CalculateObliqueMatrix inplement

CalculateObliqueMatrix

// https://forum.unity.com/threads/oblique-near-plane-clipping.194722/
public class Clipper : MonoBehaviour
{
    Matrix4x4 projection;
    Camera offscreenCam;
   
    void Start ()
    {
        projection            = camera.projectionMatrix;
    }
   
    void OnPreRender()
    {
        camera.projectionMatrix      = projection;
        Matrix4x4 obliqueProjection  = projection; 
       
        Vector4 cameraSpaceClipPlane = CameraSpacePlane(camera, new Vector3(0.0f, 0.0f, 0.0f), Vector3.up, 1.0f);
        CalculateObliqueMatrix(ref obliqueProjection, cameraSpaceClipPlane);
        camera.projectionMatrix = obliqueProjection;
    }
   
    static Vector4 CameraSpacePlane(Camera cam, Vector3 pos, Vector3 normal, float sideSign)
    {
        Vector3 offsetPos = pos + normal * 0.07f;
        Matrix4x4 m = cam.worldToCameraMatrix;
        Vector3 cpos = m.MultiplyPoint(offsetPos);
        Vector3 point = m.inverse.MultiplyPoint(new Vector3(0.0f, 0.0f, 0.0f));
        cpos -= new Vector3(0.0f, point.y, 0.0f);
        Vector3 cnormal = m.MultiplyVector( normal ).normalized * sideSign;
        return new Vector4(cnormal.x, cnormal.y, cnormal.z, -Vector3.Dot(cpos,cnormal));
    }  
       
    static void CalculateObliqueMatrix(ref Matrix4x4 projection, Vector4 clipPlane)
    {
        Vector4 q = projection.inverse * new Vector4(
            sgn(clipPlane.x),
            sgn(clipPlane.y),
            1.0f,
            1.0f
        );
        Vector4 c = clipPlane * (2.0F / (Vector4.Dot (clipPlane, q)));
        // third row = clip plane - fourth row
        projection[2] = c.x - projection[3];
        projection[6] = c.y - projection[7];
        projection[10] = c.z - projection[11];
        projection[14] = c.w - projection[15];
    }
   
    private static float sgn(float a)
    {
        if (a > 0.0f) return 1.0f;
        if (a < 0.0f) return -1.0f;
        return 0.0f;
    }
}