When joining thick lines there are usually some options
In a bevel join, the joint is 'flat'. But what does that mean precisely?
There seem to be two options:
The two points are orthogonal to their respective segments and half the thickness away from them:
The distance from the 'flat' to the center line is half the line thickness:
Which of them is actually a true bevel-join? And is there a name for the other (I'm thinking it might be some kind of truncated miter).