Calculate instance values for openTypeOS2WidthClass, openTypeOS2WeightClass?

[LettError]

Under some conditions it should be possible to calculate meaningful values for openTypeOS2WidthClass and openTypeOS2WeightClass, when generating instances.

openTypeOS2WidthClass

wdth conditions:

• https://learn.microsoft.com/en-us/typography/opentype/spec/os2#uswidthclass
• value between 50 - 120 ?

openTypeOS2WeightClass

wght conditions:

• https://learn.microsoft.com/en-us/typography/opentype/spec/os2#usweightclass
• value between 100 - 900

General considerations:

• Anisotropic locations should not be a problem as these are horizontal attributes.
• Extrapolation: values could be clipped to prevent unexpected outcomes like negative values.
• Values are expected to be user space values, so appropriate mapping has to happen.

Pro: these can be fussy values to set in a build script. Width may be more difficult to get right as it only has 9 values it can be. Weight, on the other hand, has a bigger range. Con: not every designspace will fit the conditions and that means ufoOperator will not always to the math. And that may be confusing.

[typemytype]

fontMath already does something with those info attributes: https://github.com/robotools/fontMath/blob/777dd889e4431d5b4666f032bdf1269d2ccf37b0/Lib/fontMath/mathInfo.py#L381

Is there something else necessary

[LettError]

Fontmath will indeed interpolate the values in the source files. I suppose the idea is that the source values can be inferred from the location in the designspace, independent from what is stored in the source.

[typemytype]

mmm I see

something that overwrites those interpolated values (extracted from the sources, which could be different then the location of the instance) based on the wght and wdth locations...