Overview
I'm working on a shader in Unity 2017.1 to enable UnityEngine.UI.Image components to blur what is behind them.
As some of the approaches in this Unity forum topic, I use GrabPasses, specifically a tex2Dproj(_GrabTexture, UNITY_PROJ_COORD(<uv with offset>)) call to look up the pixels that I use in my blur summations. I'm doing a basic 2-pass box blur, and not looking to optimize performance right now.
This works as expected:
I also want to mask the blur effect based on the image alpha. I use tex2D(_MainTex, IN.uvmain) to look up the alpha color of the sprite on the pixel I am calculating the blur for, which I then combine with the alpha of the blur.
This works fine when working with just a single UI.Image object:
The Problem
However when I have multiple UI.Image objects that share the same Material created from this shader, images layered above will cut into the images below:
I believe this is because objects with the same material may be drawn simultaneously and so don't appear in each other's GrabPasses, or at least something to that effect.
That at least would explain why, if I duplicate the material and use each material on its own object, I don't have this problem.
Here is the source code for the shader: https://gist.github.com/JohannesMP/8d0f531b815dfad07823d44bc12b8112
The Question
Is there a way to force objects of the same material to draw consecutively and not in parallel? Basically, I would like the result of a lower object's render passes to be visible to the grab pass of subsequent objects.
I could imagine creating a component that dynamically instantiates materials to force this, or using render textures, but I would really like a solution that doesn't require adding components or creating multiple materials to swap out.
I would love a solution that is entirely self-contained within one shader/one material but is unsure if this is possible. I'm still only starting to get familiar with shaders so I'm positive there are some features I am not familiar with.
