Question Help understanding Update and FixedUpdate

As I understand, Update runs on each frame update. Depending on the scene and speed of the computer, the Update may happen at irregular intervals. FixedUpdate will run at fixed interval, specified in project settings. And it's also recommended to run physics/collisions in FixedUpdate.

So my question is this: I have FixedUpdate set to 0.02 seconds, aka 50 times/second aka 50FPS. When I put my inputs under FixedUpdate, a lot of them are completely missed and not registered by FixedUpdate. Why is it that even when checking 50 times/second, FixedUpdate is not seeing my inputs? If instead my game checked for inputs under Update, and Update ran at 50FPS, would Update also miss my inputs?

I have tried using time interval for FixedUpdate to run every 0.0025 seconds aka 400FPS, and inputs works smoothly at that setting. But it seems FixedUpdate shouldn't be running that fast, or at least that isn't the purpose of it.

Also, my current test runs at 400FPS when using Update. The collision detection is very jittery. But when I place collision detection under FixedUpdate, which is running at only .02 seconds aka 50FPS, the collision detection is very smooth. Why is this? How is the faster update at 400FPS resulting in more jittery collisions?

 

I'm sure some other folks here can give more in depth answers, but in general inputs (namely, inputs using the old built-in system) should be done under regular Update. As you've experienced, reading inputs under FixedUpdate is unreliable, particularly as it happens frame independently.

And anything to do with collisions and physics should be done under FixedUpdate. As I understand, the engine's physics calculations happen on the FixedUpdate timestep. Thus, if you move something with physics/collision during update (and in particular, don't use the Rigidbody movement methods), you will be moving it outside of the physics engine calculations. As in, you will be moving objects every frame, which can potentially overlap colliders, before the physics engine catches up and fixes the position of everything.

Which is why you get that jittery motion. You're overlapping colliders outside of the physics engine, then they get moved back to where they should, which happens again ad nauseum.

Putting your physics related movement under FixedUpdate means they get moved in sync with the physics engine, preventing this aforementioned issue.

Hopefully that helps a bit.

 

Very good explanation

 

And to go with it, here is some timing diagram help:

https://docs.unity3d.com/Manual/ExecutionOrder.html