Add support for encoding/decoding WebP images.

blimp/common/compositor/blimp_image_serialization_processor.cc

Index: blimp/common/compositor/blimp_image_serialization_processor.cc
diff --git a/blimp/common/compositor/blimp_image_serialization_processor.cc b/blimp/common/compositor/blimp_image_serialization_processor.cc
index 36ea6af38275856c8d91fe02215e0c2b4c2b9db0..ca436ed8cdd6300f344dd9e804bf7d34bd33e687 100644
--- a/blimp/common/compositor/blimp_image_serialization_processor.cc
+++ b/blimp/common/compositor/blimp_image_serialization_processor.cc
@@ -8,14 +8,133 @@
 #include <vector>
 
 #include "base/logging.h"
+#include "blimp/common/compositor/webp_decoder.h"
+#include "third_party/libwebp/webp/encode.h"
+#include "third_party/skia/include/core/SkData.h"
 #include "third_party/skia/include/core/SkPicture.h"
 #include "third_party/skia/include/core/SkPixelSerializer.h"
+#include "third_party/skia/include/core/SkUnPreMultiply.h"
 
 namespace {
-bool NoopDecoder(const void* input, size_t input_size, SkBitmap* bitmap) {
-  // TODO(nyquist): Add an image decoder.
-  return false;
-}
+// TODO(nyquist): Make sure encoder does not serialize images more than once.
+// See crbug.com/548434.
+class WebPImageEncoder : public SkPixelSerializer {
+ public:
+  WebPImageEncoder() {}
+  ~WebPImageEncoder() override{};
+
+  bool onUseEncodedData(const void* data, size_t len) override {
+    const unsigned char* cast_data = static_cast<const unsigned char*>(data);
+    if (len < 14)
+      return false;
+    return !memcmp(cast_data, "RIFF", 4) && !memcmp(cast_data + 8, "WEBPVP", 6);
+  }
+
+  SkData* onEncode(const SkPixmap& pixmap) override {
+    // Initialize an empty WebPConfig.
+    WebPConfig config;
+    if (!WebPConfigInit(&config))
+      return nullptr;
+
+    // Initialize an empty WebPPicture.
+    WebPPicture picture;
+    if (!WebPPictureInit(&picture))
+      return nullptr;
+
+    // Ensure width and height are valid dimensions.
+    if (!pixmap.width() || pixmap.width() > WEBP_MAX_DIMENSION)
+      return nullptr;
+    picture.width = pixmap.width();
+    if (!pixmap.height() || pixmap.height() > WEBP_MAX_DIMENSION)
+      return nullptr;
+    picture.height = pixmap.height();
+
+    // Import picture from raw pixels.
+    DCHECK(pixmap.alphaType() == kPremul_SkAlphaType);
+    auto pixel_chars = static_cast<const unsigned char*>(pixmap.addr());
+    if (!PlatformPictureImport(pixel_chars, &picture))
+      return nullptr;
+
+    // Create a buffer for where to store the output data.
+    std::vector<unsigned char> data;
+    picture.custom_ptr = &data;
+
+    // Use our own WebPWriterFunction implementation.
+    picture.writer = &WebPImageEncoder::WriteOutput;
+
+    // Setup the configuration for the output WebP picture. This is currently
+    // the same as the default configuration for WebP, but since any change in
+    // the WebP defaults would invalidate all caches they are hard coded.
+    config.quality = 75.0;  // between 0 (smallest file) and 100 (biggest).
+    config.method = 4;  // quality/speed trade-off (0=fast, 6=slower-better).
+
+    // Encode the picture using the given configuration.
+    bool success = WebPEncode(&config, &picture);
+
+    // Release the memory allocated by WebPPictureImport*(). This does not free
+    // the memory used by the picture object itself.
+    WebPPictureFree(&picture);
+
+    if (!success)
+      return nullptr;
+
+    // Copy WebP data into SkData. |data| is allocated only on the stack, so
+    // it is automatically deleted after this.
+    return SkData::NewWithCopy(&data.front(), data.size());
+  }
+
+ private:
+  // WebPWriterFunction implementation.
+  static int WriteOutput(const uint8_t* data,
+                         size_t size,
+                         const WebPPicture* const picture) {
+    std::vector<unsigned char>* dest =
+        static_cast<std::vector<unsigned char>*>(picture->custom_ptr);
+    dest->insert(dest->end(), data, data + size);
+    return 1;
+  }
+
+  // For each pixel, un-premultiplies the alpha-channel for each of the RGB
+  // channels. As an example, for a channel value that before multiplication was
+  // 255, and after applying an alpha of 128, the premultiplied pixel would be
+  // 128. The un-premultiply step uses the alpha-channel to get back to 255. The
+  // alpha channel is kept the unchanged.

[urvang, 2016/02/17 18:48:34]

s/the unchanged/unchanged/

[nyquist, 2016/02/17 22:39:00]

Done.

+  void UnPremultiply(const unsigned char* in_pixels,
+                     unsigned char* out_pixels,
+                     size_t pixel_count) {
+    const SkUnPreMultiply::Scale* table = SkUnPreMultiply::GetScaleTable();
+    for (; pixel_count-- > 0; in_pixels += 4) {
+      unsigned char alpha = in_pixels[3];
+      if (alpha == 255) {  // Full opacity, just blindly copy.
+        *out_pixels++ = in_pixels[0];
+        *out_pixels++ = in_pixels[1];
+        *out_pixels++ = in_pixels[2];
+        *out_pixels++ = alpha;
+      } else {
+        SkUnPreMultiply::Scale scale = table[alpha];
+        *out_pixels++ = SkUnPreMultiply::ApplyScale(scale, in_pixels[0]);
+        *out_pixels++ = SkUnPreMultiply::ApplyScale(scale, in_pixels[1]);
+        *out_pixels++ = SkUnPreMultiply::ApplyScale(scale, in_pixels[2]);
+        *out_pixels++ = alpha;
+      }
+    }
+  }
+
+  bool PlatformPictureImport(const unsigned char* pixels,
+                             WebPPicture* picture) {
+    // Need to unpremultiply each pixel, each pixel using 4 bytes (RGBA).
+    size_t pixel_count = picture->height * picture->width;
+    std::vector<unsigned char> unpremul_pixels(pixel_count * 4);
+    UnPremultiply(pixels, unpremul_pixels.data(), pixel_count);
+
+    // Each pixel uses 4 bytes (RGBA) which affects the stride per row.
+    int row_stride = picture->width * 4;
+
+    if (SK_B32_SHIFT)  // Android
+      return WebPPictureImportRGBA(picture, unpremul_pixels.data(), row_stride);
+    return WebPPictureImportBGRA(picture, unpremul_pixels.data(), row_stride);
+  }
+};
 
 }  // namespace
 
@@ -24,12 +143,12 @@ namespace blimp {
 BlimpImageSerializationProcessor::BlimpImageSerializationProcessor(Mode mode) {
   switch (mode) {
     case Mode::SERIALIZATION:
-      pixel_serializer_ = nullptr;
+      pixel_serializer_.reset(new WebPImageEncoder);
       pixel_deserializer_ = nullptr;
       break;
     case Mode::DESERIALIZATION:
       pixel_serializer_ = nullptr;
-      pixel_deserializer_ = &NoopDecoder;
+      pixel_deserializer_ = &WebPDecoder;
       break;
     default:
       NOTREACHED() << "Unknown serialization mode";