Compositing images during output (something else we do) can save a lot of rendering time, but again, you need to carefully prepare your work and give exact instructions.
Every image containing an alpha channel will be layered on top of the images in the lower numbered channels. For example, the image in channel 2 will be composited on top of the image in channel 1, the image in channel 3 will be composited on top of images in channels 1 and 2, etc. up to 49 channels. If an image does not contain an alpha channel, it will cover all of the lower numbered channels. Make some test composites in Photoshop before you send your work.
The graphics below explain the process. Channel 1 in our example contains a background PICT file of a star field. Channel 2 is a PICT file of Mars and channel 3 a PICT file of the Earth. Mars and Earth were rendered in Electric Image on a black background. The files have an alpha channel. Channel 4 is an Electric Image Movie (could also be a PICS file) of a X29 crossing the scene. The file contains 120 frames and is also rendered with an alpha channel. The same applies for channel 5, an animation file of a shuttle traveling across the scene. In the graphic below you can see that the scene begins with the star field, Earth and Mars. The X29 appears followed by the shuttle. The scene ends with the stars, Earth and Mars. The output of this file can be seen on our demo tape.
