A layer mask is used to reveal a portion of a layer while the rest of the layer remains hidden. This means that you can use a mask layer to 'delete' areas of a layer that you don't want.
In Affinity Photo, two types of masking are possible:
Masks are applied as a separate layer, allowing them to be freely edited and moved. Mask layers affect any layer below them within the Layers panel. They can also be clipped to individual layers so that only that layer is affected. Alternatively, mask layers can be added to layer groups so they only affect that group.
Do one of the following:
From the Layers panel, click Mask Layer.
The added mask will hide areas outside a selection (if a selection is in place) or display the entire layer (if no selection is in place).
From the Layer menu, select New Empty Mask Layer.
The added mask will hide the entire layer regardless of any selection in place.
From the Layers panel, -click a layer and select Mask to Below.
The selected layer becomes a mask for the layer below it.
To return to normal view, click the thumbnail or press the .
Paint on the page using the Erase Brush Tool.
By default, the Assistant will add a layer mask to the selected layer to accommodate your paint strokes. Once the mask is in place, you can 'restore' the mask using the Paint Brush Tool.