Automatic combined style map names are passed to ufo2ft with undesirable values

[NightFurySL2001]

When using fontmake to instantiate a UFO from designspace with multilingual names, the instance style map family names include additional undesirable names that should not be included.

For example, if I include this Python code in ufo2ft:

        print(instance.localisedFamilyName)
        print(instance.localisedStyleName)
        print(instance.localisedStyleMapFamilyName)
        print(instance.localisedStyleMapStyleName)

before https://github.com/googlefonts/ufo2ft/blob/9cda1f16f25d7941f0f8a82b5ae1788150bf18d1/Lib/ufo2ft/instantiator.py#L782.

Using this designspace:

<?xml version='1.0' encoding='utf-8'?>
<designspace format="4.0">
    <axes>
        <axis
            minimum="100"
            maximum="700"
            default="100"
            tag="wght"
            name="weight"
        >
            <labelname xml:lang="en">Weight</labelname>
            <labelname xml:lang="zh">字重</labelname>
            <map output="100" input="100"/>
            <map output="165" input="200"/>
            <map output="275" input="300"/>
            <map output="360" input="400"/>
            <map output="425" input="450"/>
            <map output="505" input="500"/>
            <map output="602" input="600"/>
            <map output="700" input="700"/>
            <labels>
                <label uservalue="100" name="Thin"/>
                <label uservalue="200" name="ExtraLight"/>
                <label uservalue="300" name="Light"/>
                <label uservalue="400" name="Regular"/>
                <label uservalue="450" name="Text"/>
                <label uservalue="500" name="Medium"/>
                <label uservalue="600" name="Semibold"/>
                <label uservalue="700" name="Bold"/>
            </labels>
        </axis>
    </axes>
    <sources>
        <source
            familyname="Frex Sans GB VF"
            stylename="Bold"
            filename="masters/FrexSansGBVF-Bold.ufo"
            name="Bold"
        >
            <familyname xml:lang="zh">械黑 GB VF</familyname>
            <location>
                <dimension xvalue="700" name="weight"/>
            </location>
        </source>
        <source
            familyname="Frex Sans GB VF"
            stylename="Thin"
            filename="masters/FrexSansGBVF-Thin.ufo"
            name="Thin"
        >
            <familyname xml:lang="zh">械黑 GB VF</familyname>
            <lib copy="1"/>
            <groups copy="1"/>
            <info copy="1"/>
            <features copy="1"/>
            <location>
                <dimension xvalue="100" name="weight"/>
            </location>
        </source>
    </sources>
    <instances>
        <instance
            familyname="Frex Sans GB"
            stylename="Thin"
            filename="instances/FrexSansGB-Thin.ufo"
            name="Thin"
        >
            <location>
                <dimension xvalue="100" name="weight"/>
            </location>
            <familyname xml:lang="zh">械黑 GB</familyname>
            <stylename xml:lang="zh">Thin</stylename>
        </instance>
        <instance
            familyname="Frex Sans GB"
            stylename="Regular"
            filename="instances/FrexSansGB-Regular.ufo"
            name="Regular"
        >
            <location>
                <dimension xvalue="360" name="weight"/>
            </location>
            <familyname xml:lang="zh">械黑 GB</familyname>
            <stylename xml:lang="zh">Regular</stylename>
        </instance>
        <instance
            familyname="Frex Sans GB"
            stylename="Bold"
            filename="instances/FrexSansGB-Bold.ufo"
            name="Bold"
        >
            <location>
                <dimension xvalue="700" name="weight"/>
            </location>
            <familyname xml:lang="zh">械黑 GB</familyname>
            <stylename xml:lang="zh">Bold</stylename>
        </instance>
    </instances>
</designspace>

Running fontmake -i Thin -o ufo -m test.designspace, the output is follow:

INFO:fontmake.font_project:Loading 2 DesignSpace source UFOs
INFO:fontmake.font_project:Interpolating master UFOs from designspace
INFO:fontmake.font_project:Generating instance UFO for 'Thin'
{'zh': '械黑 GB'}
{'zh': 'Thin'}
{'en': 'Frex Sans GB VF Thin', 'zh': '械黑 GB VF Thin'} // <-- where do this come from???
{}
INFO:fontmake.font_project:Saving instances\FrexSansGB-Thin.uf

Pretty sure this isn't on ufo2ft as I've written this part and there is a test case testing this exact scenario.

[anthrotype]

I haven't checked but I suspect it might be fontTools.designspaceLib.statNames.getStatNames() which is called by fontTools.designspaceLib.split.splitVariableFonts() (actually by _extractSubSpace, in turn called by the latter)

fontmake uses splitVariableFonts to support building multiple <variable-font> elements within a DS format v5.

@NightFurySL2001 do you mind verifying this is the case?

[NightFurySL2001]

Looks so from comment. Also is where the bug in googlefonts/ufo2ft#942 came from. Is this an desirable behaviour e.g. in the previous designspace example? I would expect that to work and the localised family name will be without "VF", i.e. following <familyname xml:lang="zh">械黑 GB</familyname>.

[anthrotype]

that comment in getStatNames is telling something maybe:

Note: this method does not consider info attached to the instance, like
family name. The user needs to override all names on an instance that STAT
information would compute differently than desired.

[NightFurySL2001]

The problem is if it's intended to work like that with the given designspace.

[anthrotype]

I think so, the comment says "this method does not consider info attached to the instance, like family name". In your DS, the sources have family names with " VF" suffix and those seem to be used by getStatNames().

@belluzj wrote that code, maybe he can confirm?

[belluzj]

Hello, yes sorry. I think the function getStatNames() is trying it best to generate names as the STAT table would do (from my understanding). Maybe the problem is higher in the call tree, in the code that uses that function to make up the localisedStyleMapFamilyName instead of using instance data. I'll have a look.

There might also be an option for you @NightFurySL2001, in the spirit of my comment "The user needs to override all names on an instance that STAT information would compute differently than desired": can you add stylemapfamilyname="Frex Sans GB Thin" to the <instance> element, and possibly a child <stylemapfamilyname xml:lang="zh">械黑 GB Thin</stylemapfamilyname>, and let us know if that helps?

The documentation for those is here: https://fonttools.readthedocs.io/en/latest/designspaceLib/xml.html#stylename-familyname-stylemapstylename-stylemapfamilyname-elements-localised-names-for-instances

[NightFurySL2001]

Adding the stylemap*name in the instance will work, but I think it looks more verbose than it should have been. If designspacelib had left the field empty, downstream programs (e.g. ufo2ft) would have generated the names more elegantly with the instance family name.