Alpha Channels, Masks and Mattes

Week 11

Alec shared this tutorial with us for using rgb channels as mattes in Nuke.


I wondered if there was a similar method to use in After Effects and found this:

Ask DT: After Effects – How to Composite Together a RGB matte, or ID Pass

This method uses keying>colour key to key out a particular colour. I found that this did not work very well for masking my grass as the edge was not very accurate even after adjusting the ‘edge thin’.

I found that this method using the ‘shift channel’ effect was more accurate. Not only can the transparency be shifted to show only luminance/white but also to red, green or blue and then the selected area/alpha can be used as a matte for another layer.


How to key out black


While searching for information about using masks in After Effects I came across this article:

After Effects Help/Alpha channels, masks and mattes

I thought that these notes in particular were interesting:

“The term alpha channel technically refers to the fourth (A) channel in an RGBA image file, regardless of whether that channel is used for communicating transparency information. However, since that fourth channel is used so often to communicate transparency information, the terms alpha and transparency have become nearly synonymous in common usage. It’s important to remember, though, that this connection is essentially arbitrary. Some formats may use other channels for transparency information, and other formats may use the fourth channel for something other than transparency information.”

…”A matte is a layer (or any of its channels) that defines the transparent areas of that layer or another layer. White defines opaque areas, and black defines transparent areas. An alpha channel is often used as a matte, but you can use a matte other than the alpha channel if you have a channel or layer that defines the desired area of transparency better than the alpha channel does, or in cases where the source image doesn’t include an alpha channel.”

“…A mask in After Effects is a path that is used as a parameter to modify layer attributes, effects, and properties. The most common use of a mask is the modification of an alpha channel of a layer, which determines the transparency of the layer at each pixel.”

One thought on “Alpha Channels, Masks and Mattes

  1. Pingback: Paint Effects for Grass and Trees | Natasha Crowley

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