gpu: gpu decoder sets viewport size same to the size of the default framebuffer.

gpu/command_buffer/service/gles2_cmd_decoder.cc

 // Copyright (c) 2012 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 "gpu/command_buffer/service/gles2_cmd_decoder.h"
 
 #include <stdio.h>
 
 #include <algorithm>
 #include <list>
@@ skipping 2663 matching lines @@
     offscreen_saved_color_texture_->Create();
 
     // Allocate the render buffers at their initial size and check the status
     // of the frame buffers is okay.
     if (!ResizeOffscreenFrameBuffer(size)) {
       LOG(ERROR) << "Could not allocate offscreen buffer storage.";
       Destroy(true);
       return false;
     }
 
+    // |size| is set if the GL context is offscreen.
+    state_.viewport_width = size.width();
+    state_.viewport_height = size.height();
+
     // Allocate the offscreen saved color texture.
     DCHECK(offscreen_saved_color_format_);
     offscreen_saved_color_texture_->AllocateStorage(
         gfx::Size(1, 1), offscreen_saved_color_format_, true);
 
     offscreen_saved_frame_buffer_->AttachRenderTexture(
         offscreen_saved_color_texture_.get());
     if (offscreen_saved_frame_buffer_->CheckStatus() !=
         GL_FRAMEBUFFER_COMPLETE) {
       LOG(ERROR) << "Offscreen saved FBO was incomplete.";
@@ skipping 41 matching lines @@
 
   has_robustness_extension_ =
       context->HasExtension("GL_ARB_robustness") ||
       context->HasExtension("GL_KHR_robustness") ||
       context->HasExtension("GL_EXT_robustness");
 
   if (!InitializeShaderTranslator()) {
     return false;
   }
 
-  state_.viewport_width = size.width();
-  state_.viewport_height = size.height();

[no sievers, 2015/03/02 20:37:12]

What's the bug?

We call InitState() in line 2758, which calls glViewport() with the value in
|state_|, so the driver's state will match our initial state.

Your patch changes the initial state for onscreen surface from 0,0 to 0,1 (w,h),
why is better?

If we want to match the spec better (viewport should match window size
initially) we could try surface_->GetSize() for onscreen contexts. But not sure
that will always work or fix anything. We probably rely on the client to do the
right thing.

[dshwang, 2015/03/03 09:20:29]

There's no bug. I just confused why onscreen viewport is initially 0, 0.
previous patch changes from 0,0 to 1,1
That's good point! onscreen viewport should match surface_->GetSize()
it's always work because surface must be created before this function.
It doesn't fix anything but it's what code is supposed to be.

-
   GLint viewport_params[4] = { 0 };
   glGetIntegerv(GL_MAX_VIEWPORT_DIMS, viewport_params);
   viewport_max_width_ = viewport_params[0];
   viewport_max_height_ = viewport_params[1];
 
   state_.scissor_width = state_.viewport_width;
   state_.scissor_height = state_.viewport_height;
 
   // Set all the default state because some GL drivers get it wrong.
   state_.InitCapabilities(NULL);
@@ skipping 9369 matching lines @@
   }
 }
 
 // Include the auto-generated part of this file. We split this because it means
 // we can easily edit the non-auto generated parts right here in this file
 // instead of having to edit some template or the code generator.
 #include "gpu/command_buffer/service/gles2_cmd_decoder_autogen.h"
 
 }  // namespace gles2
 }  // namespace gpu