alpha mask
The cutout you already made. The mask rides the source layer’s own transparency — effects land inside the subject and nowhere else, no rebuilding required.
What it’s for
A matte painter cuts the foreground figure once and then treats it as sacred — everything afterward respects that silhouette. The alpha mask is that discipline made automatic. It reads the source layer’s own alpha channel, the transparency that already defines the cutout, and turns it straight into the mask.
You rarely need to touch much here, but the handles are
there. low is the alpha floor and
high the ceiling, so you can tighten a soft
matte or rescue a feathered edge. gamma shapes
the falloff in between — firm up a fringe, or let a
held-out edge breathe.
Reach for this when: you want “only inside the subject” effects without rebuilding the cutout; a glow that hugs a transparent PNG; a grade that stops dead at the edge of whatever was already keyed for you.
Where it lives in the app
Add from the Masks panel. The mask appears as its own layer
on the timeline; assign it to one or more effects by setting
their target to point at this mask layer.
Parameters
| Key | Range | Default | Effect |
|---|---|---|---|
low | 0–1 | 0 | Alpha floor |
high | 0–1 | 1 | Alpha ceiling |
gamma | 0.1–4 | 1 | Curve |
Pairs well with
- phosphor CRT glow — a cut-out subject that glows at its own edges, like it was lit from behind.
- chromatic aberration — colour fringing that stops exactly at the silhouette. The subject vibrates; the background stays still.
- text-glitch — glitch confined to lettering that was keyed out elsewhere. The words break; their surroundings don’t.
- color grade — push the subject one way and leave the plate untouched, like a figure stepping out of its own scene.