Steve Wright

https://www.linkedin.com/pulse/why-oh-premultiply-steve-wright/

James Pratt

https://jamesprattvfx.wordpress.com/2018/11/08/premult-unpremult/

The simple definition of premult is to multiply the alpha and the RGB of the input together.

Un-Premult suggests that this does the opposite operation to the premult node. Therefore instead of multiplying the RGB values by the alpha, it divides instead.

Alan Martinez

https://www.linkedin.com/posts/alan-martinez-1a7a60234_unpremult-and-premult-are-terms-used-activity-7089270470889394176-OVXE

“Unpremult” and “premult” are terms used in digital compositing that are relevant for both those working with computer-generated graphics (CG) and those working with live-action plates.

“Unpremult” is short for “unpremultiply” and refers to the action of undoing the multiplication of a pixel by its alpha value. It is commonly used to avoid halos or unwanted edges when combining images. (This by making sure that edits to a layer are added independently from edges’ opacity levels.)

“Premult” is short for “premultiply” and is the opposite process of “unpremult.” In this case, each pixel in an image is multiplied by its alpha value.
In simple terms, premult crops the RGB by its alpha, while unpremult does the opposite.

It’s important to perform color corrections on CG renders in a sort of sandwich approach. First, divide the image to extend the edges fully of the RGB channels. Then, apply the necessary color corrections. Finally, pre-multiply the image again to avoid artifacts on the edges.

Typically, most 3D rendered images are premultiplied. As a rule of thumb, if the background is black or even just very dark, the image may be premultiplied. Additionally, most of the time, the 3D render has antialiasing in the edges.

Aaron Strasbourg

https://www.aaronstrasbourgvfx.com/post/2017/06/23/002-unpremult-and-premult