Use the Matte shader with camera back plate textures or videos. See Setting a camera Front or Back Plane. 
         
          To create a 3D matte:
         
         
            - Drag the Matte shader from the Shaders folder in the Asset browser, and drop it onto your model. 
- Select Replace by Type from the context menu that appears. 
- In the Matte shader settings, select Highlight Matted Objects to surround the object with a faint outline if you are having
               trouble viewing the matted objects in your scene. 
            
- Activate Enable 3D Masking option to apply the 3D Matte shader to the selected object. When Enable 3D Masking is not active,
               the shader appears as a wireframe. 
            
- Activate Adjust Alpha to control the video Alpha channel sent either through your video option board or viewed in the Render
               window. 
            
- Use the Alpha slider to manipulate the transparency of the Alpha channel. This lets you, for example, make the output slightly
               transparent so you can pre-visualize characters interacting with live Actors. 
            
 Except where otherwise noted, this work is licensed under a Creative Commons Attribution-NonCommercial-ShareAlike 3.0 Unported License
 Except where otherwise noted, this work is licensed under a Creative Commons Attribution-NonCommercial-ShareAlike 3.0 Unported License