Impeller really wants premultiplied alpha

lib/ui/BUILD.gn

@@ -237,6 +237,7 @@ if (enable_unittests) {
       "fixtures/hello_loop_2.gif",
       "fixtures/hello_loop_2.webp",
       "fixtures/FontManifest.json",
+      "fixtures/unmultiplied_alpha.png",
       "fixtures/WideGamutIndexed.png",
       "//flutter/third_party/txt/third_party/fonts/Roboto-Medium.ttf",
     ]

lib/ui/painting/image_decoder_impeller.cc

@@ -205,6 +205,25 @@ DecompressResult ImageDecoderImpeller::DecompressTexture(
     bitmap->setImmutable();
   }
 
+  // If the image is unpremultiplied, fix it.
+  if (alpha_type == SkAlphaType::kUnpremul_SkAlphaType) {
+    // Single copy of ImpellerAllocator crashes.
+    auto premul_allocator = std::make_shared<ImpellerAllocator>(allocator);
+    auto premul_bitmap = std::make_shared<SkBitmap>();
+    premul_bitmap->setInfo(bitmap->info().makeAlphaType(kPremul_SkAlphaType));
+    if (!premul_bitmap->tryAllocPixels(premul_allocator.get())) {
+      std::string decode_error(
+          "Could not allocate intermediate for premultiplication conversion.");
+      FML_DLOG(ERROR) << decode_error;
+      return DecompressResult{.decode_error = decode_error};
+    }
+    // readPixels() handles converting pixels to premultiplied form.
+    bitmap->readPixels(premul_bitmap->pixmap());
+    premul_bitmap->setImmutable();
+    bitmap_allocator = premul_allocator;
+    bitmap = premul_bitmap;
+  }
+
   if (bitmap->dimensions() == target_size) {
     std::shared_ptr<impeller::DeviceBuffer> buffer =
         bitmap_allocator->GetDeviceBuffer();

lib/ui/painting/image_decoder_no_gl_unittests.cc

@@ -5,6 +5,7 @@
 #include "flutter/lib/ui/painting/image_decoder_no_gl_unittests.h"
 
 #include "flutter/fml/endianness.h"
+#include "include/core/SkColorType.h"
 
 namespace flutter {
 namespace testing {
@@ -186,5 +187,41 @@ TEST(ImageDecoderNoGLTest, ImpellerWideGamutIndexedPng) {
 #endif  // IMPELLER_SUPPORTS_RENDERING
 }
 
+TEST(ImageDecoderNoGLTest, ImepllerUnmultipliedAlphaPng) {
+#if defined(OS_FUCHSIA)
+  GTEST_SKIP() << "Fuchsia can't load the test fixtures.";
+#endif
+  auto data = flutter::testing::OpenFixtureAsSkData("unmultiplied_alpha.png");
+  auto image = SkImages::DeferredFromEncodedData(data);
+  ASSERT_TRUE(image != nullptr);
+  ASSERT_EQ(SkISize::Make(11, 11), image->dimensions());
+
+  ImageGeneratorRegistry registry;
+  std::shared_ptr<ImageGenerator> generator =
+      registry.CreateCompatibleGenerator(data);
+  ASSERT_TRUE(generator);
+
+  auto descriptor = fml::MakeRefCounted<ImageDescriptor>(std::move(data),
+                                                         std::move(generator));
+
+#if IMPELLER_SUPPORTS_RENDERING
+  std::shared_ptr<impeller::Allocator> allocator =
+      std::make_shared<impeller::TestImpellerAllocator>();
+  std::optional<DecompressResult> result =
+      ImageDecoderImpeller::DecompressTexture(
+          descriptor.get(), SkISize::Make(11, 11), {11, 11},
+          /*supports_wide_gamut=*/true, allocator);
+  ASSERT_EQ(result->image_info.colorType(), kRGBA_8888_SkColorType);
+
+  const SkPixmap& pixmap = result->sk_bitmap->pixmap();
+  const uint32_t* pixel_ptr = static_cast<const uint32_t*>(pixmap.addr());
+  // Test the upper left pixel is premultiplied and not solid red.
+  ASSERT_EQ(*pixel_ptr, (uint32_t)0x1000001);
+  // Test a pixel in the green box is still green.
+  ASSERT_EQ(*(pixel_ptr + 11 * 4 + 4), (uint32_t)0xFF00FF00);
+
+#endif  // IMPELLER_SUPPORTS_RENDERING
+}
+
 }  // namespace testing
 }  // namespace flutter