Rasterization is slow when you have lots of small (often sub-pixel) triangles. Mapping a big triangle into screen space is fast, but mapping many triangles per

Rasterization is slow when you have lots of small (often sub-pixel) triangles. Mapping a big triangle into screen space is fast, but mapping many triangles per pixel (and hopefully throwing out the 99+% of the geometry that won't show up at all) into screen space and blending them together in a convincing way takes a while. Real-time graphics does the best it can with a few big triangles (by e.g. texturing and bump-mapping them) out of necessity, but people are moving towards cases that are harder on the rasterizer.