Vertex Position to Screen Space and Back Again.

Hi Shader Pros,

I wrote a complex Fragment shader recently that modified the vertex positions relative to screen space. I'm trying to see if I can do the same thing in a Surface Shader. I understand the different spaces and how to use the built-in matrices to move up the chain e.g. M*V*P. What I struggle with is moving in the other direction.

here's the example scenario:

Let's say I want to move every vertex 5 screen-space pixels to the right. In a Fragment shader since my end goal is to convert the vertex position from object space to clip space (1 to -1 on the screen), i simple get the vertex screen position bump it to the right and I'm on my way:

 float offset = 5;

 float4 scrnSpacePosit = mul( UNITY_MATRIX_MVP, currVertex );
 scrnSpacePosit.y = scrnSpacePosit.y*_ProjectionParams.x;
 scrnSpacePosit.xy = float2(scrnSpacePosit.xy/scrnSpacePosit.w);
 float4 clipSpaceOffset = float4(offset/_ScreenParams.xy, 0, 0);
 float4 vectorRight = float4(1,0,0,0);
                     
 o.pos = scrnSpacePosit + (vectorRight * clipSpaceOffset);

However, for a Surface Shader it appears that vertex modification is done in Object Space. So my question is: How can I reverse the ScreenSpace Position back into Object Space. Or alternatively, I need to pass my (vectorRight * clipSpaceOffset) back into objectSpace.

Any Help is much appreciated.

-Mike