VR: Using rigidbody for movement, and getting fixed time in sync with frame rate

Question

I have a rigidbody that my VR camera rig is attached to. To move the player I add force to the rigidbody. For this to be smooth I have to set FixedDeltaTime to 1 / VR refresh rate. (I'm controlling it in FixedUpdate and using Interpolate on the rigidbody)

When I do just this it becomes silk smooth most of the time, but every 3-6 minutes it starts to judder badly for about 10 seconds, then goes back to being smooth again. The scene is quite basic, and I can see from the oculus debug tool, and unity profiler that this is not performance related. It seems to be a timing issue with fixed time. By debugging I can see that when it judders, it is doing 2 updates, then 2 fixedUpdates repeatedly (and 1 of each when smooth).

After experimenting I tried setting fixedDeltaTime to deltaTime at the beginning of an Update(). This seems to sync them up. I don't get the judder and with Interpolate set to Extrapolate on the rigidbody, it's nice and smooth.

This doesn't feel like the best solution though. I will be adding multi player soon. Will my fixed time being variable from frame to frame cause issues down the line?

I suspect that the problem here might be because the refresh rate of the headset is 90. 1 / 90 is 0.0111111111111111. But unity rounds this to 8 decimal places (0.01111111). This must be a common problem but I can't find much about it online.

Is there a better way I can sync fixedDeltaTime to always be in time with the frame rate, but with it still being constant?

Answer 1

In my VR-game (supports Quest, Rift, and SteamVR) i have Fixed Timestep set to 0.01 (100Hz), and SteamVR does what you do by default, having a variable fixed update, which even sounds wrong.

You have to do:

 SteamVR.settings.lockPhysicsUpdateRateToRenderFrequency = false;

I'm not having multiplayer (yet), but I do have a replay-recording and playing functionality which demand a fixed update at a predefined value.

The index headset can run up to 144Hz, the Quest as low as 60 but also 72Hz and 80, Rift 90, Rift S 80. It's not practical to try and sync this, leave the framerate alone and set a fixed fixed update.

You must combat the choppynes in some other way.

My code in my CameraController follows at an offset a ship (player) with a Rigidbody2D attached, and it works very smooth, no sign of the problem you are experience. But I don't know exactly why it works, maybe Interpolate and LateUpdate does the trick(?).

 GameObject oPlayer; //has RigidBody2D attached to it
 private Vector3 vCamOffset;
 private Vector3 vMapSize;
 internal static Vector3 vCamPos = new Vector3(0, 0, -4.5f);
 
 public void InitForGame(GameLevel i_oMap, GameObject i_oPlayer)
 {
     oPlayer = i_oPlayer;
     oMap = i_oMap;
     vCamPos = new Vector3(0, 0, -10.0f); //set it away from the player,
     // transform.position will then be set first Update
     vCamOffset = new Vector3(0, 0.3f, -1.90f);
     vMapSize = oMap.GetMapSize();
 }
 
 void LateUpdate()
 {
     Vector3 v = oPlayer.transform.position;
     float fLeftLimit = -(vMapSize.x / 20.0f) + 0.5f;
     float fRightLimit = (vMapSize.x / 20.0f) - 0.5f;
     if (v.x < fLeftLimit) v.x = fLeftLimit;
     if (v.x > fRightLimit) v.x = fRightLimit;
     float fTopLimit = (vMapSize.y / 20.0f) - 0.3f;
     float fBottomLimit = -(vMapSize.y / 20.0f) + 0.5f;
     if (v.y < fBottomLimit) v.y = fBottomLimit;
     if (v.y > fTopLimit) v.y = fTopLimit;
     v += vCamOffset; //add this after limiting
     
     transform.position = v; //camera holder
     vCamPos = v;
 }

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VR: Using rigidbody for movement, and getting fixed time in sync with frame rate

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