Guard band clipping is only really applicable to "edge function" type rasterizers. For the classic scanline-based algorithm, sure, you can easily clip to the ri

Guard band clipping is only really applicable to "edge function" type rasterizers. For the classic scanline-based algorithm, sure, you can easily clip to the right and bottom edges of the viewport while rasterizing, but the top and left edges are trickier. Clipping in clip space, before rasterization, is more straightforward, given that you have to frustum cull primitives anyway.