DO NOT MERGE Support WebP Alpha (Lossless) images

Android/Skia supports pre-multiplied Alpha. WebP image format supports
Alpha via color modes MODE_RGBA & MODE_RGBA_4444 (corresponding ones for
premultiplied alpha are MODE_rgbA & MODE_rgbA_4444).
The pre-requisite change for this one is the libwebp patch:
https://googleplex-android-review.googlesource.com/#/c/219333/

Change-Id: I30b0d8d4b0325eb3c925371ac8f3a9b7d4801639

1 files changed, 18 insertions, 12 deletions

diff --git a/src/images/SkImageDecoder_libwebp.cpp b/src/images/SkImageDecoder_libwebp.cpp
index b2aea24..3ca1254 100644
--- a/src/images/SkImageDecoder_libwebp.cpp
+++ b/src/images/SkImageDecoder_libwebp.cpp
@@ -34,7 +34,6 @@ extern "C" {
 // If moving libwebp out of skia source tree, path for webp headers must be
 // updated accordingly. Here, we enforce using local copy in webp sub-directory.
 #include "webp/decode.h"
-#include "webp/decode_vp8.h"
 #include "webp/encode.h"
 }
 
@@ -146,16 +145,20 @@ static bool return_false(const SkBitmap& bm, const char msg[]) {
     return false; // must always return false
 }
 
-static WEBP_CSP_MODE webp_decode_mode(SkBitmap* decodedBitmap) {
+static WEBP_CSP_MODE webp_decode_mode(SkBitmap* decodedBitmap, int hasAlpha) {
     WEBP_CSP_MODE mode = MODE_LAST;
     SkBitmap::Config config = decodedBitmap->config();
+    // For images that have alpha, choose appropriate color mode (MODE_rgbA,
+    // MODE_rgbA_4444) that pre-multiplies RGB pixel values with transparency
+    // factor (alpha).
     if (config == SkBitmap::kARGB_8888_Config) {
-      mode = MODE_RGBA;
+      mode = hasAlpha ? MODE_rgbA : MODE_RGBA;
     } else if (config == SkBitmap::kARGB_4444_Config) {
-      mode = MODE_RGBA_4444;
+      mode = hasAlpha ? MODE_rgbA_4444 : MODE_RGBA_4444;
     } else if (config == SkBitmap::kRGB_565_Config) {
       mode = MODE_RGB_565;
     }
+    SkASSERT(mode != MODE_LAST);
     return mode;
 }
 
@@ -211,8 +214,8 @@ static bool webp_idecode(SkStream* stream, WebPDecoderConfig& config) {
 
 static bool webp_get_config_resize(WebPDecoderConfig& config,
                                    SkBitmap* decodedBitmap,
-                                   int width, int height) {
-    WEBP_CSP_MODE mode = webp_decode_mode(decodedBitmap);
+                                   int width, int height, int hasAlpha) {
+    WEBP_CSP_MODE mode = webp_decode_mode(decodedBitmap, hasAlpha);
     if (mode == MODE_LAST) {
         return false;
     }
@@ -239,10 +242,12 @@ static bool webp_get_config_resize(WebPDecoderConfig& config,
 
 static bool webp_get_config_resize_crop(WebPDecoderConfig& config,
                                         SkBitmap* decodedBitmap,
-                                        SkIRect region) {
+                                        SkIRect region, int hasAlpha) {
 
-    if (!webp_get_config_resize(config, decodedBitmap,
-                                region.width(), region.height())) return false;
+    if (!webp_get_config_resize(
+        config, decodedBitmap, region.width(), region.height(), hasAlpha)) {
+      return false;
+    }
 
     config.options.use_cropping = 1;
     config.options.crop_left = region.fLeft;
@@ -360,7 +365,7 @@ bool SkWEBPImageDecoder::onDecodeRegion(SkBitmap* decodedBitmap,
 
     SkAutoLockPixels alp(*bitmap);
     WebPDecoderConfig config;
-    if (!webp_get_config_resize_crop(config, bitmap, rect)) {
+    if (!webp_get_config_resize_crop(config, bitmap, rect, hasAlpha)) {
         return false;
     }
 
@@ -418,7 +423,8 @@ bool SkWEBPImageDecoder::onDecode(SkStream* stream, SkBitmap* decodedBitmap,
     SkAutoLockPixels alp(*decodedBitmap);
 
     WebPDecoderConfig config;
-    if (!webp_get_config_resize(config, decodedBitmap, origWidth, origHeight)) {
+    if (!webp_get_config_resize(config, decodedBitmap, origWidth, origHeight,
+                                hasAlpha)) {
         return false;
     }
 
@@ -560,7 +566,7 @@ bool SkWEBPImageEncoder::onEncode(SkWStream* stream, const SkBitmap& bm,
 static SkImageDecoder* DFactory(SkStream* stream) {
     int width, height, hasAlpha;
     if (!webp_parse_header(stream, &width, &height, &hasAlpha)) {
-        return false;
+        return NULL;
     }
 
     // Magic matches, call decoder