Rendering with Replaced Shaders
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Rendering with Replaced Shaders

Some rendering effects require rendering a scene with a different set of shaders. For example, good edge detection would need a texture with scene normals, so it could detect edges where surface orientations differ. Other effects might need a texture with scene depth, and so on. To achieve this, it is possible to render the scene with replaced shaders of all objects.

Shader replacement is done from scripting using Camera.RenderWithShader or Camera.SetReplacementShader functions. Both functions take a shader and a replacementTag.

It works like this: the camera renders the scene as it normally would. the objects still use their materials, but the actual shader that ends up being used is changed:

So if all shaders would have, for example, a "RenderType" tag with values like "Opaque", "Transparent", "Background", "Overlay", you could write a replacement shader that only renders solid objects by using one subshader with RenderType=Solid tag. The other tag types would not be found in the replacement shader, so the objects would be not rendered. Or you could write several subshaders for different "RenderType" tag values. Incidentally, all built-in Unity shaders have a "RenderType" tag set.

Shader replacement tags in built-in Unity shaders

All built-in Unity shaders have a "RenderType" tag set that can be used when rendering with replaced shaders. Tag values are the following: