Actual clipping is expensive so indeed a "guard band" is used : inside the region allowed by the internal precision of the rasterizer, outside pixels are simply

Actual clipping is expensive so indeed a "guard band" is used : inside the region allowed by the internal precision of the rasterizer, outside pixels are simply "ignored". See e.g. https://fgiesen.wordpress.com/2011/07/05/a-trip-through-the-graphics-pipeline-2011-part-5/ https://fgiesen.wordpress.com/2011/07/05/a-trip-through-the-... for a nice detailed explanation.