How do i chroma key with a black background?

[Todd Kopriva]

Keylight is for keying out hues/colors, especially green and blue.

For creating transparency based on luminance, you are better off using the Extract effect:
https://www.sternfx.com/tutorials/125

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Todd Kopriva, Adobe Systems Incorporated
After Effects quality engineering
After Effects team blog
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[Jonachan Chainy]

thank you so much, I appreciate your help on this and I’ve been looking for this for a while…

[John Cuevas]

Along with Extract, you might want to try the Luma Key effect or the free Unmult Plugin.

Unmult’s was created for creating alpha channels for Knoll lens flares, it just drops out black and creates an alpha. Generally when I used it, I would add a levels effect after it, switch to the alpha channel and crush the blacks and increase the whites a little more to strengthen the alpha.

Johnny Cuevas, Editor
Thinkck.com

“I have not failed 700 times. I have succeeded in proving that those 700 ways will not work. When I have eliminated the ways that will not work, I will find the way that will work.”
—THOMAS EDISON on inventing the light bulb.