Alpha channels are the basis of digital compositing, but there are two different types – straight and premultiplied. Understanding the difference can make or break your composites – if the alpha channel is wrong, your composition will look wrong.

This video is a refresher course on alpha channels that covers a broad range of topics. As well as visually demonstrating the difference between straight and premultiplied alpha channels, we look at the pros and cons of each, what colour matting is and how to avoid problems with it, and we also look at what the word “premultiplied” actually means.

The video also shows common problems caused by the incorrect interpretation of alpha channels- most commonly dark or light halos around the edge of layers, but also colour casts in semi-transparent areas.

While most visual fx software only allows alpha channels to be premultiplied with black or white, After Effects is unusual because it can premultiply alphas with any colour – whatever the background colour of your composition is set to. This can also lead to unexpected problems with importing clips rendered in After Effects into other applications.

Basically – alpha channels are important. It’s important to understand what they are and how they work. A tutorial on alpha channels may not be as much fun as one on particles or other cool looking visual fx, but taking the time to cover some of the basics can save you from producing work that looks wrong.


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