Ganbar R. - NUKE 101. Professional Compositing and Visual Effects - 2011

ToURING THE INTERFACE WITH A BASIC CoMPoSITE 27
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Never color correct premultiplied images. Chapter 4 is all about color correction.
Always transform (move around) and filter (blur and so forth) premultiplied
images.
You control premultiplication with two nodes in the Merge toolbox: Premult and
Unpremult.
The rule says that if an image’s RGB channels aren’t black where the alpha channel
is black, then it isn’t a premultiplied image. When you looked at Read2 you noticed
exactly this. The black areas in the alpha channel were a light purple color in the RGB
channels (FigUrE 2.10). This means this image is a … what? A straight image!
Because the Merge node expects a premultiplied image as its foreground input, you
need to premultiply the image first.
1. While Read2 is selected, press Tab and type pre. A drop-down menu displays
with the Premult option. Use the down arrow key to navigate to it and press Enter/
Return. Alternatively, you can choose the Premult node from the Merge toolbox
(FigUrE 2.11).
There’s now a Premult node called Premult1 connected to Read2. The area outside
the doll’s outline is now black (FigUrE 2.12). It is now a premultiplied image.
You can proceed to place this image over the background.
FIGURE 2.10 This
image shows both
the alpha and rgB
channels.
FIGURE 2.11 The Premult node sits in between
read2 and Merge1.
FIGURE 2.12 The area that
was light purple is now black.
2. Click Merge1 and press 1 on your keyboard.
All that purple nastiness has disappeared. You are now seeing a composite of the
foreground image over the background (FigUrE 2.13 next page).