To create a mask from curves

1. Select the image layer you want to mask.

See Select an image layer.

1. Choose Paint > Shape > Make Mask Shape.
2. Click each curve to include in the mask shape. You may click a curve more than once, in several different locations. Make sure you click the part(s) of the curve that you want to include in the mask boundary.

   These curves will define the inner and outer boundaries of the shape.

   To remove a curve from the selection, click it.

3. Click Accept.

You can also set Global Auto-Shape Type to Mask in the Paint Panel. Curves that you draw will then automatically create mask shapes.