kivy/examples/kv/app_stencil.kv

Widget:
    canvas:

        StencilPush

        # create a rectangle mask, from pos 100, 100, with a 100, 100 size.
        Rectangle:
            pos: 100, 100
            size: 100, 100

        StencilUse

        # we want to show a big green rectangle, however, the previous stencil
        # mask will crop us :)
        Color:
            rgb: 0, 1, 0
        Rectangle:
            size: 900, 900

        StencilUnUse

        # Remove the mask previously set
        Rectangle:
            pos: 100, 100
            size: 100, 100

        StencilPop