Improve rationale for [com.google.fonts/check/metadata/os2_weightclass]

[vv-monsalve]

Splitting #2872 into individual Issues for each Fail

Observed behaviour

The Fail was reported after FB checks performed from DIspatcher. The message for the FAIL (below) doesn't offer a clear explanation of what could be the cause. The OS/2 usWeightClass value (300) is ok for the font.

FAIL
OS/2 usWeightClass (300:"Light") does not match weight specified at METADATA.pb (400:"Regular").
code mismatch

Expected behaviour

Having a more clear or explanatory rationale on what is happening and how to proceed.
Related to googlefonts/gftools#200

Resources and exact process needed to replicate

Link to the Dispatcher Fontbakery results
Link to Last Cl FB QA folder
Upstream https://github.com/vv-monsalve/Varta/tree/QA-VartaST

[m4rc1e]

We need to modify the metadata usWeightClass check for VFs. For static fonts, we expect each weightClass metadata value to be the same as the font. For VFs, we expect each weightClass value to be 400 or near 400.

This was discussed in the following issue, #2683

Apologies but I should've flagged this issue sooner. I have already updated gftools add-font

[vv-monsalve]

Thanks @m4rc1e. From what I see, the metadata on this project seems to match with what you mention there in your example. Thus I'd like some clarification.

This is the METADATA.pb of the FBD process

name: "Varta"
designer: "Joana Correia, Viktoriya Grabowska, Eben Sorkin"
license: "OFL"
category: "SANS_SERIF"
date_added: "2015-05-13"
fonts {
  name: "Varta"
  style: "normal"
  weight: 400      ----------------------------------- it is 400
  filename: "Varta[wght].ttf"
  post_script_name: "Varta-Light"
  full_name: "Varta Light"
  copyright: "Copyright 2019 The Varta Project Authors (https://github.com/SorkinType/Varta)"
}
subsets: "latin"
subsets: "latin-ext"
subsets: "menu"
subsets: "vietnamese"
axes {
  tag: "wght"
  min_value: 300.0
  max_value: 700.0
}

Should the Default Value be 400 in the fvar table?
I changed it to 300 after I got a Fail in the firsts CL FB checks. Would it be the check that would you modify? [com.google.fonts/check/usweightclass] (below)

fvar table

<?xml version="1.0" encoding="UTF-8"?>
<ttFont sfntVersion="\x00\x01\x00\x00" ttLibVersion="4.9">

  <fvar>

    <!-- Weight -->
    <Axis>
      <AxisTag>wght</AxisTag>
      <Flags>0x0</Flags>
      <MinValue>300.0</MinValue>
      <DefaultValue>300.0</DefaultValue>
      <MaxValue>700.0</MaxValue>
      <AxisNameID>256</AxisNameID>
    </Axis>

    <!-- Light -->
    <NamedInstance flags="0x0" subfamilyNameID="257">
      <coord axis="wght" value="300.0"/>
    </NamedInstance>

    <!-- Regular -->
    <NamedInstance flags="0x0" subfamilyNameID="258">
      <coord axis="wght" value="400.0"/>
    </NamedInstance>

    <!-- SemiBold -->
    <NamedInstance flags="0x0" subfamilyNameID="259">
      <coord axis="wght" value="600.0"/>
    </NamedInstance>

    <!-- Bold -->
    <NamedInstance flags="0x0" subfamilyNameID="260">
      <coord axis="wght" value="700.0"/>
    </NamedInstance>
  </fvar>

</ttFont>

Previously reported fail

🔥 FAIL: Checking OS/2 usWeightClass.

• com.google.fonts/check/usweightclass

• 🔥 FAIL OS/2 usWeightClass expected value for 'Regular' is 400 but this font has 300.
  GlyphsApp users should set a Custom Parameter for 'Axis Location' in each master to ensure that the information is accurately built into variable fonts. [code: bad-value]

[m4rc1e]

Should the Default Value be 400 in the fvar table?

Nope. Keep the font as is.

Your setup looks good.

[vv-monsalve]

I see, ok I'll wait for the check to be solved then. Thanks @m4rc1e