Grabpass refraction masking

Hey everyone,

I am trying to solve the grabpass front object issue with some depth masking but I am facing some difficulties in forward rendering. Here are some interesting parts : Code (CSharp):

 // Vertex function
 UNITY_INITIALIZE_OUTPUT(Input, o);
 float4 pos = mul (UNITY_MATRIX_MVP, v.vertex);
 o.depthCoord = ComputeScreenPos(pos);
 COMPUTE_EYEDEPTH(o.depthCoord.z);
 o.grabCoord = ComputeGrabScreenPos(pos);
  
 // Surface function
 // Offset
 half2 offset = o.Normal * _Refraction * _Grab_TexelSize.xy;
 // Masking
 IN.depthCoord.xy = offset * IN.depthCoord.z + IN.depthCoord.xy;
 half mask = step(IN.depthCoord.z, LinearEyeDepth(SAMPLE_DEPTH_TEXTURE_PROJ(_CameraDepthTexture, UNITY_PROJ_COORD(IN.depthCoord))));
 // Refraction
 offset *= mask;
 IN.grabCoord.xy = offset * IN.grabCoord.z + IN.grabCoord.xy;
 half3 refraction = tex2Dproj(_Grab, UNITY_PROJ_COORD(IN.grabCoord));

This works well in deferred rendering but in forward it fails as the grabpass y coordinates are inverted. Any idea on how to solve this easely ? I would like to avoid creating a full image effect shader mask.

Thank a lot !