It looks like Glyphs app isn’t exporting the value for underlinePosition tha I’m putting in? It’s compensating and adding half the height of the underlineThickness?
Are you talking about static or variable fonts? If I recall correctly, the defined underline cannot (or is not) exported for variable fonts.
Static. The exported value in the OTF is different to the value specified in my .glyphs file
That has historic reasons:
From the OpenType post table spec:
This is the suggested distance of the top of the underline from the baseline (negative values indicate below baseline).
The PostScript definition of this FontInfo dictionary key (the y coordinate of the center of the stroke) is not used for historical reasons. The value of the PostScript key may be calculated by subtracting half the underlineThickness from the value of this field.
Can’t reproduce … for me, the underlinePosition value is exported to TTF and OTF exactly as entered in the master custom parameter. (Glyphs 3.2 3177)
I wonder if it changed between 3151 and 3177?