1. A method of compositing a plurality of graphic objects with a compositing buffer, said plurality of graphic objects forming a group being attenuated by group opacity and being composited from a top object to a bottom object, the method comprising the steps of: generating, based on a first mask and the group opacity, a second mask, the first mask storing a remaining possible contribution for further graphic objects below and including said plurality of graphic objects; processing said plurality of graphic objects in a top down order, the processing comprising the sub-steps of, for each graphic object of the plurality of graphic objects: (a) determining a contribution value for the graphic object using the second mask, the contribution value representing a contribution of the graphic object to the compositing buffer; (b) compositing a colour value of the graphic object with the compositing buffer using said contribution value; and (c) updating the second mask using said contribution value; and updating the first mask using said second mask and the group opacity, wherein said updated first mask is configured for further compositing of objects below said plurality of graphic objects.