Alright, that’s a long title, but that’s the shortest I could come up with to describe this tutorial.
It’s not rare that someone, somewhere, provides us with an element (logo, UI, motion graphics, precomp, anything really) with a nice alpha, but with a white or colored background. Of course it’s not ideal, but that’s nothing we can’t get around if we know how.
Then, by using the same logic, I’ll try to explain how to invert almost any other comp operation.
Additionally, although it’s demonstrated here for Nuke, this would work in most compositing packages.
Transparent Element is not on a Black Background.
I just came across a little Gizmo on Nukepedia called ColorPremult. I was surprised to realize it didn’t seem to be there earlier, seing how often I have been given elements on a colored background. While the Gizmo seem to work as advertised, I thought we could go a step farther and make it work on ANY background, not only with a solid color.
Breaking it down.
Let’s assume we have been given this beautiful TV news banner to drop onto some footage of a TV show, along with its alpha:
If we try to just premult the two together, we will obtain the following result, which is wrong (you can still see the background in it):
What we should have been given was the banner onto a black background along with its alpha. This, instead is that element already comped over a background. To obtain our element, we basically need to “un-comp” it from the background.