The layer mask is the non-destructive tool par excellence. It lets you hide or reveal parts of a layer without erasing a single pixel.

The black-and-white principle

• Painting black on the mask: you hide that area.
• Painting white: you reveal that area.
• Grey creates partial transparency (a soft fade).

In practice

Click the mask icon at the bottom of the Layers panel, then paint with the brush. Made a mistake? Paint back over it in white and everything returns: nothing is ever destroyed.

Key point: « black conceals, white reveals ». That one sentence sums up how masks work.