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A superimposed green screen clip on track V2 of the Timeline, above a
background plate on track V1, ready for compositing
To create a chroma-key composite in the Color page:
1 Right-click anywhere within the gray area of the Node Editor and choose Add Alpha Output to add
the Alpha Output at the right of the node graph.
An Alpha Output appears underneath the Node Tree output at the right of the Node Editor.
The image Node Tree output
with Alpha Output below
2 You’ll want to use Node 1 to color correct the image to optimize it for keying. In this example, the
foreground plate is log-encoded, so a LUT is added to Node 1 to normalize it.
3 Add a Serial node after Node 1 and connect its key output to the Alpha Output. This is a fast
way to build this composite if you’re confident that the corrections you’ve made in Node 1 won’t
adversely affect the key (or if you know they’ll improve it by expanding the contrast of a flat-
contrast source clip).
Alternately, you could create a second processing branch by adding a disconnected node (Node
2), connecting the Source node to its RGB input, and then connecting its key output to the Alpha
Output you just added. Creating a second image processing branch from which to pull your key
lets you key directly from the source (assuming the source is fit to key without adjustment),
avoiding any problems that grading the first node could introduce to the key.
Node 1 grades the clip, expanding contrast to improve both the image, and its
ability to be keyed. Node 2 uses Qualifier controls to pull the key.
4 Use the Qualifier controls of Node 2 to key out the green background, and then turn on the Invert
checkbox to create the proper composite. In this example, we’re using the 3D Qualifier mode to
pull a high-quality key.
Chapter 143Channel Splitting and Image Compositing 2833
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