Catch shadows for an alpha channel

For more information about shadows, see Shadow in Maya.

The following is a simple example of how to use the Use Background material to catch shadows.

To capture only shadows in an alpha channel

1. In Hypershade, create a Use Background material and assign it to the shadow catching object(s).
2. Select the objects(s) in the scene casting shadows. In the Render Stats section of the objects’ Attribute Editor, turn Primary Visibility off.
3. Render the scene.

   Shadow information is captured in the alpha channel.

To assign Use Background to stand-in geometry

1. Use this workflow to make a 3D object look like it’s in a real environment. The real environment usually comes from a snapshot or live action shot that is assigned to an image plane. The object can cast shadows onto the seemingly invisible plane, which acts as the shadow catcher.
2. Create an image plane using the scene in which you want to place the stand-in geometry by doing the following.
3. Select View > Image Plane > Import Image from the current view.
4. Browse to the file you want and click Import.

   A placement icon appears in either the current view or all views, depending on the Display mode you set, and the Image Plane Attribute Editor opens.

5. Select an Image File or Texture from the Type drop-down list, then click the folder icon next to Image Name to load an image plane into the view.
   Note

   Make sure you select the Image Plane Attached to Camera option to position the plane where you want it before you create a stand-in object. For more information about image planes, see Image plane.

6. Set the following attributes in the Image Plane Attributes section:
   • Display–looking through camera
   • Image Plane–attached to Camera
7. Adjust the perspective camera so that the construction grid plane aligns approximately where you want to place the object.
   
8. Create and position a NURBS or polygonal plane on which to cast shadows and reflections.
9. Position the object you want to cast shadows onto the stand-in geometry on the grid plane.
   
10. In Hypershade, create a Use Background material and assign it to the plane.
11. Create and position lights in the scene. Turn on the Use Depth Map Shadows attribute in the Shadows section of the light’s Attribute Editor to see the shadows in the rendered result.
    
12. Render the scene in Render View (for best results, do not IPR render).
    
13. Adjust the location of the geometry, the lights, and the Use Background attributes if necessary and then re-render the scene at any time.
    

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