筆記: 自定義物理 Physics.ComputePenetration

在編寫遊戲時經常需要做一些違返物理的事.
這時候很容易的就想用 tweening 或者自己計算的路經來覆寫物理引擎的結果.但是在實際運作時又會陷入想使用物理引擎的情況.
這時候就會想 : “如果有方法可以得知物理引擎的運算結果就好了.” 的想法

We always want to redefine physics during game development.
usually implement it by tweening or custom path to complete ignore the physic engine result.
however at some point we still wanted to respect the result from physic engine.

在 Unity3D 裡的 Physics.ComputePenetration 就是一個對這情況使用的接口.
Ref : https://docs.unity3d.com/ScriptReference/Physics.ComputePenetration.html
在官方的文件中有標明這是個佔用資源的 API,
並建議儘量於 Physics.Overlap 後使用.

以下為我所做的小測試, 方便明白這個 API 在做甚麼.

in Unity3D, there is an API : Physics.ComputePentration to handle this situation.
Ref : https://docs.unity3d.com/ScriptReference/Physics.ComputePenetration.html

In official document, there is a warning about this API.

This function is useful to write custom depenetration functions. One particular example is an implementation of a character controller where a specific reaction to collision with the surrounding physics objects is required. In this case, one would first query for the colliders nearby using OverlapSphere and then adjust the character’s position using the data returned by ComputePenetration.
PhysicsComputePenetration test (red arrow represent the final result of all overlap object vector(s))

可以看見隨著 Capsule 的位置改變, ComputePenetration 的結果總是指向沒有障礙物的空間.
那是物理引擎避免物件互相交疊而指出的建議路經.
測試中, 由於每支向量是單獨對於該 Collider 的所以最後的指向及距離需要自己來算.

As you can see based on the capsule position changed, the result from ComputePenetration are always pointing to the avoid collision direction.
that’s what physic engine will do on your object to avoid those collide overlap each others.
however you need to handle each collision vector by yourself, it return direction and distance and only able to return one object,
so for multiple objects you will need to calculate the final direction & distance after combine all of it.

Here is the implement of above test script.

using UnityEngine;
using Kit;

[RequireComponent(typeof(CapsuleCollider))]
public class PhyTest : MonoBehaviour
{
	[SerializeField] CapsuleCollider m_Self = null;
	[SerializeField, Range(0f, 89f)] float m_Angle = 3f;
	[SerializeField, Range(0f, 1f)] float m_Length = .3f;
	[SerializeField] Color m_DebugColor = Color.yellow;

	[SerializeField] Color m_Color = new Color(1f, 1f, 1f, 0.3f);
	[SerializeField] Color m_HitColor = new Color(1f, 0f, 0f, 0.4f);
	private Collider[] m_OverlapColliders = new Collider[128];
	private int HitCount { get; set; } = 0;

	private Vector3 SuggestedDirectionFromEngine(Collider[] objArr, int validRange)
	{
		Vector3 finalVector = Vector3.zero;
		float distance = 0f;
		int total = 0;
		for (int i = 0; i < validRange; i++)
		{
			Collider obstacle = objArr[i];
			if (obstacle == m_Self)
				continue;
			Vector3 pushBackDirection;
			float pushBackDistance;
			bool isCollision = Physics.ComputePenetration(m_Self, transform.position, transform.rotation,
				obstacle, obstacle.transform.position, obstacle.transform.rotation, out pushBackDirection, out pushBackDistance);
			if (!isCollision)
				continue;

			total++;
			distance += pushBackDistance;
			finalVector += pushBackDirection;

			Debug.DrawRay(transform.position, pushBackDirection * pushBackDistance, Color.magenta, 0f, false);
		}

		return finalVector.normalized * (distance / (float)total);
	}

	private void Reset()
	{
		if (m_Self == null)
			m_Self = GetComponent<CapsuleCollider>();
	}

	private void OnDrawGizmos()
	{
		if (KitBox.IsEditorMode)
		{
			Matrix4x4 matrix = Matrix4x4.TRS(transform.position, transform.rotation, transform.lossyScale);
			float half = Mathf.Clamp((m_Self.height / 2f) - m_Self.radius, 0f, float.PositiveInfinity);
			Vector3 p0 = matrix.MultiplyPoint3x4(m_Self.center + new Vector3(0f, half, 0f));
			Vector3 p1 = matrix.MultiplyPoint3x4(m_Self.center + new Vector3(0f, -half, 0f));
			HitCount = Physics.OverlapCapsuleNonAlloc(p0, p1, m_Self.radius, m_OverlapColliders);
		}
		
		Vector3 dir = SuggestedDirectionFromEngine(m_OverlapColliders, HitCount);
		GizmosExtend.DrawRay(transform.position, dir, Color.green);

		GizmosExtend.DrawArrow(transform.position, dir, m_DebugColor, m_Angle, m_Length);
	}
}
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