Incorrect contour extraction and rendering of DFKai-SB glyphs

[drott]

https://rtsh.es/test/dfkai_pdf.html hosts a PDF.js view of a PDF that displays glyphs from Microsofts's DFKai-SB font
https://learn.microsoft.com/en-us/typography/font-list/dfkai-sb

Two of the glyphs appear compressed with incomplete contour shapes.

vs expected:

I'll try to provide a further reduction.

The font is available as an attachment of the upstream bug report: https://issues.chromium.org/387045808

[drott]

The font is available as an attachment of the upstream bug report: https://issues.chromium.org/387045808

Sorry, the font is not, only the PDF that embeds it. I am working on extracting it.

[drott]

Font extracted from PDF by setting breakpoints on CSS FontFace creation in the pdf.js code.

dfkai.zip

<name>
    <namerecord nameID="0" platformID="1" platEncID="0" langID="0x0" unicode="True">
      (c) Copyright DynaLab Inc. 1992-1998
    </namerecord>
    <namerecord nameID="1" platformID="1" platEncID="0" langID="0x0" unicode="True">
      DFKai-SB
    </namerecord>
[...]
    <namerecord nameID="5" platformID="1" platEncID="0" langID="0x0" unicode="True">
      Version 5.01
    </namerecord>
    <namerecord nameID="6" platformID="1" platEncID="0" langID="0x0" unicode="True">
      DFKaiShu-SB-Estd-BF
    </namerecord>
[...]
  </name>

[drott]

Fauntlet diff:

https://gist.github.com/drott/4380519da3492a948ef8e0411cb52b61

[drott]

According to https://learn.microsoft.com/en-us/typography/fonts/windows_11_font_list#traditional-chinese and https://learn.microsoft.com/en-us/typography/font-list/dfkai-sb#products-that-supply-this-font 5.01 seems to be the latest version of this font.

[dfrg]

So it turns out that this is one of the "tricky" fonts that requires the bytecode interpreter to generate a correct outline. If I run these through skrifa with the target set to Mono, I get the intended results.

Since our API doesn't handle hinting internally, this isn't something we can just fix. What we can do is copy FreeType's tricky font detection and expose it as a method (maybe OutlineGlyphCollection::require_interpreter()). If that returns true then the user should apply hinting (with the Mono target). This method won't be fast since it requires checking the name table and potentially computing checksums so for Skia, this should be done when creating a typeface.