Section 7 · Image Construction & Compositing

Layer Masks

Definition

A grayscale channel attached to a layer controlling which parts of it are visible — black conceals, white reveals, gray produces partial transparency — without deleting pixels.

Why it matters

Masking is non-destructive. Erasing is permanent; masking is a decision you can revisit at any point, which is exactly what iterative compositing requires.

Example

Painting black into a mask to hide a source image’s background, then painting white to bring back an edge removed too aggressively.

Teaching note

The eraser tool is the enemy here and students reach for it instinctively. A demonstration where an erased edge cannot be recovered and a masked one can, on the same image, is worth the five minutes.