WebVR: add angular velocity estimate to pose

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 edcbb68f0fe05e2c0f75e96229e281b9e33a5b32..13d13bd866330577a634307b87fe44d62d23d57c 100644
--- a/chrome/browser/android/vr_shell/vr_shell.cc
+++ b/chrome/browser/android/vr_shell/vr_shell.cc
@@ -655,7 +655,9 @@ void VrShell::RegisterGamepadDataFetcher(
 }
 
 /* static */
-device::mojom::VRPosePtr VrShell::VRPosePtrFromGvrPose(gvr::Mat4f head_mat) {
+device::mojom::VRPosePtr VrShell::VRPosePtrFromGvrPose(gvr::Mat4f head_mat,
+                                                       gvr::Mat4f head_mat_2,
+                                                       int64_t epsilon_nanos) {
   device::mojom::VRPosePtr pose = device::mojom::VRPose::New();
 
   pose->orientation.emplace(4);
@@ -682,6 +684,36 @@ device::mojom::VRPosePtr VrShell::VRPosePtrFromGvrPose(gvr::Mat4f head_mat) {
     pose->position.value()[2] = decomposed_transform.translate[2];
   }
 
+  // The angular velocity is a 3-element vector pointing along the
+  // rotation axis with magnitude equal to rotation speed in
+  // radians/second.
+  //
+  // The spec isn't very clear on details, clarification requested in
+  // https://github.com/w3c/webvr/issues/212 .
+  //
+  // Assume we're using right-hand rule for direction and a
+  // headset-attached frame of reference with +X to the right, +Y up,
+  // and +Z backwards (towards back of head).
+  //
+  // Examples:
+  // - "look up" motion: (positive, 0, 0)
+  // - "turn left" motion: (0, positive, 0)
+  // - "lean head left" motion: (0, 0, positive)
+  //
+  // Assuming that pose prediction is simply based on adding a time *
+  // angular velocity rotation to the pose, we can approximate the
+  // angular velocity from the difference between two successive
+  // poses. This is a first order estimate that assumes small enough
+  // rotations so that we can do linear approximation.
+  pose->angularVelocity.emplace(3);
+  double epsilon_seconds = epsilon_nanos * 1e-9;

[cjgrant, 2017/03/24 21:15:40]

Should protect against epsilon_nanos being 0 to avoid a possible div-by-zero if
input is bad (on startup, shutdown, corner case, etc).

[klausw, 2017/03/24 22:13:29]

That can't happen accidentally since it's a compile-time constant we pick.
Adding a comment to clarify.

+  pose->angularVelocity.value()[0] =
+      (head_mat_2.m[1][2] - head_mat.m[1][2]) / epsilon_seconds;
+  pose->angularVelocity.value()[1] =
+      -(head_mat_2.m[0][2] - head_mat.m[0][2]) / epsilon_seconds;
+  pose->angularVelocity.value()[2] =
+      (head_mat_2.m[0][1] - head_mat.m[0][1]) / epsilon_seconds;
+
   return pose;
 }