[ddren]

I wonder how this is implemented in the GPU. From my time working on a 3D renderer a long time ago, triangles with offscreen vertices would be clipped into smaller triangles, so in the end you would still be rendering multiple triangles anyway. I imagine it would also be possible to clip the scanlines instead.

[obl]

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-... for a nice detailed explanation.