Godot 4 physics bug or am I missing something!

carterlilley

Hey all - I'm having an issue with Godot 4's new Move and Slide function.

I'm getting these strange one-frame hitches where my model lowers itself into the ground and bounces back - see here for a whole bunch of examples.

All movement is processed appropriately in physics process, while model/camera rotations are in process. Physics process is set to 60fps. This issue was not present in Godot 3.

I used to process the movement like this: velocity = move_and_slide_with_snap(velocity, _snap_vector,Vector3.UP,true)

But now I process it like this: motion_velocity = _velocity + Vector3.DOWN move_and_slide()

Any ideas whats going on? Is it my fault or do I need to report a bug on the Godot 4 Git?

Thanks!

cybereality

I think you need to check if is_on_floor() is true and not apply gravity in this case (set velocity y to zero). However, there is an issue/bug is is_on_floor() that makes it return errant results. So you need to save the last 3 results of is_on_floor() and see which answer got the majority.

carterlilley

https://media.giphy.com/media/yoF6f2lIEFsDGDrJL0/giphy.gif

Here is a gif to make it more clear what is happening. The actual collider seems to randomly sink into the ground. Perhaps something to do with bullet?

carterlilley

Just so this thread is aware, I wanted to make sure my code was sound and that this was indeed a bug before reporting as such.

FWIW - I believe I've found a fix. Since I was porting this code from GD3, I was storing all velocity data in my own vector 3 named _velocity. Then, in GD4, I pass that to the new motion_velocity().

For some reason, storing the exact same values in motion_velocity from the beginning - and skipping the step of "passing" my velocity vector to motion_velocity resolves this issue.

That would lead me to assume it is a bug of some sort. I will open a report! Thanks all!

cybereality

I'm not sure if it's a bug, but it's possible. From my understanding, motion_velocity gets updated by the physics engine. This is why in Godot 3.x you would do:

your_velocity = move_and_slide(your_velocity, ...)

Because you have to keep track of the changes the physics engine makes to the velocity, otherwise they will be overwritten or conflict. If you were tracking your own velocity variable, without having it being updated in the right place to the current value of motion_velocity, then that is likely the issue and not a bug.

carterlilley

Hey - I think you're completely right about this, actually!

cybereality

So I think using motion_velocity directly is easier, at least for new projects. But if you already have a bunch of code written, you can do this:

motion_velocity = velocity
move_and_slide()
velocity = motion_velocity