The making of Feedback: The Riverbank scene. A pilot for a 3D computer animated film.

Compositing refers to the layering of two-dimensional image plates on top of one-another to create a single image. It is used to weld together the different render layers created in the previous stage to create a final frame. Layers can be added together in a number of ways, but the most common is through the use of a transparency channel, otherwise known as an alpha channel or a mask. This black and white image is white where the picture is opaque and black where it is transparent. The compositing package reads this channel and uses it to “cut out” the image. The "cut-out" can then be "stuck" over those below it. The average rendered frame consists of four different channels: the red, green, blue and alpha (transparency) channels In our compositing package, image sequences are stacked in layers, as though one was looking down at a stack of pictures. You can see the whole of the top one, then parts of the second one through holes in the top one, and so on. For example, the final frame below is a stacked combination of all the layers below it.
	Although transparency is the most common method of layering image sequences, it is not the only one. For the shot on the left, the lens flare was rendered onto a black plate with no transparency channel. It was then "added" to the images below it. An "additive" function adds the intensity of the image to those below it. The intensity of the black pixels is zero, so they add nothing while the lighter pixels increase the brightness of the final frame.
" Post effects” are added to the individual layers to help them integrate in the final frame. The most basic of these is colour correction. It is very difficult to obtain the exact look you’re after within the Maya renderer. The shadows may not be dark enough, or the light bright enough. By applying effects to layers during the compositing stage, these discrepancies can be sorted out without having to render the image sequences again! Post-render colour-correction can made an enormous difference to the look of our shots. In the shot above-right, the foreground and matte have been blurred, a lens-flare added and subtle colour-correction applied to all the layers. Suddenly the shot stands out! We used relatively few effects in compositing the Riverbank scene. We focused on using the colour correction tools to sharpen the images, and the blur tools to create a sense of depth.
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