Black conceals, white reveals
A layer mask is a greyscale image attached to a layer. Where the mask is white, the layer shows. Where it is black, the layer hides. Where it is grey, the layer shows partially. That is the whole mechanism — everything else is technique.
Why masking beats erasing
The Eraser destroys pixels. A mask hides them, and hiding is reversible: paint white and the content comes back, months later, in a file someone else opens. There is no editing situation where the Eraser is the better choice on a layer you might revisit.
Working on a mask
Click the mask thumbnail to target it — the brush now paints into the mask rather than the image. Useful keys:
- D then X — reset to black/white and swap. You will press these constantly.
- Alt-click the mask — view the mask itself full-screen, to spot leaks and soft edges.
- \ (backslash) — view the mask as a red rubylith over the image.
- Shift-click the mask — disable it temporarily to compare.
- Ctrl/Cmd-I — invert. Adding a black mask and painting the effect in is often faster than masking it out.
Soft brushes, low flow
Most amateur masking looks amateur because of hard edges. Use a soft brush at 10–30% flow and build the transition in several passes. Ninety percent of natural-looking local edits come down to this and nothing else.
Clipping masks are a different thing
Layer mask and clipping mask sound alike and are unrelated. A clipping mask uses the transparency of the layer below as the boundary. Alt-click between two layers to create one. Use a layer mask when you are painting a shape by hand; use a clipping mask when the shape already exists as a layer.
Refine the mask, not the selection
Beginners try to make one perfect selection. Experienced editors make a rough selection, turn it into a mask, and fix the mask with a brush. The mask is where the accuracy actually lives — see making selections.