1. A computer-implemented method, comprising: receiving input at a graphics processing unit (GPU) of a host computer system, wherein the input is sent to the GPU according to at least one central processing unit (CPU) of the host computer system, wherein the input comprises a knockout group and a background, wherein the knockout group comprises a plurality of objects that are ordered by paint order, wherein the plurality of objects comprises a respective topmost object and one or more additional objects at each of one or more locations enclosed by the knockout group; drawing the background into a frame buffer of the GPU; and drawing the plurality of objects in the knockout group into the frame buffer of the GPU, comprising, for each pixel in the frame buffer at the one or more locations, determining a color and an opacity of the pixel based on a color and an opacity of the respective topmost object at the pixel and based on a color and an opacity of the background at the pixel and not based on a color and an opacity of any of the one or more additional objects which are overlapped by the topmost object at the pixel.