Apply shader to all objects in the scene

I would like to render a scene using a specific geometry and fragment shader for each object in that scene. The problem is that all the objects already have materials and textures of their own. My shader would need to access those material's properties (each texture channel), together with normals, world position coordinates etc.

What would be the best way to achieve this? Replacing all the materials in the scene with my shader+original material by hand is not possible, as there are a lot of assemblies with child objects which kind of break when I try to modify them.

 

https://docs.unity3d.com/Manual/SL-ShaderReplacement.html

 

Thank you, I tried them and work as expected, both geometry and fragment shaders.

However, I encountered another problem: some objects in my scene have textures, and others don't (only use the Albedo color). In the replacement fragment shader, I can either use the _Color or _MainTex property for the output pixel value. Is there any way of using the texture when the object has a texture, and revert to its color when there's no texture assigned to it?

 

Is this really not possible?

I was thinking to test in my script if the material has a texture, and if not to create one with the existing color. But this seems too over-the-top for what it's supposed to do...

 

I doubt you still need help but just in case other people find this post, the solution is to multiply the color and maintex values in your shader.