为何自定义网格放入MeshCollider后显示为盒状碰撞体？

我在Unity中实现按住鼠标左键绘制UI选择框，将其投影到世界空间的MeshCollider以检测框选物体。通过RaycastAll将选择框四个角的屏幕坐标转换为世界坐标（这部分功能正常），但生成的MeshCollider显示为3D盒状，而非预期的4顶点平面网格。Inspector显示已加载自定义网格，双击网格能看到预期的平面，但场景中碰撞体显示为8顶点盒状。

主脚本代码 #

////////// SELECTION BOX ////////////////
//set vertex p0 to the position where the player first clicked
if (Input.GetMouseButtonDown(0))
{
    clickPos = Input.mousePosition;

    RaycastHit[] hits0 = Physics.RaycastAll(Camera.main.ScreenPointToRay(clickPos));
    foreach (RaycastHit hit in hits0)
    {
        if (hit.collider.tag == "chunk")
        {
            p0 = new Vector3(hit.point.x, -2f, hit.point.z);
        }
     }
}

//everything disappear if player let go the mouse button
if (Input.GetMouseButtonUp(0))
{
    selectionBox.gameObject.SetActive(false);
    GetComponent<MeshCollider>().sharedMesh?.Clear();
    GetComponent<MeshCollider>().enabled = false;
}
//if mouse button is held down, the UI box appear and the other vertices(p1, p2, p3) are determined accordingly
if (Input.GetMouseButton(0))
{
    mousePos = Input.mousePosition;
    if ((clickPos - mousePos).magnitude > 20f)
    {
        // selection box UI
        selectionBox.gameObject.SetActive(true);
        selectionBox.rectTransform.sizeDelta = new Vector2(Mathf.Abs(clickPos.x - mousePos.x), Mathf.Abs(clickPos.y - mousePos.y));
        selectionBox.rectTransform.anchoredPosition = clickPos + new Vector3((mousePos.x - clickPos.x) / 2, (mousePos.y - clickPos.y) / 2, 0f);

        //selection box meshcollider
        RaycastHit[] hits1 = Physics.RaycastAll(Camera.main.ScreenPointToRay(mousePos));
        foreach (RaycastHit hit in hits1)
        {
            if (hit.collider.tag == "chunk")
            {
                p1 = new Vector3(hit.point.x, -2f, hit.point.z);
            }
        }
        RaycastHit[] hits2 = Physics.RaycastAll(Camera.main.ScreenPointToRay(new Vector2(clickPos.x, mousePos.y)));
        foreach (RaycastHit hit in hits2)
        {
            if (hit.collider.tag == "chunk")
            {
                p2 = new Vector3(hit.point.x, -2f, hit.point.z);
            }
        }
        RaycastHit[] hits3 = Physics.RaycastAll(Camera.main.ScreenPointToRay(new Vector2(mousePos.x, clickPos.y)));
        foreach (RaycastHit hit in hits3)
        {
            if (hit.collider.tag == "chunk")
            {
                p3 = new Vector3(hit.point.x, -2f, hit.point.z);
            }
        }

//meshGen is a custom class that takes in four Vector3 and output the corresponding mesh, then the mesh is added to the meshcollider.
        mesh = meshGen.GenerateMeshFromWorldPoint(p0, p1, p2, p3);                
        GetComponent<MeshCollider>().sharedMesh = mesh;
        GetComponent<MeshCollider>().convex = true;
        GetComponent<MeshCollider>().enabled = true;
    }
}

MeshGen类代码 #

public class MeshGen
{
    Vector3[] _vertices;
    int[] _triangles;

    public Mesh GenerateMeshFromWorldPoint(Vector3 p0, Vector3 p1, Vector3 p2, Vector3 p3)
    {        
        _vertices = new Vector3[]
        {
            p0, p1, p2, p3
        };
        _triangles = new int[]
        {
            0, 1, 2, 0, 3, 1
        };
        Mesh mesh = new Mesh();
        mesh.name = "Custom Mesh Collider";
        mesh.vertices = _vertices;
        mesh.triangles = _triangles;
        return mesh;
    }
}

解决思路 #

• 关闭Convex选项：核心原因是开启了MeshCollider的convex = true，Unity为了让凸碰撞体满足封闭体积要求，会自动将平面网格拉伸为薄盒状。直接移除GetComponent<MeshCollider>().convex = true代码，或设置为false即可恢复平面碰撞体。
• 完善Mesh数据：虽然不影响碰撞功能，但补充法线和UV可让网格更规范，避免潜在显示问题：
  在MeshGen的GenerateMeshFromWorldPoint方法中添加：

  // 添加统一向上的法线，匹配平面朝向
  mesh.normals = new Vector3[] { Vector3.up, Vector3.up, Vector3.up, Vector3.up };
  // 添加UV坐标
  mesh.uv = new Vector2[] { Vector2.zero, Vector2.right, Vector2.up, Vector2.one };
  

• 优化Mesh清理逻辑：当前sharedMesh?.Clear()可能损坏Mesh资源，建议改为直接赋值null：

  GetComponent<MeshCollider>().sharedMesh = null;
  

• 检查物体变换参数：确保承载MeshCollider的GameObject没有非均匀缩放或异常偏移，避免网格被扭曲显示。

内容的提问来源于stack exchange，提问作者MathMith