I was rendering some graphics in After Effects for use in Premiere Pro and started thinking about Alpha interpretation. I have been doing graphics professionally since I started editing the behind the scenes footage for Lord of the Rings , The Fellowship of the Ring and over the years I worked out that Straight alphas almost always worked out better for me, with better edges. Now I have read that Final Cut Pro prefers a premultiplied alpha, but I have certainly had better luck with with a straight alpha. I wanted to dive into what the differences really are, and a great definition is in Help for After Effects.
Image files with alpha channels store transparency information in one of two ways: straight or premultiplied. Although the alpha channels are the same, the color channels differ.
With straight (or unmatted) channels, transparency information is stored only in the alpha channel, not in any of the visible color channels. With straight channels, the results of transparency aren’t visible until the image is displayed in an application that supports straight channels.
With premultiplied (or matted) channels, transparency information is stored in the alpha channel and also in the visible RGB channels, which are multiplied with a background color. Premultiplied channels are sometimes said to be matted with color. The colors of semitransparent areas, such as feathered edges, are shifted toward the background color in proportion to their degree of transparency.
Some software lets you specify the background color with which the channels are premultiplied; otherwise, the background color is usually black or white.
Straight channels retain more accurate color information than premultiplied channels. Premultiplied channels are compatible with a wider range of programs, such as Apple QuickTime Player. Often, the choice of whether to use images with straight or premultiplied channels has been made before you receive the assets to edit and composite. Adobe Premiere Pro and After Effects recognize both straight and premultiplied channels, but only the first alpha channel they encounter in a file containing multiple alpha channels.
Setting the alpha channel interpretation correctly can prevent problems when you import a file, such as undesirable colors at the edge of an image or a loss of image quality at the edges of the alpha channel. For example, if channels are interpreted as straight when they are actually premultiplied, semitransparent areas retain some of the background color. If a color inaccuracy, such as a halo, appears along the semitransparent edges in a composition, try changing the interpretation method.
And that explains it, since with premultiplies the edges can get some of the background color leaking into them in some cases, which is not the case with straight alphas.
And while quicktime and Apple may prefer premultiplied alphas that does not make them better or desirable, Apple already has it’s whole gamma shift issue with quicktime that is a disaster, so adhering to what they want isn’t always the best idea.
