Question Jitter Calculation in HDRP

Code (CSharp):

1.  
2. var planes = origProj.decomposeProjection;
3.  
4. float vertFov = Math.Abs(planes.top) + Math.Abs(planes.bottom);
5. float horizFov = Math.Abs(planes.left) + Math.Abs(planes.right);
6.  
7. var planeJitter = new Vector2(jitterX * horizFov / actualWidth,
8.     jitterY * vertFov / actualHeight);
9.  
10. planes.left += planeJitter.x;
11. planes.right += planeJitter.x;
12. planes.top += planeJitter.y;
13. planes.bottom += planeJitter.y;
14.  
15. // Reconstruct the far plane for the jittered matrix.
16. // For extremely high far clip planes, the decomposed projection zFar evaluates to infinity.
17. if (float.IsInfinity(planes.zFar))
18.     planes.zFar = frustum.planes[5].distance;
19.  
20. proj = Matrix4x4.Frustum(planes);
21.  
22.  

How did you guys end up something like this.
I know this jitter gives up very nice result. But what the hell is this!
I hate plane calculation and died immediately, now I'm a ghost. Maybe someone very smart can tell me the detail of this calculation mathematically?

Especially these lines look important. But so obscure for me.

Code (CSharp):

1. float vertFov = Math.Abs(planes.top) + Math.Abs(planes.bottom);
2. float horizFov = Math.Abs(planes.left) + Math.Abs(planes.right);

 

https://github.com/Unity-Technologies/PostProcessing/commit/51877f63fa427a746627420b41c14156930bfea6

I've found original implementation with detail.
I noticed that I have no knowledge of components in projection matrix, and it was the problem. Thanks!