Currently I have a water plane that has refraction effect in it using GrabPass, and I also have a script that does post processing in OnRenderImage(). This is very slow on mobile, so I am trying to skip the GrabPass in the material, and do a Graphics.Blitz() to a global _GrabTexture from the source inside the OnRenderImage() in my post processing script. However, doing so will require me to draw the water plane back to the source rendertexture before any other post process chain in this OnRenderImage() or else the water plane will not show up in the final image. So is there a way I can draw a mesh directly onto the source rendertexture in between the post process chain?
Graphics.DrawMeshNow did the trick.