[Aura] Use GLHelper support to validate format before processing texture

content/common/gpu/client/gl_helper_readback_support.cc

Index: content/common/gpu/client/gl_helper_readback_support.cc
diff --git a/content/common/gpu/client/gl_helper_readback_support.cc b/content/common/gpu/client/gl_helper_readback_support.cc
index 4412c9546dece7a44a74d070574a6f082ef7b8d0..f5b9fc64186a9ac85a9ce497298cc56c149cea57 100644
--- a/content/common/gpu/client/gl_helper_readback_support.cc
+++ b/content/common/gpu/client/gl_helper_readback_support.cc
@@ -25,6 +25,8 @@ void GLHelperReadbackSupport::InitializeReadbackSupport() {
   for (int i = 0; i <= kLastEnum_SkColorType; ++i) {
     format_support_table_[i] = GLHelperReadbackSupport::NOT_SUPPORTED;
   }
+
+  CheckForReadbackSupport(kAlpha_8_SkColorType);
   CheckForReadbackSupport(kRGB_565_SkColorType);
   CheckForReadbackSupport(kARGB_4444_SkColorType);
   CheckForReadbackSupport(kRGBA_8888_SkColorType);
@@ -39,6 +41,9 @@ void GLHelperReadbackSupport::CheckForReadbackSupport(
     case kRGB_565_SkColorType:
       supports_format = SupportsFormat(GL_RGB, GL_UNSIGNED_SHORT_5_6_5);
       break;
+    // kAlpha_8_SkColorType is supported here via a special encoding
+    // using a 32-bit texture to represent an 8-bit image.
+    case kAlpha_8_SkColorType:
     case kRGBA_8888_SkColorType:
       // This is the baseline, assume always true.
       supports_format = true;
@@ -140,6 +145,7 @@ GLHelperReadbackSupport::GetReadbackConfig(SkColorType color_type,
         return GLHelperReadbackSupport::SUPPORTED;
       }
       break;
+    case kAlpha_8_SkColorType:

[piman, 2014/09/17 19:52:31]

I think we need more logic to handle this.

In the GLHelper::ReadbackTextureSync or Async path, there's no logic to move the
alpha components into a decimated RGBA texture, like there is in
CropScaleReadbackAndCleanTexture. So if we return GL_RGBA in that case, it will
do the wrong thing.

I don't think we want to encode the decimation logic into this class, which
should be strictly about glReadPixels capabilities. Note that
CropScaleReadbackAndCleanTexture calls this with kRGBA_8888_SkColorType for the
alpha case.


That said, glReadPixels does support reading back alpha only - see
https://www.khronos.org/opengles/sdk/docs/man/xhtml/glReadPixels.xml - so we
could support that case.

[sivag, 2014/09/18 13:19:03]

Done.

     case kRGBA_8888_SkColorType:
       *format = GL_RGBA;
       if (can_swizzle) {