That's a clean solution! BTW, I've been using this one from [fonttools](https://github.com/fonttools/fonttools/issues/3055) by Behdad Esfahbod, which avoids `log` but uses `asinh`.

Nice! Good find. I adapted the font tools version. It's maybe 20% slower than my translation from the analytical formula, and it seems to properly handle degenerate cases. The general form of the Font Tools version makes me 30% more confident about not having weird singularities or numerical instabilities.