The Pitfalls of Exchanging Files: Alpha Channels, Data Range, and More

Alpha channels can be particularly tricky depending on which application they’re coming from. So let’s consider what an alpha channel is before digging into the potential issues they can cause.

Any time an image or video asset file—rather than a project file—has an embedded alpha channel, it typically contains three RGB channels, often called a Fill and a black and white alpha channel often called a Matte.

A straight alpha channel means that only the alpha channel (matte) contains transparency data and any RGB (fill) elements are opaque. With a straight alpha channel, the fill can look strange on its own, since it’s not being limited by the alpha channel (as you can see in the second example below).

A premultiplied alpha channel means that the RGB channels contain transparency information as well as alpha channel. The RGB values are combined with a specified matte color (usually black) that defines the amount of transparency. This matte color is subsequently removed from the RGB values by the software that you open the file with.
In simpler terms, the transparency value for a pixel is stored as a value between 0 and 1 (with 0 being completely transparent and 1 being completely opaque). For premultiplied transparency, the software simply multiplies the RGB values by the alpha value to calculate the result—hence the term premultiply. So an alpha value of 1 results in a fully opaque pixel, a value of 0 results in a completely transparent pixel, and a value of 0.5 would provide a 50 percent transparent pixel.

In programs like this, alpha channels are handled in the background without much user control. This can work well for artists who are more comfortable with text or graphic animations as there isn’t as much need for alpha management or interpretation for these elements. So it’s as simple as selecting an alpha channel interpretation when you render.