Adding FeatureVariations to variable fonts ↩

• What is FeatureVariations?
• Make it work.
  • Create rvrn feature
  • Create the lookup
  • Generate the variable font
  • TTX

What is FeatureVariations?

FeatureVariations is the none interpolatable variations, A.K.A. Discontinuous variation. This is about the famous dollar.nostroke vs dollar.withstroke.

Small animation of this thing in action ($), one axis!

In the example the feature is activated if the axis value is greater than someValue. This is a one axis example but there can be as many axis involved as your design requires.

Animation, Example of multi axis. Pan?

Make it work.

• create rvrn feature
• create lookup
• generate variable OpenType font
• ttx
  • fvar
    • index axes
  • GSUB
    • change version number
    • index feature rvrn & lookup
    • add FeatureVariations table with correct indexes
• recompile

Create rvrn feature

The registered feature tag for this is rvrn. In this feature substitute the activated glyphs by the default glyphs. In the example of the dollar:

feature rvrn {
	sub dollar.nostroke by dollar.withstroke;
} rvrn;

The official OT Feature specs

Create the lookup

Also make a lookup where the substitutions is reversed. Make up a name for it:

lookup DollarNoStrokeActive {
  sub dollar.withstroke by dollar.nostroke;
} DollarNoStrokeActive;

Generate the variable font

Generate the variable font. The FeatureVariations are not working yet.

TTX

With TTX you can access the font tables (decompile), edit them and recompile the font.

1. Open Terminal
2. Navigate to the directory of your variable font.
3. ttx -t GSUB -t fvar <path-of-your-otvar.ttf> This generates a ttx file with only the GSUB and fvar tables, these are the only ones we need. Open the ttx file in your favourite code editor.
4. Change the GSUB version number 0x00010000 > 0x00010001

5. Look for the index number of rvrn feature. And for the index number of the lookup.

    …
    <FeatureList>
      …
      <FeatureRecord index="6">
        <FeatureTag value="rvrn"/>
        …
    <LookupList>
      <Lookup index="8">
        <LookupType value="1"/>
        <LookupFlag value="0"/>
        <!-- SubTableCount=1 -->
        <SingleSubst index="0" Format="2">
          <Substitution in="dollar.withstroke" out="dollar.nostroke"/>
        …
    …
   
   

6. add FeatureVariations block
7. with indexes …

8. recompile

    <fvar>
   
        <!-- Weight -->
        <Axis>
          <AxisTag>wght</AxisTag>
          <Flags>0x0</Flags>
          <MinValue>0.0</MinValue>
          <DefaultValue>20.0</DefaultValue>
          <MaxValue>30.0</MaxValue>
          <AxisNameID>256</AxisNameID>
        </Axis>
   
        <!-- Angle -->
        <Axis>
          <AxisTag>ANGL</AxisTag>
          <Flags>0x0</Flags>
          <MinValue>0.0</MinValue>
          <DefaultValue>180.0</DefaultValue>
          <MaxValue>360.0</MaxValue>
          <AxisNameID>257</AxisNameID>
        </Axis>
   
        <!-- Steps -->
        <Axis>
          <AxisTag>STEP</AxisTag>
          <Flags>0x0</Flags>
          <MinValue>10.0</MinValue>
          <DefaultValue>20.0</DefaultValue>
          <MaxValue>60.0</MaxValue>
          <AxisNameID>258</AxisNameID>
        </Axis>
   
        <!-- Style_1 -->
        <NamedInstance flags="0x0" subfamilyNameID="259">
          <coord axis="wght" value="20.0"/>
          <coord axis="ANGL" value="180.0"/>
          <coord axis="STEP" value="20.0"/>
        </NamedInstance>
    </fvar>
   

Just van Rossum - Robothon 2009, TTX