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Alpha Multiply [AML]
The Alpha Multiply node
Alpha Multiply Node Introduction
As the name gives away, the Alpha Multiply’s sole purpose is to multiply an image’s color channels by
its alpha channel. When you color correct an image that contains a premultiplied alpha channel, first
apply an Alpha Divide node before any color correction node to create a non-premultiplied image.
Then you can perform the color correction. After the color correction, add an Alpha Multiply node to
return the image to its premultiplied state.
Inputs
The Alpha Multiply node includes two inputs in the Node Editor.
– Input: The orange input accepts a 2D image with a “straight” or non-premultiplied alpha.
– Effect Mask: The optional blue input expects a mask shape created by polylines, basic
primitive shapes, paint strokes, or bitmaps masks. Connecting a mask to this input limits
the pixels where the Alpha multiply occurs. An effects mask is applied to the tool after the
tool is processed.
Basic Node Setup
The Alpha Multiply node is placed after color correction is done to an image with a premultiplied Alpha
channel. Here the MediaIn node is assumed to have a premultiplied Alpha channel. The Alpha Divide
node is inserted, and then color correction nodes operate on the “straight” Alpha. An Alpha Multiply
node is placed at the end of the chain to premultiply the Alpha channel again. If only a single color
correction node is used, then the Pre-Divide/Post-Multiply checkbox on the Options tab can be used
in place of the Alpha Divide/Alpha Multiple nodes.
An Alpha Multiply node is inserted after color correcting an image with premultiplied alpha.
Inspector
This node has no controls.
Chapter 107Matte Nodes 2068
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