Why use "Do Not Premultiply Alpha"?

3deeguy · Post by **3deeguy** » Thu Jul 21, 2011 1:40 pm

Can it be used for things like scene changes? Instead of putting individual scenes together in an external movie editor can separate videos be lined-up in the sequencer and re-rendered? At least I would then be able to sync all my clips to a single music track.

I started studying particle layers last night wondering what it would take to create an 'explosion in space'. This is such a large subject that I might have to put it off for a while.

slowtiger · Post by **slowtiger** » Thu Jul 21, 2011 2:43 pm

"Pre-Multiply Alpha" is a term used for rendering images on a transparent background. The two options are "Pre-Multiply Alpha" and "No Pre-Multiply Alpha". The first one is useful for darker outlines, the second for lighter outlines, and they'll prevent to have thin white or dark rims later in compoisting images on a background image.

3deeguy · Post by **3deeguy** » Thu Jul 21, 2011 3:12 pm

Sounds like a way to keep composited images from looking like cut-outs. Thanks slowtiger. It's time to give myself another mental break then come back and do some tests.

hayasidist · Post by **hayasidist** » Fri Jul 22, 2011 1:43 pm

hi. also if you have soft edge then without "do not premultiply" the soft edge is blended with the bg colour ... as I found out the hard way ...

viewtopic.php?p=105407

with "do not premultuiply" the bg does appear not to be blended into partially transparent areas. (and apols to Ramón cos I forgot to say thanks back then!)