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 c81d27c609d415b0d5647747ae1897dd094e13cc..d8b72a5c37e3517e548154478e8f7604116184b6 100644
--- a/chrome/browser/android/vr_shell/vr_shell.cc
+++ b/chrome/browser/android/vr_shell/vr_shell.cc
@@ -274,6 +274,23 @@ void VrShell::UpdateController(const gvr::Vec3f& forward_vector) {
   }
 }
 
+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();
 
@@ -291,6 +308,14 @@ 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_ << ")";
+
+    int mod = POSE_QUEUE_SIZE;
+    head_pose = webvr_head_pose_[(webvr_pose_frame % mod + mod) % mod];
   } else {
     DrawVrShell(head_pose);
   }