A method of compositing a plurality of graphic objects with a compositing buffer, is disclosed. The plurality of graphic objects forming a group is attenuated by group opacity and is composited from a top object to a bottom object. Based on a first mask and the group opacity, a second mask is generated. The first mask stores a remaining possible contribution for further graphic objects below and including the plurality of graphic objects. The plurality of graphic objects in a top down order is processed. In particular, for each graphic object of the plurality of graphic objects: (a) a contribution value for the graphic object using the second mask is determined, the contribution value representing a contribution of the graphic object to the compositing buffer; (b) a colour value of the graphic object is composited with the compositing buffer using the contribution value; and (c) the second mask is updated using the contribution value. The first mask is then updated using the second mask and the group opacity. The updated first mask is configured for further compositing of objects below the plurality of graphic objects.