Oblique Frustum Clipping

Hi there,

I've been implementing oblique frustum clipping for our water to clip everything above water level. It works great :) For some reason, I can't for the life of me get it to work the other way around, ie: clip everything below water level. I can get it to work with the main camera at 0, but as soon as I try and incorporate the camera's position in any way, it screws up. The draw order reverses and it's almost like the frustum stays at 0 while the camera's moving.

Here's an example of the code I'm using for below, which works fine with clipping everything below the plane.

 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];
     }
 }

Any help would be much appreciated!

Thanks! Sam