[Maths/Physics] Lose gravity when setting linear velocity

noirefield

Hi everyone,

So, I'm making a small game which the players control the ball to move around and shoot the enemies. But I'm stuck with an issue that is:

If you don't press any movement directional input and let it roll freely down the ramp it will gain speed from its gravity but if I press a directional buttons (like move forward, left or right) suddenly the gravity is lost and the ball slowdowns, how to keep the gravity?

Issue demo video:

I'm not very good at math so this is what I did: (Please check the attachment for the project source code, Size: 3mb, File: Main.tscn)

Thank you.

TwistedTwigleg

Welcome to the forums @noirefield!

The issue is that you are overriding the linear_velocity when the rigidbody moves, and this is what the RigidBody node uses to calculate gravity (among other things). For the most part, there are some exceptions, instead of directly modifying the velocity of a Rigidbody node, you want instead use forces and impulses to move the node around. By using forces, you shouldn't be overriding the linear velocity, which should allow gravity to work as expected. :smile:

noirefield

Thanks @TwistedTwigleg

Okay so I changed it to this

The gravity issue is solved but new issues come: 1. The speed increase continuously non-stop, not a constant speed for a player 2. It takes time to reverse the moving direction, like I'm moving forward but suddenly start moving backward and it takes time to reduce the speed before moving backwards. So how do I make it reverse the moving direction instantly?

These 2 issues don't appear in the previous way of moving (using set_linear_velocity)

Thanks.

TwistedTwigleg

Great, I'm glad the gravity issue is fixed at least.

As for the other two issues:

1: Not sure on this one just yet. I was looking at doing a constant movement speed myself a bit ago, but couldn't figure it out and didn't have the time to experiment. I'm sure there is a way though, I'm just not sure what it is, sorry!

2: If you are only moving horizontally, one thing you can try is checking to see if the velocity is going in the opposite direction of the direction you are wanting to move in, and if it is, setting the velocity to zero on the X axis and then setting the impulse. Something like this (untested):

if currentVelocity.x < 0 and direction.x < 0:
	currentVelocity.x = 0
	linear_velocity.x = 0
elif currentVelocity.x > 0 and direction.x > 0:
	currentVelocity.x = 0
	linear_velocity.x = 0

Something else you may want to look into is using a KinematicBody2D instead of a RigidBody2D for your character, unless you need something RigidBody2D specific. The KinematicBody2D node is much more easily controllable and would solve the first two issues relatively easily. There is also a documentation page with a simple character controller that you could use as a basis.

noirefield

@TwistedTwigleg

Thanks for your answer.

As for Issue #1, which can be solved similar to Unity: https://answers.unity.com/questions/265810/limiting-rigidbody-speed.html

As for Issue #2, I dont think I should switch to KinematicBody so I'll stay with RididBody however, I doubt your solution will work for relative velocity (move based on the direction of the camera and player)?

TwistedTwigleg

@noirefield said: @TwistedTwigleg

Thanks for your answer.

As for Issue #1, which can be solved similar to Unity: https://answers.unity.com/questions/265810/limiting-rigidbody-speed.html

Great!

As for Issue #2, I dont think I should switch to KinematicBody so I'll stay with RididBody however, I doubt your solution will work for relative velocity (move based on the direction of the camera and player)?

It might still work, as long as Gravity is on a single axis, whether it be X, Y, or Z, and movement (with the exception of jumping) doesn't affect that axis. You could also look into applying a force of equal force to the current velocity on the horizontal axes when you change directions and see if that works.