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706 Chapter 24 Keying
Keylight
Keylight is an Academy Award-winning keyer from Framestore CFC based in England. It
accurately models the interaction of the bluescreen or greenscreen light with the
foreground elements, and replaces it with light from the new background. With this
approach, blue spill and green spill removal becomes an intrinsic part of the process, and
provides a much more natural look with less tedious trial-and-error work. Soft edges,
such as hair, and out-of-focus edges are pulled quite easily with the Keylight node.
To ensure the best results, try to always pull the key on raw plates. In the Keylight
parameters, there is a colourspace control to indicate if the plate is in log, linear, or
video color space. Therefore, you should not perform color correction on film plates
when feeding the plates into Keylight.
For a hands-on example of using the Keylight node, see Tutorial 5, “Using Keylight,” in
the Shake 4 Tutorials.
Parameters
This node displays the following controls in the Parameters tab:
output
You have the option to do your composite within the Keylight node, but there are other
output options as well should you want to composite the foreground image using
other nodes:
• comp: Renders the final composite, against the assigned background.
• on Black: Renders the foreground objects over black, creating a premultiplied
output.
• on Replace: Renders the foreground objects over the replaceColour. This is a good
mode to test your composite. Choosing a bright color allows you to instantly see if
there are unwanted transparent areas in the foreground subject.
• unpremult: Renders the foreground without premultiplying the matte. Use this mode
when you want to add transformations and color corrections after pulling the key.
You then apply either a MMult node or enable preMultiply in an Over node to create
the final composite.
Float Support in Keylight
The Keylight node now supports the preservation of 32-bit data. As a result, float
images may require different keyer settings than 8-bit images. For example,
highlights in the foreground subject of float images may produce unexpected areas
of translucency due to barely perceptible green or blue casts that are preserved in
float, but which would be clipped in 8 bit or 16 bit. This can be addressed by lowering
the highlightGain parameter (to approximately -0.25) to strengthen weak areas in the
interior of the key.