Add support for encoding/decoding WebP images.

blimp/common/compositor/webp_decoder.cc

Index: blimp/common/compositor/webp_decoder.cc
diff --git a/blimp/common/compositor/webp_decoder.cc b/blimp/common/compositor/webp_decoder.cc
new file mode 100644
index 0000000000000000000000000000000000000000..88737d8712cb6be39b5305a5ec0647d8fec6dd20
--- /dev/null
+++ b/blimp/common/compositor/webp_decoder.cc
@@ -0,0 +1,85 @@
+// 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 {
+#if SK_B32_SHIFT  // Output little-endian RGBA pixels (Android).
+inline WEBP_CSP_MODE outputMode(bool hasAlpha) {
+  return hasAlpha ? MODE_rgbA : MODE_RGBA;

[urvang, 2016/02/13 00:24:36]

I'm thinking you can always use MODE_RGBA here (and MODE_BGRA below). Was there
a specific reason for the if/else?

Note: In Chrome, both premultiplied and non-premultiplied modes had to be
supported. And also, there's ICC profile to support. Hence the complexity there.

[nyquist, 2016/02/16 23:37:10]

Done.

+}
+#else  // Output little-endian BGRA pixels.
+inline WEBP_CSP_MODE outputMode(bool hasAlpha) {
+  return hasAlpha ? MODE_bgrA : MODE_BGRA;
+}
+#endif
+
+}  // namespace
+
+namespace blimp {
+
+bool WebPDecoder(const void* input, size_t input_size, SkBitmap* bitmap) {
+  DCHECK(bitmap);
+

[urvang, 2016/02/13 00:24:36]

Also DCHECK() that AlphaType is as expected (opaque or non-premultiplied)

[nyquist, 2016/02/16 23:37:10]

So, I added
DCHECK(config.input.has_alpha);

Is that enough?

+  // 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;
+  }

[urvang, 2016/02/13 00:24:36]

You can ensure ur assumption in code:
DCHECK_EQ(config.input.has_animation, 0);

[nyquist, 2016/02/16 23:37:10]

Done.

+
+  // 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;
+  decoderBuffer.colorspace = outputMode(config.input.has_alpha);
+  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;
+  }
+
+  // Set the bitmap's opaqueness based on what we saw.
+  bitmap->setAlphaType(config.input.has_alpha ? kPremul_SkAlphaType

[urvang, 2016/02/13 00:24:36]

As discussed, assume and use non-premultiplied mode everywhere. So:

bitmap->setAlphaType(config.input.has_alpha ? kUnpremul_SkAlphaType :
kOpaque_SkAlphaType;

[nyquist, 2016/02/16 23:37:10]

Done.

+                                              : kOpaque_SkAlphaType);
+
+  return true;
+}
+
+}  // namespace blimp