Image format where RGB values are pre-multiplied by alpha channel — faster compositing, no halos around transparent objects. Professional standard in VFX pipelines.
In compositing, you constantly work with layers that are partially transparent—and this is where beginners diverge from those who know what they are doing. The question is: do you store the color values independently of their transparency, or do you multiply them in from the start? Premultiplied images have already multiplied the RGB channels by the alpha channel. This sounds technically dry, but it prevents real problems on the set of your compositing process.
Imagine you have a shot with a keyed-out background—like a person in front of a green screen. The edges are partially transparent because the keying software is uncertain. In unpremultiplied (straight) images, each pixel still stores its original RGB value, regardless of whether the alpha transparency there is at 10%. This leads to "halos" in later compositing—bright or dark fringes around the figure, because color information with low opacity is placed over other layers. This looks cheap and is difficult to correct afterward. Premultiplied images solve this problem: the RGB values are already calculated down to their true visibility. A pixel with Alpha=0.5 and original Red=255 is stored as Red=127.5. When compositing over other layers? No false halo artifacts. The edges are clean.
In practice, this means: your compositor will thank you if you deliver mattes, keys, and VFX plates as premultiplied. Nuke, After Effects, Fusion—all major compositing suites work internally with premultiplied pipelines. Of course, you sometimes need unpremultiplied raw data for certain color correction operations, but the norm in professional VFX houses is clear: premultiplied saves time, prevents errors, and reduces computational load. The reason is simple—the multiplication is already done, not just necessary during compositing. This is also one of the reasons why OpenEXR in its 16-bit or 32-bit variant with an alpha channel has become the standard—the format can store both variants, and the pipeline lead defines what goes out.
Common confusion: Premultiplication is distinct from matting (the separate storage of a matte) or from alpha compositing in general. It is a specific mathematical storage and calculation method. When delivering to the next department? Always clarify whether premultiplied or straight is expected—the difference is small in file size, huge in quality.