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

content/common/gpu/client/gl_helper_readback_support.cc

 // Copyright 2014 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 "content/common/gpu/client/gl_helper_readback_support.h"
 #include "base/logging.h"
 #include "gpu/GLES2/gl2extchromium.h"
 #include "third_party/skia/include/core/SkImageInfo.h"
 
 namespace content {
 
 GLHelperReadbackSupport::GLHelperReadbackSupport(gpu::gles2::GLES2Interface* gl)
     : gl_(gl) {
   InitializeReadbackSupport();
 }
 
 GLHelperReadbackSupport::~GLHelperReadbackSupport() {}
 
 void GLHelperReadbackSupport::InitializeReadbackSupport() {
   // We are concerned about 16, 32-bit formats only.  The below are the most
   // used 16, 32-bit formats.  In future if any new format support is needed
   // that should be added here.  Initialize the array with
   // GLHelperReadbackSupport::NOT_SUPPORTED as we dont know the supported
   // formats yet.
   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);
   CheckForReadbackSupport(kBGRA_8888_SkColorType);
   // Further any formats, support should be checked here.
 }
 
 void GLHelperReadbackSupport::CheckForReadbackSupport(
     SkColorType texture_format) {
   bool supports_format = false;
   switch (texture_format) {
     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;
       break;
     case kBGRA_8888_SkColorType:
       supports_format = SupportsFormat(GL_BGRA_EXT, GL_UNSIGNED_BYTE);
       break;
     case kARGB_4444_SkColorType:
       supports_format = false;
       break;
@@ skipping 81 matching lines @@
   switch (color_type) {
     case kRGB_565_SkColorType:
       if (format_support_table_[color_type] ==
           GLHelperReadbackSupport::SUPPORTED) {
         *format = GL_RGB;
         *type = GL_UNSIGNED_SHORT_5_6_5;
         *bytes_per_pixel = 2;
         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) {
         // If GL_BGRA_EXT is advertised as the readback format through
         // GL_IMPLEMENTATION_COLOR_READ_FORMAT then assume it is preferred by
         // the implementation for performance.
         GetAdditionalFormat(*format, *type, &new_format, &new_type);
 
         if (new_format == GL_BGRA_EXT && new_type == GL_UNSIGNED_BYTE) {
           *format = GL_BGRA_EXT;
@@ skipping 17 matching lines @@
       return GLHelperReadbackSupport::NOT_SUPPORTED;
     default:
       NOTREACHED();
       break;
   }
 
   return GLHelperReadbackSupport::NOT_SUPPORTED;
 }
 
 }  // namespace content