We will start with a very simple surface shader which shows you how the results of your shader calculations are passed back to the renderer. We don't worry about input parameters yet but you will learn that RenderMan uses a graphics state which includes shading attributes like the current color and current opacity. This attributes are known inside the shader as if they would have been defined globally (outside the shader). Mental ray uses another mechanism. It passes the state explicitly to the shaders and they grab the information needed by calling functions to extract the needed bits.