Speedup WebP lossless compression for low effort (m=0) mode with following:

- Disable Cross-Color transform.
- Evaluate predictors #11 (paeth), #12 and #13 only.

Change-Id: I857264c85c61c3957d4fb45ae32d261d947c8bed

src/dsp/lossless.c

@@ -758,9 +758,12 @@ static WEBP_INLINE void UpdateHisto(int histo_argb[4][256], uint32_t argb) {
 
 static int GetBestPredictorForTile(int width, int height,
                                    int tile_x, int tile_y, int bits,
+                                   int low_effort,
                                    const int accumulated[4][256],
                                    const uint32_t* const argb_scratch) {
   const int kNumPredModes = 14;
+  const int kPredModePaeth = 11;
+  const int start_mode = low_effort ? kPredModePaeth : 0;
   const int col_start = tile_x << bits;
   const int row_start = tile_y << bits;
   const int tile_size = 1 << bits;
@@ -769,7 +772,7 @@ static int GetBestPredictorForTile(int width, int height,
   float best_diff = MAX_DIFF_COST;
   int best_mode = 0;
   int mode;
-  for (mode = 0; mode < kNumPredModes; ++mode) {
+  for (mode = start_mode; mode < kNumPredModes; ++mode) {
     const uint32_t* current_row = argb_scratch;
     const VP8LPredictorFunc pred_func = VP8LPredictors[mode];
     float cur_diff;
@@ -839,7 +842,7 @@ static void CopyTileWithPrediction(int width, int height,
   }
 }
 
-void VP8LResidualImage(int width, int height, int bits,
+void VP8LResidualImage(int width, int height, int bits, int low_effort,
                        uint32_t* const argb, uint32_t* const argb_scratch,
                        uint32_t* const image) {
   const int max_tile_size = 1 << bits;
@@ -870,7 +873,7 @@ void VP8LResidualImage(int width, int height, int bits,
         all_x_max = width;
       }
       pred = GetBestPredictorForTile(width, height, tile_x, tile_y, bits,
-                                     (const int (*)[256])histo,
+                                     low_effort, (const int (*)[256])histo,
                                      argb_scratch);
       image[tile_y * tiles_per_row + tile_x] = 0xff000000u | (pred << 8);
       CopyTileWithPrediction(width, height, tile_x, tile_y, bits, pred,

src/dsp/lossless.h

@@ -98,7 +98,7 @@ void VP8LColorIndexInverseTransformAlpha(
     const struct VP8LTransform* const transform, int y_start, int y_end,
     const uint8_t* src, uint8_t* dst);
 
-void VP8LResidualImage(int width, int height, int bits,
+void VP8LResidualImage(int width, int height, int bits, int low_effort,
                        uint32_t* const argb, uint32_t* const argb_scratch,
                        uint32_t* const image);
 

src/enc/vp8l.c

@@ -341,7 +341,7 @@ static int AnalyzeAndInit(VP8LEncoder* const enc, WebPImageHint image_hint) {
   if (!enc->use_palette_) {
     if (image_hint == WEBP_HINT_PHOTO) {
       enc->use_predict_ = 1;
-      enc->use_cross_color_ = 1;
+      enc->use_cross_color_ = (method > 0);
     } else {
       double non_pred_entropy, pred_entropy;
       if (!AnalyzeEntropy(pic->argb, width, height, pic->argb_stride,
@@ -350,7 +350,7 @@ static int AnalyzeAndInit(VP8LEncoder* const enc, WebPImageHint image_hint) {
       }
       if (pred_entropy < 0.95 * non_pred_entropy) {
         enc->use_predict_ = 1;
-        enc->use_cross_color_ = 1;
+        enc->use_cross_color_ = (method > 0);
       }
     }
   }
@@ -919,14 +919,15 @@ static void ApplySubtractGreen(VP8LEncoder* const enc, int width, int height,
 }
 
 static WebPEncodingError ApplyPredictFilter(const VP8LEncoder* const enc,
-                                            int width, int height, int quality,
+                                            int width, int height,
+                                            int quality, int low_effort,
                                             VP8LBitWriter* const bw) {
   const int pred_bits = enc->transform_bits_;
   const int transform_width = VP8LSubSampleSize(width, pred_bits);
   const int transform_height = VP8LSubSampleSize(height, pred_bits);
 
-  VP8LResidualImage(width, height, pred_bits, enc->argb_, enc->argb_scratch_,
+  VP8LResidualImage(width, height, pred_bits, low_effort, enc->argb_,
-                    enc->transform_data_);
+                    enc->argb_scratch_, enc->transform_data_);
   VP8LPutBits(bw, TRANSFORM_PRESENT, 1);
   VP8LPutBits(bw, PREDICTOR_TRANSFORM, 2);
   assert(pred_bits >= 2);
@@ -1207,6 +1208,7 @@ WebPEncodingError VP8LEncodeStream(const WebPConfig* const config,
                                    VP8LBitWriter* const bw) {
   WebPEncodingError err = VP8_ENC_OK;
   const int quality = (int)config->quality;
+  const int low_effort = (config->method == 0);
   const int width = picture->width;
   const int height = picture->height;
   VP8LEncoder* const enc = VP8LEncoderNew(config, picture);
@@ -1279,7 +1281,8 @@ WebPEncodingError VP8LEncodeStream(const WebPConfig* const config,
       }
     }
 #endif  // WEBP_EXPERIMENTAL_FEATURES
-    err = ApplyPredictFilter(enc, enc->current_width_, height, quality, bw);
+    err = ApplyPredictFilter(enc, enc->current_width_, height, quality,
+                             low_effort, bw);
     if (err != VP8_ENC_OK) {
       WebPSafeFree(copy_buffer);
       goto Error;