Update varLib to current designspace spec

@behdad, does varLib need to be updated to implement the current designspace spec? When building a variable font for Noto Sans Cherokee with fontmake, varLib crashes like this:

  File "/home/sascha/src/fontmake/Lib/fontmake/font_project.py", line 200, in build_variable_font
    font, _, _ = varLib.build(designspace_path, finder)
  File "/home/sascha/src/fontmake/env/src/fonttools/Lib/fontTools/varLib/__init__.py", line 380, in build
    model = models.VariationModel(master_locs)
  File "/home/sascha/src/fontmake/env/src/fonttools/Lib/fontTools/varLib/models.py", line 137, in __init__
    keyFunc = self.getMasterLocationsSortKeyFunc(locations, axisOrder=axisOrder)
  File "/home/sascha/src/fontmake/env/src/fonttools/Lib/fontTools/varLib/models.py", line 148, in getMasterLocationsSortKeyFunc
    assert {} in locations, "Base master not found."
AssertionError: Base master not found.

When compiling NotoSansCherokee, the locations variable has the value [{u'wght': -1.0}, {u'wght': -1.0}, {u'wght': -0.7}], which seems a bit surprising given the generated designspace:

<?xml version='1.0' encoding='utf-8'?>
<designspace format="3">
    <axes>
        <axis default="400" maximum="900.0" minimum="100.0" name="weight" tag="wght">
            <map input="100.0" output="26" />
            <map input="200.0" output="39" />
            <map input="300.0" output="58" />
            <map input="400.0" output="90" />
            <map input="500.0" output="108" />
            <map input="600.0" output="128" />
            <map input="700.0" output="151" />
            <map input="800.0" output="169" />
            <map input="900.0" output="190" />
            <labelname xml:lang="en">Weight</labelname>
        </axis>
    </axes>
    <sources>
        <source familyname="Noto Sans Cherokee" filename="NotoSansCherokee-Light.ufo" name="Noto Sans Cherokee Light" stylename="Light">
            <location>
                <dimension name="weight" xvalue="26.000000" />
            </location>
        </source>
        <source familyname="Noto Sans Cherokee" filename="NotoSansCherokee-Regular.ufo" name="Noto Sans Cherokee Regular" stylename="Regular">
            <lib copy="1" />
            <groups copy="1" />
            <features copy="1" />
            <info copy="1" />
            <location>
                <dimension name="weight" xvalue="90.000000" />
            </location>
        </source>
        <source familyname="Noto Sans Cherokee" filename="NotoSansCherokee-Bold.ufo" name="Noto Sans Cherokee Bold" stylename="Bold">
            <location>
                <dimension name="weight" xvalue="190.000000" />
            </location>
        </source>
    </sources>
    <instances>
        <instance familyname="Noto Sans Cherokee" filename="../instance_ufo/NotoSansCherokee-Thin.ufo" name="Noto Sans Cherokee Thin" stylename="Thin">
            <location>
                <dimension name="weight" xvalue="26.000000" />
            </location>
            <info />
            <kerning />
        </instance>
        <instance familyname="Noto Sans Cherokee" filename="../instance_ufo/NotoSansCherokee-ExtraLight.ufo" name="Noto Sans Cherokee ExtraLight" stylename="ExtraLight">
            <location>
                <dimension name="weight" xvalue="39.000000" />
            </location>
            <info />
            <kerning />
        </instance>
        <instance familyname="Noto Sans Cherokee" filename="../instance_ufo/NotoSansCherokee-Light.ufo" name="Noto Sans Cherokee Light" stylename="Light">
            <location>
                <dimension name="weight" xvalue="58.000000" />
            </location>
            <info />
            <kerning />
        </instance>
        <instance familyname="Noto Sans Cherokee" filename="../instance_ufo/NotoSansCherokee-Regular.ufo" name="Noto Sans Cherokee Regular" stylename="Regular">
            <location>
                <dimension name="weight" xvalue="90.000000" />
            </location>
            <info />
            <kerning />
        </instance>
        <instance familyname="Noto Sans Cherokee" filename="../instance_ufo/NotoSansCherokee-Medium.ufo" name="Noto Sans Cherokee Medium" stylename="Medium">
            <location>
                <dimension name="weight" xvalue="108.000000" />
            </location>
            <info />
            <kerning />
        </instance>
        <instance familyname="Noto Sans Cherokee" filename="../instance_ufo/NotoSansCherokee-SemiBold.ufo" name="Noto Sans Cherokee SemiBold" stylename="SemiBold">
            <location>
                <dimension name="weight" xvalue="128.000000" />
            </location>
            <info />
            <kerning />
        </instance>
        <instance familyname="Noto Sans Cherokee" filename="../instance_ufo/NotoSansCherokee-Bold.ufo" name="Noto Sans Cherokee Bold" stylename="Bold">
            <location>
                <dimension name="weight" xvalue="151.000000" />
            </location>
            <info />
            <kerning />
        </instance>
        <instance familyname="Noto Sans Cherokee" filename="../instance_ufo/NotoSansCherokee-ExtraBold.ufo" name="Noto Sans Cherokee ExtraBold" stylename="ExtraBold">
            <location>
                <dimension name="weight" xvalue="169.000000" />
            </location>
            <info />
            <kerning />
        </instance>
        <instance familyname="Noto Sans Cherokee" filename="../instance_ufo/NotoSansCherokee-Black.ufo" name="Noto Sans Cherokee Black" stylename="Black">
            <location>
                <dimension name="weight" xvalue="190.000000" />
            </location>
            <info />
            <kerning />
        </instance>
    </instances>
</designspace>

I won’t be able to fix this during the coming days (need to prepare for a Google committee; then a full day of committee meetings; then there’s the Easter holiday in Switzerland). So it would be great if @behdad or somebody else could look into this.

[LettError]

To be honest, I find this requirement the weakest part of the specification. Looking at https://github.com/googlei18n/fontmake/issues/167, @brawer summarizes: For example, Light must be at wght=300, Regular at wght=400, Black at wght=900. — this is going to be very difficult to enforce.

• Whether or not a typeface can map to these numbers this depends on its design. In order for the font not to be on spec it would be enough for someone could have to look and say “that’s not regular at all”.
• What if the designspace only has a thin and bold, the spec would then state that whatever is at 400 is the regular. This is not necessarily the right weight.
• Weight progression is curved, even in linear interpolation between 2 masters.
• Using avar to correct for this is unwanted: avar is to expose certain areas of an axis at a different rate, for instance, gulp, for sliders. It is not intended to correct for realities in a particular typeface and the expectation of the spec.
• Regardless of what the spec says, there are going to be many different typefaces expressing many ideas about how weight grows, using the full range of this axis. We might as well start preparing for it.
• As I understand, this requirement is motivated by a desire to have some interoperability between different fonts, so that if one is missing, another might have a reasonable replacement weight. As a typographer I have my reservations about how useful this is.
• I think the bottom line is the OpenType spec should not make any normative statements on the design of typefaces. I recommend reconsidering this.

[behdad]

That normakuzation is just multiplication and addition applied to all coordinates the same way, per axis

Really, that’s exactly what my code does…