fundamental problem working with alpha channel in photoshop and after effects

Report · Oct 10, 2019

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. Notice the screenshot of the image of the white testing text on a black background for the alpha channel. 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 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 dont understand is when I turn on the alpha matte 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? I hope I amnot faulty in what i have done.

Report · Oct 10, 2019

Use the "Alpha Iverted Matte", because what you are saying to the layer below is that it use the alpha of the layer above and that the white leave it matte. In this case the layer does not use its own alpha but uses that of the layer above.

Track Matte in After Effects

Report · Oct 10, 2019

I am not sure I follow. I know I can use inverted alpha but the logic makes no sense. Why is black transparent in one treatment of alpha but white transparent in another treatment.

Report · Oct 10, 2019

Imagine that you make these two layers in only one layer, that is, imagine you have a single layer (the one below) with the same alpha as the top layer embedded, as an RGBA file, in that case white is matte and black is transparent. Does it seem logical now? This is how Track Matte works.

Report · Oct 10, 2019

It is as if you had the alpha channel separately in another layer and when applying the Track Matte you can use it in 4 different ways.

Report · Oct 10, 2019

I still don't understand..

Report · Oct 10, 2019

Maybe this will be easier if you turn on the Alpha Chanel in the Comp panel so you are only looking at the alpha channel. Start by adding a blue shape layer to the timeline. Next, try creating two shape layers using rectangles in the same shape layer. Fill one with white and the other with black. Just for fun fil a third with 50% Gray. Put a big gray stroke around the rectangles and maybe even a blue one. You get something like this:

set the view options to Alpha in the comp panel so you can see what is going on with the alpha channel.

Then set the shape layer as a track matte and look at just the alpha channel.

Everything that is white is going to show be opaque, everything that is black will be transparent. If you were to turn on the shape layer and solo it you would see the same thing. Track matte makes the color values and the alpha channels from the source (top) layer control the opacity of the layer below. Cycle through all of the options to see how the values of the Alpha Channel change for each option:

Then as a final check, turn on all of the channels and enable the transparency grid so you can see the actual transparency. If you are going to work with layers it is very important that you have a clear understanding of how "track mattes," alpha, and color channels work. There is nothing odd or unusual about the behavior and it is common across any app that has the ability to use alpha channels for transparency, even Photoshop.

Report · Oct 11, 2019

Thanks for the help but transparency is not consistent between layers. I have this one image which summarizes my arguement. Look at this transparency grid of the alpha matte. The background of the tiff is showing transparent but its the text that is keyed. The alpha matte keys the white text treating it as transparent despite the black background being transparent. I am sorry to be struglling with this but could you please clarify.