Fix blits from multisampled renderbuffers to alpha:false WebGL back buffer.

third_party/WebKit/Source/platform/graphics/gpu/DrawingBuffer.cpp

Index: third_party/WebKit/Source/platform/graphics/gpu/DrawingBuffer.cpp
diff --git a/third_party/WebKit/Source/platform/graphics/gpu/DrawingBuffer.cpp b/third_party/WebKit/Source/platform/graphics/gpu/DrawingBuffer.cpp
index 155558732ef1c1bfd811b51a7aaa3b5d72de3334..d4f384554dc9c51736d3c58b20f7f35ee702cc70 100644
--- a/third_party/WebKit/Source/platform/graphics/gpu/DrawingBuffer.cpp
+++ b/third_party/WebKit/Source/platform/graphics/gpu/DrawingBuffer.cpp
@@ -576,6 +576,19 @@ DrawingBuffer::CreateOrRecycleColorBuffer() {
   return CreateColorBuffer(size_);
 }
 
+DrawingBuffer::ScopedRGBEmulationForBlitFramebuffer::
+    ScopedRGBEmulationForBlitFramebuffer(DrawingBuffer* drawing_buffer)
+    : drawing_buffer_(drawing_buffer) {
+  doing_work_ = drawing_buffer->SetupRGBEmulationForBlitFramebuffer();
+}
+
+DrawingBuffer::ScopedRGBEmulationForBlitFramebuffer::
+    ~ScopedRGBEmulationForBlitFramebuffer() {
+  if (doing_work_) {
+    drawing_buffer_->CleanupRGBEmulationForBlitFramebuffer();
+  }
+}
+
 DrawingBuffer::ColorBuffer::ColorBuffer(
     DrawingBuffer* drawing_buffer,
     const ColorBufferParameters& parameters,
@@ -599,6 +612,10 @@ DrawingBuffer::ColorBuffer::~ColorBuffer() {
   if (image_id) {
     gl->BindTexture(parameters.target, texture_id);
     gl->ReleaseTexImage2DCHROMIUM(parameters.target, image_id);
+    if (rgb_workaround_texture_id) {
+      gl->BindTexture(parameters.target, rgb_workaround_texture_id);
+      gl->ReleaseTexImage2DCHROMIUM(parameters.target, image_id);
+    }
     gl->DestroyImageCHROMIUM(image_id);
     switch (parameters.target) {
       case GL_TEXTURE_2D:
@@ -618,6 +635,7 @@ DrawingBuffer::ColorBuffer::~ColorBuffer() {
     gpu_memory_buffer.reset();
   }
   gl->DeleteTextures(1, &texture_id);
+  gl->DeleteTextures(1, &rgb_workaround_texture_id);
 }
 
 bool DrawingBuffer::Initialize(const IntSize& size, bool use_multisampling) {
@@ -1258,6 +1276,78 @@ GLenum DrawingBuffer::GetMultisampledRenderbufferFormat() {
   return GL_RGB8_OES;
 }
 
+bool DrawingBuffer::SetupRGBEmulationForBlitFramebuffer() {
+  // We only need to do this work if:
+  //  - The user has selected alpha:false and antialias:false
+  //  - We are using CHROMIUM_image with RGB emulation
+  // macOS is the only platform on which this is necessary.
+
+  if (want_alpha_channel_ || anti_aliasing_mode_ != kNone)
+    return false;
+
+  if (!(ShouldUseChromiumImage() &&
+        ContextProvider()->GetCapabilities().chromium_image_rgb_emulation))
+    return false;
+
+  if (!back_color_buffer_)
+    return false;
+
+  // If for some reason the back buffer doesn't have a CHROMIUM_image,
+  // don't proceed with this workaround.
+  if (!back_color_buffer_->image_id)
+    return false;
+
+  // Before allowing the BlitFramebuffer call to go through, it's necessary
+  // to swap out the RGBA texture that's bound to the CHROMIUM_image
+  // instance with an RGB texture. BlitFramebuffer requires the internal
+  // formats of the source and destination to match when doing a
+  // multisample resolve, and the best way to achieve this without adding
+  // more full-screen blits is to hook up a true RGB texture to the
+  // underlying IOSurface. Unfortunately, on macOS, this rendering path
+  // destroys the alpha channel and requires a fixup afterward, which is
+  // why it isn't used all the time.
+
+  GLuint rgb_texture = back_color_buffer_->rgb_workaround_texture_id;
+  GLenum target = GC3D_TEXTURE_RECTANGLE_ARB;
+  if (!rgb_texture) {
+    gl_->GenTextures(1, &rgb_texture);
+    gl_->BindTexture(target, rgb_texture);
+    gl_->TexParameteri(target, GL_TEXTURE_MAG_FILTER, GL_LINEAR);
+    gl_->TexParameteri(target, GL_TEXTURE_MIN_FILTER, GL_LINEAR);
+    gl_->TexParameteri(target, GL_TEXTURE_WRAP_S, GL_CLAMP_TO_EDGE);
+    gl_->TexParameteri(target, GL_TEXTURE_WRAP_T, GL_CLAMP_TO_EDGE);
+
+    // Bind this texture to the CHROMIUM_image instance that the color
+    // buffer owns. This is an expensive operation, so it's important that
+    // the result be cached.
+    gl_->BindTexImage2DWithInternalformatCHROMIUM(target, GL_RGB,
+                                                  back_color_buffer_->image_id);
+    back_color_buffer_->rgb_workaround_texture_id = rgb_texture;
+  }
+
+  gl_->FramebufferTexture2D(GL_DRAW_FRAMEBUFFER_ANGLE, GL_COLOR_ATTACHMENT0,
+                            target, rgb_texture, 0);
+  return true;
+}
+
+void DrawingBuffer::CleanupRGBEmulationForBlitFramebuffer() {
+  // This will only be called if SetupRGBEmulationForBlitFramebuffer was.
+  // Put the framebuffer back the way it was, and clear the alpha channel.
+  DCHECK(back_color_buffer_);
+  DCHECK(back_color_buffer_->image_id);
+  GLenum target = GC3D_TEXTURE_RECTANGLE_ARB;
+  gl_->FramebufferTexture2D(GL_DRAW_FRAMEBUFFER_ANGLE, GL_COLOR_ATTACHMENT0,
+                            target, back_color_buffer_->texture_id, 0);
+  // Clear the alpha channel.
+  gl_->ColorMask(GL_FALSE, GL_FALSE, GL_FALSE, GL_TRUE);
+  gl_->Disable(GL_SCISSOR_TEST);
+  gl_->ClearColor(0, 0, 0, 1);
+  gl_->Clear(GL_COLOR_BUFFER_BIT);
+  DCHECK(client_);
+  client_->DrawingBufferClientRestoreScissorTest();
+  client_->DrawingBufferClientRestoreMaskAndClearValues();
+}
+
 DrawingBuffer::ScopedStateRestorer::ScopedStateRestorer(
     DrawingBuffer* drawing_buffer)
     : drawing_buffer_(drawing_buffer) {