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J Montreuil

When to use Straight vs Premultiplied?

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Avid people need straight alpha.

 

Final Cut people want premultiplied.

 

*I think* if you have soft edges, premultiplied will matte a background color to the edge. If you see a black halo around your soft edged objects, or some discoloration in translucent materials, try switching to straight.

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Isn't Straight Alpha better...? in C4D it renders out a little extra edge of the colour of the object so when you bring it into AE there's less chance of getting black edges where transparency should be. Correct me if I'm wrong.

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The difference between straight and premultiplied is not actually in the alpha channel; in both cases, the alpha channel is identical. The difference is in the color channels. With straight channels, there is no change whatsoever to the color channels because of the alpha channel. With premultiplied channels, the color channels are modified by combining the background color with the foreground color. So, straight channels are pure color channels; premultiplied color channels need to have an operation done to them to remove the background color that they have been sullied with.

 

BTW, this is pretty much the same explanation as in After Effects Help (which I wrote 9 years ago when I was the documentation guy).

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BTW, this is pretty much the same explanation as in After Effects Help (which I wrote 9 years ago when I was the documentation guy).

 

You know what's great about C4D? You can right click on nearly anything anywhere in the program and hit Show Help and it will open the help manual and take you to that page. Can you make AE do that?

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Todd, is there any reason that AE defaults to Premultiplied for most (all?) Render settings? I'd kinda like an option in preferences to set the default (I know, there's a feature request page).

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While we're on this topic and we have Todd here - Is there anyway we can make AE handle the alpha channel in the composite and NOT have it automatically premultiply? If I'm doing work on the alpha channel and switching it around with RGB channels or whatever, it always automatically premults it in the viewer. Sometimes I just want to work on the alpha channel alone, and not have it affect the RGB at all.

 

Usually this is when I'm working with .exr's, I don't know if it's the same behavior with other formats.

 

I know you can bring in 2 copies of an asset and have one interpret as premult and the other as ignore, but that just seems like a huge workaround that doubles the amount of work involved.

Edited by theta

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Todd, is there any reason that AE defaults to Premultiplied for most (all?) Render settings? I'd kinda like an option in preferences to set the default (I know, there's a feature request page).

 

It's what is desired for most exports. If you want to have it be otherwise, create your own render settings template and use that. Now that there's a Sync Settings feature for easily moving render settings templates between computers, that's even easier.

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it always automatically premults it in the viewer

 

You can choose RGB Straight in the Show Channel menu at the bottom of the Composition panel if you want to see straight channels.

 

BTW, After Effects uses straight channels throughout its compositing pipeline. The first thing that happens when you bring in an item with premultiplied channels is that After Effects does the unmultiplication to get the unmatted (straight) colors.

 

In After Effects 2.0, we had an option to choose between straight and premultiplied channels in the compositing pipeline, and we removed this because there were no real advantages to using premultiplied channels in the compositing pipeline---only for display purposes.

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So what are some instances where you'd want premultiplied? Occasionally if I'm having trouble with an alpha, I'll switch it to premultiplied to see if it makes a difference. Once in a blue moon it does, but I can't get a handle on why it's sometimes better.

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If someone gives you an asset that was created with premultiplied channels, you must interpret it as premultiplied.

 

If someone gives you an asset that was created with straight channels, you must interpret it as straight.

 

After Effects can take both.

 

As stated at the beginning of this thread, some applications can only take one or the other.

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It's what is desired for most exports. If you want to have it be otherwise, create your own render settings template and use that. Now that there's a Sync Settings feature for easily moving render settings templates between computers, that's even easier.

 

Very much looking forward to this feature.

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