Help understanding alpha transparency when exporting video for use in After Effects


Beginner, so excuse some potentially silly questions here.

I’m using the moviefileout TOP to capture video from a ‘watercolour’ type patch based on this tutorial.

I’m trying to output the coloured texture from the sketch onto a transparent background for use in After Effects, but I am having trouble. Can anybody advise me on this?

Am I right that ‘transparency’ with a video necessitates an empty alpha channel?

As per this screenshot, I would be looking for the white background to be transparent in the video output

Is it the combination of the ‘Lookup’ and ‘Ramp’ TOPs that are causing the problem here?

The last few operators are arranged as in the picture below.

hey @lunchthief if you watch your tutorial again around 10:51 bileam explains that the colors in the Ramp TOP are what’s seen in the final output, and the leftmost color in the Ramp TOP is whats used to replace the darkest colors, or the background.
So the first step is to make the leftmost color in the Ramp TOP transparent, do this by dragging its alpha slider to zero.

Tip: you can recognize transparency in TOPs if the grey checkerboard pattern is visible, so you’re going to make sure you see that checkerboard in your final output TOP before you send it to a Movie File Out TOP.

Next, the Lookup TOP. By default it its set to work in independent RBGA mode, for your case I think you need to set it to Luminance. (the Index Channel parameter) . Example attached below. (you can drag&drop the tox file into your TD window)

lookup_TOP_alpha.tox (606 Bytes)

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Nailed it nettoyeur. Many thanks for your help.

I implemented what you said and it worked!

As a follow up query, I tried all the video codecs options in the MovieFileOut TOP (except for H264/5), and found that GoPro-Cineform was the only option that gave me the alpha channel transparency that i needed (pic below). The other codecs exported with black background defaults.

Any idea why this is? When I searched it said other options gave alpha support aswell.

Screenshot 2021-04-06 at 13.14.57

from top of my head the only video codecs which have support for an alpha channel are Animation, GoPro-Cineform, HAP, and HAPQ and NotchLC

you’ll need to set the Movie Pixel Format to RBGA