Autodesk 12812-051462-9011 User Guide 3 - Page 368

marrying a photographic background with the 3D scene, including support for bump mapping, ambient occlusion, and indirect illumination. Applications include: ■ Blocking another 3D object from the camera view, thus allowing the 3D object to appear to be behind the object in the photo. ■ Allowing 3D objects to cast shadows and occlusion on and receive shadows from objects in the photo. ■ Adding reflections of 3D objects to objects in the photo. ■ Allowing the interplay of indirect light between 3D objects and objects in the photo. In all these cases the material is applied to a matte object that represents an object in the background plate, and the 3D object uses a traditional material. For additional information, see Help menu > Additional Help > mr Production Shader Library > Matte/Shadow Objects and CameraMaps, as well as the Tech Note (following). Tech Note The Matte/Shadow/Reflection shader works by doing a form of differential shading. In other words, it determines the amount of light a point would receive if it were not in shadow, compares it to the amount of light the point actually receives, and shades it by the relative difference. This means that any point that is fully lit, unshadowed by any object, returns the same color it already had, completely disregarding the actual intensity of that light. If half of the incoming light is blocked, the point will be shaded at 50 percent intensity, regardless of the full-intensity amount in an absolute sense. An important feature of the Matte/Shadow/Reflection material is that it is non-self-shadowing, non-self-occluding, non-self- reflecting, and does not cast indirect light onto itself. Because it is designed to act as a stand-in for objects present in a photographic plate, which already contains self-shadowing, self-reflection, and so on, the material automatically excludes these effects. However, it is still able to cast shadows on other objects, receive shadows from other objects, reflect other objects, and so on, withouth creating unwanted double shadows or double reflections for such effects already present in the plate. mental ray Materials | 5623