Affinity has two variants of raster mask: Mask - this has a 'white' infinite surround and is initialised with size equivalent to the document size. Empty Mask - this has a 'black' infinite surround and is initialised with no size (or possibly size 0 x 0 pixels). To get a mask in Affinity as you had in Photoshop: make selection add Empty Mask fill with 'white' while the selection is still active   Edit: altered description of Empty Mask