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[varLib] 2D designspace with 6 masters creates wrong var font

See original GitHub issue

The enclosed designspace contains two axes, “FOOO” and “BARR”. There are masters at all four corners, as well as intermediate masters in the middle of the “BARR” axis at both “FOOO” master positions.

At FOOO=0 all is well. At FOOO=1, the resulting shape at BARR=0.5 is wrong, and does not match the master at that position.

Below is the complete gvar.

What is curious is that the FOOO=0 BARR=1 delta location is given as:

        <coord axis="BARR" max="1.0" min="0.5" value="1.0"/>

yet the FOOO=1 BARR=1 delta location is given as:

        <coord axis="FOOO" value="1.0"/>
        <coord axis="BARR" value="1.0"/>

Note the missing (and therefore different) min and max values for “BARR”.

  <gvar>
    <version value="1"/>
    <reserved value="0"/>
    <glyphVariations glyph="uni0041">
      <tuple>
        <coord axis="FOOO" value="1.0"/>
        <delta pt="0" x="50" y="-233"/>
        <delta pt="1" x="-233" y="-50"/>
        <delta pt="2" x="-50" y="233"/>
        <delta pt="3" x="233" y="50"/>
        <delta pt="4" x="0" y="0"/>
        <delta pt="5" x="0" y="0"/>
        <delta pt="6" x="0" y="233"/>
        <delta pt="7" x="0" y="233"/>
      </tuple>
      <tuple>
        <coord axis="BARR" max="1.0" min="0.0" value="0.5"/>
        <delta pt="0" x="300" y="-21"/>
        <delta pt="1" x="229" y="-50"/>
        <delta pt="2" x="200" y="21"/>
        <delta pt="3" x="271" y="50"/>
        <delta pt="4" x="0" y="0"/>
        <delta pt="5" x="0" y="0"/>
        <delta pt="6" x="0" y="21"/>
        <delta pt="7" x="0" y="21"/>
      </tuple>
      <tuple>
        <coord axis="BARR" max="1.0" min="0.5" value="1.0"/>
        <delta pt="0" x="500" y="0"/>
      </tuple>
      <tuple>
        <coord axis="FOOO" value="1.0"/>
        <coord axis="BARR" max="1.0" min="0.0" value="0.5"/>
        <delta pt="0" x="190" y="64"/>
        <delta pt="1" x="64" y="-190"/>
        <delta pt="2" x="-190" y="-64"/>
        <delta pt="3" x="-64" y="190"/>
        <delta pt="4" x="0" y="0"/>
        <delta pt="5" x="0" y="0"/>
        <delta pt="6" x="0" y="-64"/>
        <delta pt="7" x="0" y="-64"/>
      </tuple>
      <tuple>
        <coord axis="FOOO" value="1.0"/>
        <coord axis="BARR" value="1.0"/>
        <delta pt="0" x="0" y="-125"/>
        <delta pt="1" x="-125" y="0"/>
        <delta pt="2" x="0" y="125"/>
        <delta pt="3" x="125" y="0"/>
        <delta pt="4" x="0" y="0"/>
        <delta pt="5" x="0" y="0"/>
        <delta pt="6" x="0" y="125"/>
        <delta pt="7" x="0" y="125"/>
      </tuple>
    </glyphVariations>
  </gvar>

If I manually edit the TTX so that last delta location contains min and max for BARR, to match the other BARR=1 delta location, and recompile, I get a font that works as expected.

fonttools-bug-report.zip

Cc. @thundernixon