reconstruct worldspace plane coordinates in post process

i want to create a postprocess-shader, which reconstructs the xy-coordinates on a plane at z=0, to then use it further.

somehow, i am not able to properly reconstruct the world positions from the Screenspace coordinates correctly.

_

my fragment shader looks like this:

Code (CSharp):

1.                     fixed4 col;
2.                     float3 rayStart = mul(unity_CameraToWorld, float4(i.uv, .0,0));
3.                     float3 rayEnd   = mul(unity_CameraToWorld, float4(i.uv, 1,0));
4.  
5.                     float3 direction = rayEnd - rayStart;
6.  
7.                     float distanceToDrawPlane = (.0 - rayStart.z) / direction.z;
8.                     float3 position = rayStart + direction * distanceToDrawPlane;
9.  
10.                     col.xyz = position;
11.  
12.                     //debug grid pattern
13.                     col.xyz = frac(position.x*10) < .5;
14.                     col.xyz *= frac(position.y * 10) < .5;
15.  
16.                     return col;

while the grid is on the correct axis, it has some problems:

- it moves with the camera, instead of keeping its position relative to the origin
- the perspective is always orthographic, even in the perspective camera.


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now, i tried different approaches, but was unable to make it work properly.

does anyone know how to do this conversion properly, or has any example in mind?

edit: attached all files for the render pass and shader

 

note: passing c

am.cameraToWorldMatrix

into the shader yield the same result,

unity_CameraInvProjection

does not work at all.

i also tried these two, but none of them seem to work:

            material.SetMatrix("_CamMatrix", cam.cameraToWorldMatrix); //is same as unity_CameraToWorld

            material.SetMatrix("_CamMatrix", cam.projectionMatrix.inverse); //does nothing at all