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In the Renderman Shading Language, the whole purpose of a shader is to determine the final pixel color for each fragment. You have complete control over what the shader does and what the final color is. If you have a scene with a mountain covered in snow, with trees in the foreground and a lake, if you want the whole image to be completely red--like (255, 0, 0) red in every single pixel--you can do that by setting the final color at the end of each shader. It can completely disregard the lights, image textures and geometry.
Is this possible in Cycles or OSL? I realize that the REYES algorithm is a very different beast than OSL closures in a unidirectional path tracer. But, can I override the physically-based-ness of Cycles in a shader?