hashers;
+
+  if (use_color_cache && !VP8LColorCacheInit(&hashers, cache_bits)) {
+    return 0;
+  }
+  VP8LClearBackwardRefs(refs);
+  for (i = 0; i < pix_count; ++i) {
+    AddSingleLiteral(argb[i], use_color_cache, &hashers, refs);
+  }
+  if (use_color_cache) VP8LColorCacheClear(&hashers);
+  return !refs->error_;
+}
+
 // Compute an LZ77 by forcing matches to happen within a given distance cost.
 // We therefore limit the algorithm to the lowest 32 values in the PlaneCode
 // definition.
@@ -919,6 +938,9 @@ static int GetBackwardReferences(int width, int height,
         res = BackwardReferencesLz77Box(width, height, argb, 0, hash_chain,
                                         &hash_chain_box, refs_tmp);
         break;
+      case kLZ77None:
+        res = BackwardReferencesNone(width, height, argb, 0, refs_tmp);
+        break;
       default:
         assert(0);
     }

src/enc/backward_references_enc.h

@@ -213,7 +213,10 @@ static WEBP_INLINE void VP8LRefsCursorNext(VP8LRefsCursor* const c) {
 enum VP8LLZ77Type {
   kLZ77Standard = 1,
   kLZ77RLE = 2,
-  kLZ77Box = 4
+  kLZ77Box = 4,
+  // With kLZ77None, LZ77 is not even tried. For now, this is only useful for
+  // 1-color images but we could use it if we are sure the image is a photo.
+  kLZ77None = 8
 };
 
 // Evaluates best possible backward references for specified quality.

src/enc/vp8l_enc.c

@@ -591,6 +591,19 @@ static int EncoderAnalyze(VP8LEncoder* const enc,
           sizeof(*enc->palette_sorted_), PaletteCompareColorsForQsort);
   }
 
+  if (use_palette && enc->palette_size_ == 1) {
+    crunch_configs[0].entropy_idx_ = kDirect;
+    crunch_configs[0].sub_configs_size_ = 1;
+    crunch_configs[0].sub_configs_[0].lz77_ = kLZ77None;
+    crunch_configs[0].sub_configs_[0].do_no_cache_ = 1;
+    *crunch_configs_size = 1;
+    // Get the entropy image as small as possible.
+    enc->histo_bits_ = MAX_HUFFMAN_BITS;
+    // The transform image is unused as we are direct.
+    enc->transform_bits_ = MAX_TRANSFORM_BITS;
+    return 1;
+  }
+
   // Empirical bit sizes.
   enc->histo_bits_ = GetHistoBits(method, use_palette,
                                   pic->width, pic->height);