note that one can avoid division if the blending formula is modifiable. default blending is linear interpolation X*(1-alpha)+Y*alpha, where X is the target and Y should be inserted with alpha. So the division of Y by alpha results in X*(1-alpha)+(Y/alpha)*alpha, so it's just X*(1-alpha)+Y
I have always understood that it has to do with semi-transparency and edge softness. Sometimes you can get a better compositing blend using pre-multiplication with an alpha on black especially for motion graphics. The drawbacks of premultiplication can sometimes be black edging because the alpha is multiplied into the RGB channels as well as the RGB channels not being accurate because of the alpha pre-multiplication. In a lot of cases, it should be fine to use it but if you're doing a bit of colour correction and/or want accurate RGB channels, you can divide the RGB by the alpha or use a straight matted version instead.
Nice and easy! I'm using after effects for the past 7 years and just now I realized I never studied alpha properly.
Glad I could help!
note that one can avoid division if the blending formula is modifiable.
default blending is linear interpolation X*(1-alpha)+Y*alpha, where X is the target and Y should be inserted with alpha.
So the division of Y by alpha results in X*(1-alpha)+(Y/alpha)*alpha, so it's just X*(1-alpha)+Y
perfect, I only have premultiplied available so this helped
Thanks
You're welcome!
Why would you use premultiplied?
I have always understood that it has to do with semi-transparency and edge softness. Sometimes you can get a better compositing blend using pre-multiplication with an alpha on black especially for motion graphics. The drawbacks of premultiplication can sometimes be black edging because the alpha is multiplied into the RGB channels as well as the RGB channels not being accurate because of the alpha pre-multiplication. In a lot of cases, it should be fine to use it but if you're doing a bit of colour correction and/or want accurate RGB channels, you can divide the RGB by the alpha or use a straight matted version instead.