Interpolating between positions unfeasible at 30 frames per second?

Question

I am working on a 3D platformer in Unity and I am targeting Android devices. The game consists of a lot of objects smoothly interpolating from the bottom of the screen to the top.

This whole time I've been testing my game at 60 fps and the performance was quite good. However, when I tested the game at 30 fps, the interpolation of the objects was no longer smooth.

I initially thought that I have way too many objects in my scene and this might be affecting the interpolating performance. So I created a new project which only has the interpolation stuff and object pooling (you can download it here). But even then, the interpolation wasn't smooth at 30 fps. Here's the code I'm using for interpolating the object:

void FixedUpdate()
{
   rb.MovePosition(transform.position + transform.up * Time.fixedDeltaTime * speed);
}

I'm using the object's Rigidbody to interpolate it. I've already set the rigidbody to "Interpolate" and I turned on "isKinematic".

I first thought that the interpolation isn't smooth since I'm using Rigidbody. So I ditched the rigidbody and used transform.Translate instead. But even then the interpolation wasn't smooth. Next I tried Vector3.Lerp. Nope. That didn't work either. The stutter was still there. I even tried Vector3.MoveTowards. That didn't work. The interpolating was still laggy. I even tried turning on GPU instancing for the objects being interpolated (since they all use the same material). That didn't do it either.

Next I started playing around with the fixed timestep (in the Time settings). I thought that maybe if I set the timestep to 0.033, then the objects will interpolate smoothly at 30 fps. But that didn't work either.

I think it's really bizarre that a simple interpolation performs so poorly at 30 fps on an Android device. If the interpolation is only for a short distance, then you don't always notice the stutter. But like I said, in my game the objects have to interpolate from the bottom of the screen all the way to the top. In this case, you can clearly see the laggy movement during interpolation.

Is it possible that long distance interpolation is simply unfeasible for Android devices at 30 fps? How do so many Android game developers manage to achieve good performance at 30 fps? Feel free to download the new project I mentioned which only has the interpolation stuff and the object pooling. It is available here. It's a very simple projects. It only has the prefab that should be interpolated, the material for the prefab, the script for interpolating the prefab, the spawning and object pooling system (for spawning the prefab), and the script for setting frame rate to 30. That's it.

Below a gif of the objects' interpolation. It's not as shaky in reality (since I had to use a bad screen recorder and compress the gif) but it's still pretty close to how the objects move at 30 fps.

Answer 1

There are 3 things you are possibly seeing happen (with a bonus fourth at the end)...

1. You are not updating the screen as often as you should
2. You are not updating the position as often as you should
3. You are just trying to do too much at 30 FPS, and 30 FPS can't keep up.

To that end...

1. Confirm you are really updating at 30 FPS
2. Confirm you are really updating the position a least 30 FPS
3. If you are doing both, then think about redesigning your game concept to slow it down

Here is one other thing to consider.

Imagine you're updating your graphics every 30 ms, and your position every 20. Frame update is on the left, position update on the right...

0 
      20    
30                  
      40
60    60            
      80
90                  
      100
120   120
      140
150
      160
180   180

You can see that sometimes you will get two moves for a frame update, which will cause a jerky motion. Despite the suggestions I have up front, I'm wondering if what you're seeing is a reflection of this kind of uneven sampling.

Answer 2

I think this article summarizes the problem you are seeing FIX YOUR (UNITY) TIMESTEP!

You could calculate your fixedDeltaTime like:

Application.targetFrameRate = 30;
Time.fixedDeltaTime = 1 / Application.targetFrameRate;

---

Just to clear up some possible misunderstandings:

The fixed time step has nothing to do with the update or with when a current frame is drawn.

The fixed time step is only for the fixed update in which the physical calculations are performed. The fixed part is to ensure that the physical objects behave more reliably, even if you drop some frames.

Order of execution for event functions

On the other hand, there is Update, where calculations are performed, and after they are completed, no matter how long it took, the frame is finally drawn.

Also, you don't need to include the Library folder when you upload a project as it can be restored locally. I would recommend using a version control system like Git and storing projects on a server to ease collaboration. There is also a gitignore file in version control that excludes all files that do not need to be tracked (uploaded and downloaded).

For the frame stats you can use the profiler and or click the stats button on the top right of your game view window.