Encode frame number in pixel data for pose sync

chrome/browser/android/vr_shell/vr_shell.cc

Index: chrome/browser/android/vr_shell/vr_shell.cc
diff --git a/chrome/browser/android/vr_shell/vr_shell.cc b/chrome/browser/android/vr_shell/vr_shell.cc
index 79d159778ccad512838f9115c28d522887474682..996da9dbee77db397bac9034856c04fe36aea014 100644
--- a/chrome/browser/android/vr_shell/vr_shell.cc
+++ b/chrome/browser/android/vr_shell/vr_shell.cc
@@ -274,6 +274,24 @@ void VrShell::UpdateController(const gvr::Vec3f& forward_vector) {
   }
 }
 
+void VrShell::SetGvrPoseForWebVr(gvr::Mat4f pose, uint32_t pose_num) {
+  webvr_head_pose_[pose_num % POSE_QUEUE_SIZE] = pose;
+  webvr_newest_pose_num_ = pose_num;
+}
+
+int32_t GetPixelEncodedFrameNumber() {
+  // Read the frame number encoded in a bottom left pixel as color values.
+  // See also third_party/WebKit/Source/modules/vr/VRDisplay.cpp which
+  // encodes the frame number, and device/vr/android/gvr/gvr_device.cc
+  // which tracks poses.
+  uint8_t pixels[4];
+  // Assume we're reading from the frambebuffer we just wrote to.
+  // That's true currently, we may need to use glReadBuffer(GL_BACK)
+  // or equivalent if the rendering setup changes in the future.
+  glReadPixels(0, 0, 1, 1, GL_RGBA, GL_UNSIGNED_BYTE, pixels);
+  return pixels[0] | (pixels[1] << 8) | (pixels[2] << 16);
+}
+
 void VrShell::DrawFrame(JNIEnv* env, const JavaParamRef<jobject>& obj) {
   buffer_viewport_list_->SetToRecommendedBufferViewports();
 
@@ -291,6 +309,13 @@ void VrShell::DrawFrame(JNIEnv* env, const JavaParamRef<jobject>& obj) {
     if (!webvr_secure_origin_) {
       DrawWebVrOverlay(target_time.monotonic_system_time_nanos);
     }
+
+    int32_t webvr_pose_frame = GetPixelEncodedFrameNumber();
+    // LOG << "klausw: newest_pose=" << webvr_newest_pose_num_ <<
+    //     " pixel=" << webvr_pose_frame <<
+    //     "(" << webvr_pose_frame - webvr_newest_pose_num_ << ")";
+
+    head_pose = webvr_head_pose_[webvr_pose_frame % POSE_QUEUE_SIZE];
   } else {
     DrawVrShell(head_pose);
   }
@@ -516,7 +541,7 @@ void VrShell::DrawWebVrEye(const gvr::Mat4f& view_matrix,
   ScaleM(icon_pos, icon_pos, small_icon_width, small_icon_height, 1.0f);
   TranslateM(icon_pos, icon_pos, 0.0f, 0.0f, -kWebVrWarningDistance);
   icon_pos = MatrixMul(
-      QuatToMatrix(QuatFromAxisAngle(1.f, 0.f, 0.f, small_icon_angle)),
+      QuatToMatrix(QuatFromAxisAngle({1.f, 0.f, 0.f}, small_icon_angle)),
       icon_pos);
   gvr::Mat4f combined = MatrixMul(projection_matrix,
                                   MatrixMul(view_matrix, icon_pos));