Encode frame number in pixel data for pose sync

chrome/browser/android/vr_shell/vr_shell.cc

 // Copyright 2016 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 "chrome/browser/android/vr_shell/vr_shell.h"
 
 #include <thread>
 
 #include "chrome/browser/android/vr_shell/ui_scene.h"
 #include "chrome/browser/android/vr_shell/vr_compositor.h"
@@ skipping 256 matching lines @@
       bool is_inside = x >= 0.0f && x < 1.0f && y >= 0.0f && y < 1.0f;
       if (is_inside) {
         closest_element = distance_to_plane;
         target_point_ = plane_intersection_point;
         target_element_ = &plane;
       }
     }
   }
 }
 
+void VrShell::SetGvrPoseForWebVr(gvr::Mat4f pose, int32_t frameNumStarted) {
+  int mod = POSE_QUEUE_SIZE;
+  webvr_head_pose_[(frameNumStarted % mod + mod) % mod] = pose;

[bajones, 2016/10/02 20:55:23]

Is there a reason this isn't simply frameNumStarted % POSE_QUEUE_SIZE?

[klausw, 2016/10/02 21:25:50]

Yes, I've been using a signed int32_t where the mod result can be negative after
wraparound, this would be unusable as-is as an array index. The double mod is a
common idiom to get around that without assumptions about the C implementation's
choice of sign.

Originally I had picked signed int for Java compatibility, but if I remember
right this is currently C++ only and I could switch it to an unsigned int.

[klausw, 2016/10/04 00:40:41]

Changed to use uint32_t which simplifies this.

+  webvr_newest_pose_num_ = frameNumStarted;
+}
+
+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];
+  //glReadBuffer(GL_BACK); // crashes, null function pointer?!

[bajones, 2016/10/02 20:55:23]

This function is only available in OpenGL ES 3.0 and up, which I doubt we're
binding the function pointers for. In OpenGL ES 2.0 glReadPixels will always
occur with the bound framebuffer, which is what we want in this case.

[klausw, 2016/10/02 21:25:50]

Good to know, and it does seem to work as expected. I'll reword the comment.

[klausw, 2016/10/04 00:40:41]

Comment updated.

+  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();
 
   gvr::Frame frame = swap_chain_->AcquireFrame();
   gvr::ClockTimePoint target_time = gvr::GvrApi::GetTimePointNow();
   target_time.monotonic_system_time_nanos += kPredictionTimeWithoutVsyncNanos;
   gvr::Mat4f head_pose = gvr_api_->GetHeadSpaceFromStartSpaceRotation(target_time);
   head_pose = gvr_api_->ApplyNeckModel(head_pose, 1.0f);
 
   // Bind back to the default framebuffer.
   frame.BindBuffer(0);
 
   if (webvr_mode_) {
     DrawWebVr();
     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_ << ")";
+
+    int mod = POSE_QUEUE_SIZE;
+    head_pose = webvr_head_pose_[(webvr_pose_frame % mod + mod) % mod];
   } else {
     DrawVrShell(head_pose);
   }
 
   frame.Unbind();
   frame.Submit(*buffer_viewport_list_, head_pose);
 }
 
 void VrShell::DrawVrShell(const gvr::Mat4f& head_pose) {
   float screen_tilt = desktop_screen_tilt_ * M_PI / 180.0f;
@@ skipping 367 matching lines @@
       content::WebContents::FromJavaWebContents(content_web_contents));
   content::ContentViewCore* ui_core = content::ContentViewCore::FromWebContents(
       content::WebContents::FromJavaWebContents(ui_web_contents));
   return reinterpret_cast<intptr_t>(new VrShell(
       env, obj, c_core,
       reinterpret_cast<ui::WindowAndroid*>(content_window_android), ui_core,
       reinterpret_cast<ui::WindowAndroid*>(ui_window_android)));
 }
 
 }  // namespace vr_shell