premultiplied alpha

[jeremybirn]

When you render with pre-multiplied alpha, be sure that your background color is black.

-jeremy

[mahmoudcg]

yes I see, thanks for the reply, so whats the benefit of working with premultiplied alphas?

[mahmoudcg]

c'mon guys I'm confused about this, when to use premultiplied alphas and when to not
can someone give me a hand on this

[jeremybirn]

I was almost tempted to quote a few pages from chapter 11 of my book, but to be brief: Pre-multiplied alphas are the default for 3D graphics. They give you a fully anti-aliased image, they give you the ability to use whatever compositing operations you want such as adding layers together without using alphas for a clean composite, and they give you perfect alignment between image and alpha so that operations like blurring the elements work as cleanly as possible at the edge pixels.

Sometimes people avoid pre-mult if they are compositing in Photoshop or After Effects, where there are sometimes some problems with edge pixels. Sometimes they avoid them for ambient occlusion passes (when they aren't full-screen) because those are only going to be used by multiplication with other layers. An if you are making sprites for a video game where you don't want the sprite edges anti-aliased against the background, I guess that's another time you could play with turning premult off.

Really, you ought to be able to deal with elements correctly either way, you just might choose different ways to composite them in a given program.

-jeremy

[mahmoudcg]

thanks a lot Jeremy for your time and explanation, I got it and will definitely read more about this

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