Constant velocity -> weird physics interaction

Question

Hi,

I am writing a subway surfer's like game which uses a character with constant movement. I also want to have instant left/right movement (aka. no force).

I am doing both with this snippet:

 Vector3 vel = this.rigidbody.velocity;
 float hAxis = CustomInput.GetHorizontalAxis();
 vel.z = _currentFowardVelocity; // 8.0f
 vel.x = hAxis * _currentHorzintalVelocity; // max 8.0f
 this.rigidbody.velocity = vel;
 // gravity
 this.rigidbody.AddForce(25f * Vector3.down);

I set the Rigidbody Collision Detection to "Continuous" to deal with the increased gravity.

But I still get issues with my rigidbody entering other fixed objects and being stuck in them: rigidbody inside static mesh

The rigidbody is a capsule collider and it's stuck in a fixed box collider in the picture abvoe. The physics material (dynamic|static) friction and bounciness is set to 0.

Questions:

Is setting the constant velocity responsible for this behaviour?
How do I fix this?

Note that I can avoid this issue by enlarging the capsule collider, but then it doesn't feel right since it's way bigger than the board which it should represent.

Thanks

Answer 1

Answer by HarshadK · Jul 23, 2014 at 12:21 PM

As per the Unity Documentation for Rigidbody.velocity it clearly states that:

In most cases you should not modify the velocity directly, as this can result in unrealistic behaviour. Don't set the velocity of an object every physics step, this will lead to unrealistic physics simulation.

For the solution rather than setting velocity you can use Rigidbody.MovePosition to move your rigidbody.

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SimonSerious · Jul 23, 2014 at 05:54 PM 0

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Thanks for your suggestion, but are there any other options?

I tried it with MovePosition and setting rigidbody.position directly with the following snippet:


    Vector3 pos = this.rigidbody.position;
    float hAxis = CustomInput.GetHorizontalAxis();
    pos.z += _currentFowardVelocity * Time.fixedDeltaTime;
    pos.x += hAxis * _currentHorzintalVelocity * Time.fixedDeltaTime;
    //this.rigidbody.MovePosition(pos);
    this.rigidbody.position = pos;
    this.rigidbody.AddForce(_GRAVITY * Vector3.down);

But the physics simulation gets totally bonkers with using either one. The player is able to completely move through other objects with horizontal movement. Sometimes the player will fly at light speed in the opposite direction, etc. This behaviour is worse than with setting velocity directly. The player get's only "stuck" with a small overlap of physics bodies with that option, but doesn't pass through objects.

Constant velocity -> weird physics interaction

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