Following the definition of glyf’s bounding box, the ymax would become y2, and the vertical origin would be y2 + t in the default instance. This is the peaceful old world we know.
However in the variable world, the advance height would become h + δh, which is simple. However the top side bearing is now t + δt, and the definition of vertical origin become ambiguous now:
- If we follow the glyf’s ymax then the vertical origin would become y2 + t + δt.
- If we follow the true bounding box, the vertical origin would become max(y1 + δy1, y2 + δy2) + t + δt. This is a complex formula and it is even not representable in the OTVar’s “value + delta” manner, as the designers expected.