Apologies for the following noob question, it's just I can't find a satisfactory answer anywhere...
When I render a simple axis-aligned quad made up of two triangles, I don't understand why there's never any visible artefacts on the diagonal edge where those triangles meet. Some of those pixels must be in both triangles, right? So if my fragshader always emits a partially-transparent color - 50% black say - then shouldn't there be a visible dark diagonal line in the resulting quad?
Obviously it's great that this isn't a real problem that has to be coded around, I just don't understand why it isn't. What GPU magic is it that makes this always work out?