Fix WebXR framebuffer discard/clear logic.

In cases when we explicitly resolved multisampled framebuffers the
resolved framebuffer was being discarded instead of the drawing
framebuffer. This likely had no ill effects but was not giving us the
benefit expected on those devices.

Additionally, framebuffers were not clearing prior to each new frame
(although discard would sometimes appear to have that effect), which
would result in data from previous frames being left behind in the
framebuffer in an unpredictable manner. This patch introduces a
consistent clear for predictability.

Bug: 863242
Cq-Include-Trybots: luci.chromium.try:linux_layout_tests_slimming_paint_v2;master.tryserver.blink:linux_trusty_blink_rel
Change-Id: Ib78c7d02812574a655220c6bdba50e02904dd483
Reviewed-on: https://chromium-review.googlesource.com/1135943Reviewed-by: Ian Vollick <vollick@chromium.org>
Reviewed-by: Klaus Weidner <klausw@chromium.org>
Commit-Queue: Ian Vollick <vollick@chromium.org>
Commit-Queue: Brandon Jones <bajones@chromium.org>
Cr-Commit-Position: refs/heads/master@{#575297}

third_party/blink/renderer/platform/graphics/gpu/xr_webgl_drawing_buffer.cc

@@ -288,12 +288,13 @@ void XRWebGLDrawingBuffer::UseSharedBuffer(
     framebuffer_complete_checked_for_sharedbuffer_ = true;
   }
 
-  if (discard_framebuffer_supported_) {
-    const GLenum kAttachments[3] = {GL_COLOR_ATTACHMENT0, GL_DEPTH_ATTACHMENT,
-                                    GL_STENCIL_ATTACHMENT};
-    gl->DiscardFramebufferEXT(GL_FRAMEBUFFER, 3, kAttachments);
+  if (WantExplicitResolve()) {
+    // Bind the drawing framebuffer if it wasn't bound previously.
+    gl->BindFramebuffer(GL_FRAMEBUFFER, framebuffer_);
   }
 
+  ClearBoundFramebuffer();
+
   DrawingBuffer::Client* client = drawing_buffer_->client();
   if (!client)
     return;
@@ -306,19 +307,14 @@ void XRWebGLDrawingBuffer::DoneWithSharedBuffer() {
   BindAndResolveDestinationFramebuffer();
 
   gpu::gles2::GLES2Interface* gl = drawing_buffer_->ContextGL();
+
+  // Discard the depth and stencil attachments since we're done with them.
+  // Don't discard the color buffer, we do need this rendered into the
+  // shared buffer.
   if (discard_framebuffer_supported_) {
-    // Discard the depth and stencil attachments since we're done with them.
-    // Don't discard the color buffer, we do need this rendered into the
-    // shared buffer.
-    if (WantExplicitResolve()) {
-      gl->BindFramebuffer(GL_FRAMEBUFFER, resolved_framebuffer_);
-    } else {
-      gl->BindFramebuffer(GL_FRAMEBUFFER, framebuffer_);
-    }
-    const GLenum kAttachments[] = {GL_DEPTH_ATTACHMENT, GL_STENCIL_ATTACHMENT};
-    gl->DiscardFramebufferEXT(GL_FRAMEBUFFER,
-                              sizeof(kAttachments) / sizeof(kAttachments[0]),
-                              kAttachments);
+    const GLenum kAttachments[3] = {GL_DEPTH_ATTACHMENT, GL_STENCIL_ATTACHMENT};
+    gl->BindFramebuffer(GL_FRAMEBUFFER, framebuffer_);
+    gl->DiscardFramebufferEXT(GL_FRAMEBUFFER, 2, kAttachments);
   }
 
   // Always bind to the default framebuffer as a hint to the GPU to start
@@ -336,6 +332,37 @@ void XRWebGLDrawingBuffer::DoneWithSharedBuffer() {
   client->DrawingBufferClientRestoreFramebufferBinding();
 }
 
+void XRWebGLDrawingBuffer::ClearBoundFramebuffer() {
+  gpu::gles2::GLES2Interface* gl = drawing_buffer_->ContextGL();
+
+  GLbitfield clear_bits = GL_COLOR_BUFFER_BIT;
+  gl->ColorMask(true, true, true, true);
+  gl->ClearColor(0.0f, 0.0f, 0.0f, 0.0f);
+
+  if (depth_) {
+    clear_bits |= GL_DEPTH_BUFFER_BIT;
+    gl->DepthMask(true);
+    gl->ClearDepthf(1.0f);
+  }
+
+  if (stencil_) {
+    clear_bits |= GL_STENCIL_BUFFER_BIT;
+    gl->StencilMaskSeparate(GL_FRONT, true);
+    gl->ClearStencil(0);
+  }
+
+  gl->Disable(GL_SCISSOR_TEST);
+
+  gl->Clear(clear_bits);
+
+  DrawingBuffer::Client* client = drawing_buffer_->client();
+  if (!client)
+    return;
+
+  client->DrawingBufferClientRestoreScissorTest();
+  client->DrawingBufferClientRestoreMaskAndClearValues();
+}
+
 void XRWebGLDrawingBuffer::Resize(const IntSize& new_size) {
   IntSize adjusted_size = AdjustSize(new_size);
 
@@ -545,12 +572,13 @@ void XRWebGLDrawingBuffer::SwapColorBuffers() {
     framebuffer_complete_checked_for_swap_ = true;
   }
 
-  if (discard_framebuffer_supported_) {
-    const GLenum kAttachments[3] = {GL_COLOR_ATTACHMENT0, GL_DEPTH_ATTACHMENT,
-                                    GL_STENCIL_ATTACHMENT};
-    gl->DiscardFramebufferEXT(GL_FRAMEBUFFER, 3, kAttachments);
+  if (WantExplicitResolve()) {
+    // Bind the drawing framebuffer if it wasn't bound previously.
+    gl->BindFramebuffer(GL_FRAMEBUFFER, framebuffer_);
   }
 
+  ClearBoundFramebuffer();
+
   client->DrawingBufferClientRestoreFramebufferBinding();
 }
 

third_party/blink/renderer/platform/graphics/gpu/xr_webgl_drawing_buffer.h

@@ -113,6 +113,8 @@ class PLATFORM_EXPORT XRWebGLDrawingBuffer
   void BindAndResolveDestinationFramebuffer();
   void SwapColorBuffers();
 
+  void ClearBoundFramebuffer();
+
   void MailboxReleased(scoped_refptr<ColorBuffer>,
                        const gpu::SyncToken&,
                        bool lost_resource);