MetaPost label positioning based on the text baseline

Question

We are all familiar with MetaPost labels:

label.top(btex $a_t$ etex, z0);

The problem with the above is that the positioning is based on the entire label, including the subscript.

How do I calculate the positioning based on the baseline of the text, i.e. only the "a" part, and not the "t" subscript?

Answer 1

Smash the box:

label.top(btex \setbox0=\hbox{$a_t$}\dp0=0pt \box0 etex, z0);

However this will ignore the depth when computing the bounding box, so you may have to add something to it if the label is near the bottom of the image.