Autodesk 12812-051462-9011 User Guide 3 - Page 450

Matte group Opaque Alpha Determines whether or not the matte material appears in the alpha channel. If you turn off Opaque Alpha, the matte object will not make an alpha channel, and the image can be used for compositing, just as if there are no matte objects in the scene. Default=off. Atmosphere group These options determine whether fog effects are applied to the matte surfaces, and how they are applied. Apply Atmosphere Turns the fogging of matte objects on and off. When applying fog, you can choose between two different methods. You can either apply fog as if the matte surface is at an infinite distance from the camera or you can apply it as if the matte surface is actually at that point on the object being shaded. In other words, you can apply the fog to the matte surface in either 2D or 3D. The following controls determine how this is applied: At Background Depth This is the 2D method. The scanline renderer on page 8116 fogs the scene, and then renders its shadows. In this case, the shadows won't be lightened by the fog. If you want to lighten the shadows, you need to turn up the shadow brightness. At Object Depth This is the 3D method. The renderer first renders the shadows, and then fogs the scene. Since this varies the amount of fog over the 3D matte surface, the generated matte/alpha channels don't blend perfectly into the background. Use At Object Depth when the matte object is meant to be a 3D object in the scene that the 2D background represents. Shadow group This group determines whether the matte surfaces receive shadows that are cast upon them, and how they receive them. Receive Shadows Renders shadows on the matte surfaces. Default=on. Affect Alpha When on, shadows cast on a matte material are applied to the alpha channel. This lets you render bitmaps with alpha channels that you can composite later. Default=on. Affect Alpha is available only when Opaque Alpha (in the Matte group box) is turned off. Matte/Shadow Material | 5705