Anisotropic interpolation produces wrong (-90°) Italic angle for instances

[frankrolf]

I filed this issue in the wrong repo:
LettError/MutatorMath#259

Copy-pasting the original issue below.
Please follow the link above for some thoughts by @LettError.

First pointed out by @kaibernau:

An anisotropic interpolation will result in an unexpected Italic angle for the instance UFOs.
Anisotropic interpolation basically splits the x- and y- axis by allowing different factors for each.
This is how to indicate it for a given instance:

        <dimension name="weight" xvalue="57" yvalue="52"/>

In the attached project, the Italic angle for both masters is -11°.
Using anisotropic interpolation, the italic angle for the instance ends up as -90°.
This is true for any value of y, including 0 and = x

I reproduced Kai’s finding using the AFDKO’s makeinstancesufo:

makeinstancesufo -ac -d designspace.designspace
# -ac switches off autohint/checkoutlinesufo

italic_angle_issue.zip

[benkiel]

@frankrolf This may now be fixed by #189. I haven't tested yet, however.