Add support for encoding/decoding WebP images.

blimp/common/compositor/webp_decoder.cc

+// Copyright 2016 The Chromium Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style license that can be
+// found in the LICENSE file.
+
+#include "blimp/common/compositor/webp_decoder.h"
+
+#include "base/logging.h"
+#include "third_party/libwebp/webp/decode.h"
+#include "third_party/libwebp/webp/demux.h"
+#include "third_party/skia/include/core/SkBitmap.h"
+
+namespace blimp {
+
+bool WebPDecoder(const void* input, size_t input_size, SkBitmap* bitmap) {
+  DCHECK(bitmap);
+
+  // Initialize an empty WebPDecoderConfig.
+  WebPDecoderConfig config;
+  if (!WebPInitDecoderConfig(&config)) {
+    LOG(WARNING) << "Failed to initialize WebP config.";
+    return false;
+  }
+
+  // Treat the input as uint8_t.
+  WebPData data = {reinterpret_cast<const uint8_t*>(input), input_size};
+
+  // Read WebP feature information into |config.input|, which is a
+  // WebPBitstreamFeatures. It contains information such as width, height and
+  // whether the WebP image has an alpha channel or not.
+  if (WebPGetFeatures(data.bytes, data.size, &config.input) != VP8_STATUS_OK) {
+    LOG(WARNING) << "Failed to get WebP features.";
+    return false;
+  }
+  // Animations are not supported.
+  DCHECK_EQ(config.input.has_animation, 0);
+
+  // Allocate correct size for the bitmap based on the WebPBitstreamFeatures.
+  bitmap->allocN32Pixels(config.input.width, config.input.height);
+
+  // Setup the decoder buffer based on the WebPBitstreamFeatures.
+  WebPDecBuffer decoderBuffer;
+
+  DCHECK(config.input.has_alpha);
+#if SK_B32_SHIFT  // Output little-endian RGBA pixels (Android).
+  decoderBuffer.colorspace = MODE_rgbA;
+#else  // Output little-endian BGRA pixels.
+  decoderBuffer.colorspace = MODE_bgrA;
+#endif
+  decoderBuffer.u.RGBA.stride = config.input.width * 4;
+  decoderBuffer.u.RGBA.size = decoderBuffer.u.RGBA.stride * config.input.height;
+
+  // Instead of using the default WebPDecBuffer output, make WebPDecode directly
+  // write into the SkBitmap.
+  decoderBuffer.is_external_memory = 1;
+  decoderBuffer.u.RGBA.rgba =
+      reinterpret_cast<uint8_t*>(bitmap->getAddr32(0, 0));
+
+  // Set the config up to use the decoding buffer we created.
+  config.output = decoderBuffer;
+
+  // Decode the input data into the bitmap buffer.
+  bool success = WebPDecode(data.bytes, data.size, &config) == VP8_STATUS_OK;
+
+  // Now free the buffer. It is safe to call this even when the buffer is
+  // external and not allocated by WebPDecode.
+  WebPFreeDecBuffer(&config.output);
+
+  if (!success) {
+    LOG(WARNING) << "Failed to decode WebP data.";
+    return false;
+  }
+
+  // The input always has an alpha-channel.
+  bitmap->setAlphaType(kUnpremul_SkAlphaType);

[urvang, 2016/02/17 01:49:08]

Given that you are assuming pre-multiplied always, shouldn't this be
"kPremul_SkAphaType"?

[nyquist, 2016/02/17 16:56:44]

Hmm... Maybe I'm not understanding correctly: We get images from Skia, encode
them, serialize them, deserialize them, and then decode them and give them to
Skia.

As part of encoding them in the WebPImageEncoder, I unpremultiplied the data and
though I had to express that here. I do see though that skia seems to always
operate on premultiplied pixels.

I have checked the end result of this, and I am physically unable to see any
difference between the two outputs, as in; they get the same SHA1 sum in a
screenshot.

Changed to premultiplied though.

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

From what I understand, the only important part is that you are decoding a WebP
image into an SkBitmap. And given that you chose colorspace MODE_rgbA or
MODE_bgrA, you should mark this SkBitmap to have premultiplied alpha type.

And actually, I just realized that allocN32Pixels() takes care of this
automatically:
https://code.google.com/p/chromium/codesearch#chromium/src/third_party/skia/i...
So, you don't explicitly have to do this.
You can just:
DCHECK(bitmap->alphatype() == kPremul_SkAlphaType)
instead.

+
+  return true;
+}
+
+}  // namespace blimp