Layer masks

• Select a layer, make a selection, then click Add Mask from Selection. The selection is cleared, as in Photoshop. Every kind of layer can have a mask.
• Paint Mask (Shift+B) paints the mask directly. Left mouse reveals, Ctrl or Alt hides, X swaps them. A layer with no mask starts fully shown.
• Show Layer Mask (Alt+\\) shows what the mask hides, in red.
• Options: on/off, invert, feather.

Masks are stored in the layer's own pixels, so they move, rotate, scale, flip and pin with the layer.