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When should one check the Convert Premul box on nodes that use alpha?

The Alpha Over node

Secondly, why is there a Premul slider on the Alpha Over node? When would you need it to be something other than 0 or 1?

I understand that pre-multiplied alpha is sometimes good and sometimes bad, and that if used improperly it can result in dark borders where transparency tapers off. But that's about all I know. Could you please enlighten me?

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    $\begingroup$ Those links could be a tad misleading, specially the one using After Effects. The "interpret footage" feature of After Effects tries to hide alpha being too smart, so you see the result after the interpretation. It has other weird aspects like the premultiplication color that just add up to the confusion. $\endgroup$
    – Gez
    Commented Jan 6, 2016 at 13:44

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When should one check the Convert Premul box on nodes that use alpha?

Only when you know that your image has unassociated (often called "straight") alpha. Or in other words, when your image's alpha is not associated (premultiplied).

why is there a Premul slider on the Alpha Over node? When would you need it to be something other than 0 or 1?

That's an unfortunate feature that brings a lot of confusion among users. It can be used to mitigate alpha errors. If you did your compositing right and didn't mix up associated and unassociated alphas, you should never use it. And even if you didn't, that feature is a hack that will only mitigate, not fix those errors. It's better to track down your alpha to spot any mistake and fix it.

I understand that pre-multiplied alpha is sometimes good and sometimes bad, and that if used improperly it can result in dark borders where transparency tapers off. But that's about all I know. Could you please enlighten me?

It's never bad. It's actually the best way transparency can be expressed in an RGB file as it allows both occlusion and emission (that means that your foreground can both block and emit light). You can do most of your compositing without moving away from associated alpha and expect good results. That doesn't necessarily mean that unassociated alpha is never useful. It has its uses, but CG and VFX are probably better suited for associated alpha. You just have to know when your image is associated or unassociated so the correct alpha-over operation is used and it produces the correct results.

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  • $\begingroup$ This isn't very helpful. So when is it? When is it (un)associated? $\endgroup$
    – Unknow0059
    Commented Jan 5 at 1:45
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    $\begingroup$ Unassociated alpha is often used in DTP (Desktop Publishing) aplications. Those applications use alpha channel that way not because it's better than associated, but because of a long tradition of using it. In essence, they used unassociated when Photoshop got alpha channel and never changed it, and everything was built on top of that convention. $\endgroup$
    – Gez
    Commented Feb 26 at 15:15
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    $\begingroup$ The reason for having unassociated alpha is probably that for some people it is useful to keep the original "plate" (the original image) untouched, using alpha as a separate mask. But that ends at the exact moment you changed a single pixel editing the image, so it's debatable. Other people, like game developers, like to encode stuff in RGBA channels that is not necessarily an image and its transparency, but something else (like technical channels). For them, unassociated is useful too as a hack. $\endgroup$
    – Gez
    Commented Feb 26 at 15:16
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    $\begingroup$ In the end, what matters is what software you're going to use, and what kind of images you're going to produce: If you're using compositing software with images rendered from a 3D software, associated (premul) is definitely the way to go. If you're using software that internally uses unassociated alpha, like Photoshop and the rest of the Adobe suite, then unassociated might spare you some headaches. If you're a game developer and you want to use an image file as a transport for technical channels, unassociated might be useful too. $\endgroup$
    – Gez
    Commented Feb 26 at 15:19

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