CharacterVirtual instability when standing on dynamic bodies

[thennequin]

Version

<= 5.5.0

Description

Hi, I am using CharacterVirtual for an FPS controller and encountered stability issues when standing on dynamic bodies.
I have reproduced similar behavior within the Jolt Physics Samples without any changes.

Observed Behavior

• Character class: Behaves as expected; when jumping onto a dynamic body (like a sphere), the character remains stable or falls off naturally according to physics.
• CharacterVirtual class: Exhibits instability in the same scenario. The character seems to "roll" or jitter off the dynamic body instead of maintaining a stable position.

Question

Are there specific settings, parameters (e.g., mMaxSlopeAngle, mPredictiveContactDistance), or callbacks for CharacterVirtual that can be tuned to better mimic the stable behavior of the standard Character class when interacting with dynamic objects?

Thank for your help

Video

0001-0168-discord.mp4

[jrouwe]

The issue here is that CharacterVirtual is a kinematic object (you control it by setting velocities). It has a special code path to push objects and be pushed by objects, but it is not really a physics simulation (it is a gameplay construct). The problem with the ball in the movie is:

• You stand on top of it
• You push it down at an off center location
• A force is applied at that point
• The ball mostly accumulates angular velocity (instead of linear velocity)
• Normally friction would result in the character being pushed off but CharacterVirtual doesn't do friction (it just follows the velocity you set)
• The character remains standing where it is and as the ball rotates towards the character it usually stays where it is
• Next frame the same thing repeats and the ball eventually starts rotating really quickly. If the ball rolls towards the character, it can push the character along if the angular velocity is converted to linear velocity due to friction with the ground.

In its current form you're responsible for programming the behavior. If you detect that you're standing on a moving body, apply a velocity to make the character slide away. Alternatively, you can register a CharacterContactListener and in OnContactAdded / Persisted you modify the CharacterContactSettings to control if the character / object push each other (e.g. small rubble should probably not push the character).

[thennequin]

Thank you for the explanation, it makes perfect sense.
I understand that since CharacterVirtual is kinematic, it lacks the natural friction to stay balanced on a rotating dynamic body.

I tried using a CharacterContactListener and setting both mCanPushCharacter and/or mCanReceiveImpulses to false in OnContactAdded/OnContactPersisted, but the rotation on the dynamic body persists and the player sometimes falls inside the body.

I will try to implement a custom logic to slide the character away based on the hit normal, which I hope should still work for cubic shapes.

If you have any code/sample to handle that case, I'm interested :) .

Thanks for your help.