How to use glyph math to create an optical bold
Glyph math is a concept used by Robofab where you take every point in a glyph and perform a mathmatical operation on each point. So if you were to multiply the glyph 'a' by 10, it would take each point, and multiply it by 10, essentially scaling the glyph 10 times. It becomes really interesting when you start doing operations on point compatible glyphs. So for example, if you were to take that same 'a' and add it to a bold version, then divide by two, you would have an average of the two glyphs (in this case a medium).
Another useful concept, though a little harder to grasp, is the idea of differences. If you subtract one glyph from another, you typically end up with some random looking squiggles. But when you apply those differences to a third glyph, things get interesting. Take for example the optical weights shown above. If you subtract the display 'a' from the caption 'a', then add those differences back on a bold display, you get a pretty good looking optical bold.
So how does it work? Let's look at just one control point to see what happens. In this case, let's look at the point on the underside of the arm of the lower case 'a'. If we take the difference of (subtract) point1 and point2 we get -20 units. If we then add these 20 units to the bold 'a', the point moves down 20 points, which is just where we want it for an caption size 'a'. Glyph math simply repeats this step for every point in the glyph.