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Changes in the OpenType spec means that fonts can now come with an unbounded number of weights, in variable and static forms.

The OpenType spec insists weights use the same scale in all metadata layers (STAT, OS/2), for static and variable fonts and that all of those must be consistent to enable substituting instances to static fonts (and vice versa).

The CSS4 spec mandates deriving naming from specific weight values.

However, that does not work in fontconfig today. For example, installing Merriweather (one of the top 20 Google fonts for the past years), without any specific tuning, will produce the following result

https://github.com/SorkinType/Merriweather/

$ fc-scan -f "%{style[0]};%{weight};%{file}\n" . |  sort -t ';' -k2,2n  | column --separator ';' -t
                        [50 210]  ./Merriweather-Italic-VF.ttf
                        [50 210]  ./Merriweather-Roman-VF.ttf
Light                   50        ./Merriweather-Light.ttf
Light Display           50        ./Merriweather-LightDisplay.ttf
Light Display Narrow    50        ./Merriweather-LightDisplayNarrow.ttf
Light Display Wide      50        ./Merriweather-LightDisplayWide.ttf
Light Heading           50        ./Merriweather-LightHeading.ttf
Light Heading Narrow    50        ./Merriweather-LightHeadingNarrow.ttf
Light Heading Wide      50        ./Merriweather-LightHeadingWide.ttf
Light Italic            50        ./Merriweather-LightItalic.ttf
Light Narrow            50        ./Merriweather-LightNarrow.ttf
Light Text              50        ./Merriweather-LightText.ttf
Light Text Narrow       50        ./Merriweather-LightTextNarrow.ttf
Light Text Wide         50        ./Merriweather-LightTextWide.ttf
Light Wide              50        ./Merriweather-LightWide.ttf
Black Text              61,6667   ./Merriweather-BlackText.ttf
Italic                  80        ./Merriweather-Italic.ttf
Light                   80        ./Merriweather-Roman-VF.ttf
Light Italic            80        ./Merriweather-Italic-VF.ttf
Regular                 80        ./Merriweather-Regular.ttf
Regular Display         80        ./Merriweather-RegularDisplay.ttf
Regular Display Narrow  80        ./Merriweather-RegularDisplayNarrow.ttf
Regular Display Wide    80        ./Merriweather-RegularDisplayWide.ttf
Regular Heading         80        ./Merriweather-RegularHeadling.ttf
Regular Heading Narrow  80        ./Merriweather-RegularHeadingNarrow.ttf
Regular Heading Wide    80        ./Merriweather-RegularHeadingWide.ttf
Regular Narrow          80        ./Merriweather-RegularNarrow.ttf
Regular Text            80        ./Merriweather-RegularText.ttf
Regular Text Narrow     80        ./Merriweather-RegularTextNarrow.ttf
Regular Text Wide       80        ./Merriweather-RegularTextWide.ttf
Regular Wide            80        ./Merriweather-RegularWide.ttf
Italic                  126,667   ./Merriweather-Italic-VF.ttf
Regular                 126,667   ./Merriweather-Roman-VF.ttf
Bold                    200       ./Merriweather-Bold.ttf
Bold Display            200       ./Merriweather-BoldDisplay.ttf
Bold Display Narrow     200       ./Merriweather-BoldDisplayNarrow.ttf
Bold Display Wide       200       ./Merriweather-BoldDisplayWide.ttf
Bold Heading            200       ./Merriweather-BoldHeading.ttf
Bold Heading Narrow     200       ./Merriweather-BoldHeadingNarrow.ttf
Bold Heading Wide       200       ./Merriweather-BoldHeadingWide.ttf
Bold Italic             200       ./Merriweather-BoldItalic.ttf
Bold Narrow             200       ./Merriweather-BoldNarrow.ttf
Bold Text               200       ./Merriweather-BoldText.ttf
Bold Text Narrow        200       ./Merriweather-BoldTextNarrow.ttf
Bold Text Wide          200       ./Merriweather-BoldTextWide.ttf
Bold Wide               200       ./Merriweather-BoldWide.ttf
Black                   210       ./Merriweather-Black.ttf
Black Display           210       ./Merriweather-BlackDisplay.ttf
Black Display Narrow    210       ./Merriweather-BlackDisplayNarrow.ttf
Black Display Wide      210       ./Merriweather-BlackDisplayWide.ttf
Black Heading           210       ./Merriweather-BlackHeading.ttf
Black Heading Narrow    210       ./Merriweather-BlackHeadingNarrow.ttf
Black Heading Wide      210       ./Merriweather-BlackHeadingWide.ttf
Black Italic            210       ./Merriweather-BlackItalic.ttf
Black Narrow            210       ./Merriweather-BlackNarrow.ttf
Black Text Narrow       210       ./Merriweather-BlackTextNarrow.ttf
Black Text Wide         210       ./Merriweather-BlackTextWide.ttf
Black Wide              210       ./Merriweather-BlackWide.ttf
Bold                    211,667   ./Merriweather-Roman-VF.ttf
Bold Italic             211,667   ./Merriweather-Italic-VF.ttf
Black                   215       ./Merriweather-Roman-VF.ttf
Black Italic            215       ./Merriweather-Italic-VF.ttf

The weights are completely inconsistent between variable and non variable (Regular is both 80 and 126,667)

The weight ordering is completely off (Black is sometimes heavier than Bold, sometimes lighter, in one instance it‘s even lighter than Regular!)

However, all those files have been generated by the same tools from the same master, which, as far as I can see, uses consistent values for all variants: https://github.com/SorkinType/Merriweather/blob/master/sources/master_ufo/Merriweather.designspace

However, it is not easy to see what is wrong because of fontconfig’s tampering with the OpenType weight scale.

Requirements

1. fontconfig should use coherent weight values for all faces of a font family regardless if they are variable or static

2. fontconfig should make it easy to identify what went wrong in case of problems. It should expose the real opentype weight value exported by font files, so it’s easy to check if the problem is in this value or in what fontconfig made of it

3. once the problem is identified, it should be easy to override problem values with the correct value using a fontconfig xml snippet (and the correct value should not take black magic to compute)

4. the result should work with the rest of the weight axis (ie it’s useless to fix the weight value of a variable instance, if all the weight range around the fixed instance is still broken)