1. A method for performing shading operations in a graphics rendering system, the method comprising: factoring terms in a shading model into separate image layers, where the shading model represents a description of shading to be applied to 3D scene geometry and the shading model is factored over a 3D object or set of 3D objects in the 3D scene geometry; separately rendering the 3D scene geometry to the separate image layers, where two or more of the layers have different rendering update rates; and combining the terms of the shading model by compositing the separate image layers using an image operator to combine the rendered image layers into an output image.