cc: Invalidate framebuffer contents when clearing

cc/output/gl_renderer.cc

 // Copyright 2010 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 "cc/output/gl_renderer.h"
 
 #include <algorithm>
 #include <limits>
 #include <set>
 #include <string>
@@ skipping 186 matching lines @@
 
   // Check for texture fast paths. Currently we always use MO8 textures,
   // so we only need to avoid POT textures if we have an NPOT fast-path.
   capabilities_.avoid_pow2_textures = context_caps.fast_npot_mo8_textures;
 
   capabilities_.using_offscreen_context3d = true;
 
   capabilities_.using_map_image =
       Settings().use_map_image && context_caps.map_image;
 
+  capabilities_.using_discard_framebuffer =
+      context_caps.discard_framebuffer;
+
   is_using_bind_uniform_ = context_caps.bind_uniform_location;
 
   if (!InitializeSharedObjects())
     return false;
 
   // Make sure the viewport and context gets initialized, even if it is to zero.
   ViewportChanged();
   return true;
 }
 
@@ skipping 69 matching lines @@
 
 void GLRenderer::ClearFramebuffer(DrawingFrame* frame) {
   // It's unsafe to clear when we have a stencil test because glClear ignores
   // stencil.
   if (client_->ExternalStencilTestEnabled() &&
       frame->current_render_pass == frame->root_render_pass) {
     DCHECK(!frame->current_render_pass->has_transparent_background);
     return;
   }
 
+  if (capabilities_.using_discard_framebuffer) {
+    GLenum attachments[] = {
+        GL_COLOR_EXT
+    };
+    context_->discardFramebufferEXT(
+        GL_FRAMEBUFFER, arraysize(attachments), attachments);
+  }
+
   // On DEBUG builds, opaque render passes are cleared to blue to easily see
   // regions that were not drawn on the screen.
   if (frame->current_render_pass->has_transparent_background)
     GLC(context_, context_->clearColor(0, 0, 0, 0));
   else
     GLC(context_, context_->clearColor(0, 0, 1, 1));
 
   bool always_clear = false;
 #ifndef NDEBUG
   always_clear = true;
@@ skipping 2844 matching lines @@
   std::string unique_context_name = base::StringPrintf(
       "%s-Offscreen-%p",
       Settings().compositor_name.c_str(),
       context_);
   offscreen_context_provider->Context3d()->pushGroupMarkerEXT(
       unique_context_name.c_str());
 }
 
 
 }  // namespace cc