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fundamental problem working with alpha channel in photoshop and after effects

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matthew macy
fundamental problem working with alpha channel in photoshop and after effects
on Oct 10, 2019 at 4:43:34 pm

I am working in Photoshop and After Effects and am experiencing an issue regarding alpha channels. I created a tiff image in photoshop with an alpha channel by going to channels and making background black with text that is white. I would post screenshots but can't here. That alpha channel in photoshop consists of white text reading "testing" on a black background. As I understand it black is transparent.
When I bring this tiff file into After Effects with a video layer stacked on the bottom the text it is solid while the background black is transparent allowing the video to come through. This is in line with the tiff's alpha channel.
What I don't understand is when I turn on the alpha matte (on the video's matte dropdown) the white of the text is keyed out allowing the video to show through.
Does alpha channel in after effects mean that white is transparent? It appears there is an inconsistency between how the alpha is treated by after effects. Is white transparent or opaque in after effects? Why is the white text keyed out with alpha matte but not so with regular stacking layers. I hope I have not been faulty.

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Greg Gesch
Re: fundamental problem working with alpha channel in photoshop and after effects
on Oct 10, 2019 at 11:50:20 pm

Hi Mathew. You are confusing two different things. An alpha channel works as you describe it, but a Track Matte is an entirely different concept. It creates its own alpha depending on the setting you select - you can use an existing alpha (as in your case) or luma (lightness/brightness). In your case you wouldn't have to have an alpha background in your tiff, you could have a solid black background and use a luma matte to get the same effect you are getting now.

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Steve Bentley
Re: fundamental problem working with alpha channel in photoshop and after effects
on Oct 13, 2019 at 12:52:18 am

Think of the white in either the image (for luma mattes) or the alpha (for alpha mattes) in the layer that is acting as the track matte as a window the video below can shine through. If you pick alpha matte whatever is white in the alpha of the track matte layer (not the layer below it) will act as a window and "project" that lower layer on top of all the layers stacked up to that point. If the lower layer has an alpha too, it will combine with the track matte layer's alpha - so a white circle in the alpha of the track matte layer will combine with a white square in the lower layer's alpha (the layer that is "passing through" the track matte layer) to create a circle cut off by some straight sides and tops (assuming he square is only a little smaller than the circle).
The same goes for the luma matte option except the track matte takes the grayscale levels in the RGB channels of the trackmatte layers and uses those as the window.
You can do all this with the inverse - so in a sense the black in the track matte layer becomes the window. That's why there are 4 choices in the track matte drop down.

Be careful too that you don't double matte your photoshop elements. If you have white text on black in the RGB channel and then use that same white text in the alpha channel to act as a cookie cutter for the RGB channel, you are really masking that image twice: once with the alpha and once with the black edges encroaching on the white text through it's antialiasing. If you want clean text or any other shape, make sure the edge of the RGB elements bleed over the edges of where the alpha element falls. (or just fill the RGB with solid color and let the alpha be a cookie cutter).

You can also just make your text or graphic a layer in photoshop and then import the file as a photoshop file into AE and choose that layer to be your artwork. It will self matte when its a layer. You can also use it as a track matte to pass lower layers through the area where the artwork is - there is an implied alpha with a photoshop layer.

Remember too that when you crush levels or curves to make an alpha channel out of something that might not have been black-and-white enough to be alpha, you will often get ratty edges as the nice smooth roll off of the antialiasing gets crushed in the levels filter.

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