Modifying previous passes in multi-pass surface shader

I have a surface shader with 3 passes.
On the final pass I would like to distort what the previous 2 passes have rendered with a normal map.
The normal map distortion is not a problem, as there are many resources on the topic online.

However, I cannot find how to grab previous passes within a surface shader.
Is this even possible?

 

I don't believe that's possible due to the isolation of each pass. One solution (albeit at the expense of an additional pass) would be to insert a named GrabPass which could then be used as the input to the third. Something like this;

Code (CSharp):

1. Pass { FirstPass }
2. Pass { SecondPass }
3. GrabPass { "InputTex" }
4. Pass { FourthPass, distort InputTex }

 

I would recommend using a CommandBuffer instead. For testing purposes a GrabPass is fine, but you're a lot more flexible when using a CommandBuffer. So render pass 1 and 2 into a RenderTexture at the right time using a CommandBuffer and render pass 3 in the normal way using the RenderTexture as input.

 

Indeed, that's the other solution. Although it's worth noting that GrabPass is perfectly fine for production purposes too! It doesn't quite add up to the cost of a regular pass as it merely blits to texture and makes it available to you. In-essence, it's the same approach, just less involved.