How to enable fake slant feature in kitty?

[0And1Story]

I'm trying to configure Chinese fonts in kitty. I prefer to use JetBrains Mono in English and Noto Sans CJK SC in Chinese.

To solve the font fallback, I use

<?xml version="1.0"?>
<!DOCTYPE fontconfig SYSTEM "urn:fontconfig:fonts.dtd">
<fontconfig>
    <match target="pattern">
        <test name="prgname" compare="eq">
            <string>kitty</string>
        </test>
        <edit mode="prepend" name="family">
            <string>Noto Sans CJK SC</string>
        </edit>
    </match>
</fontconfig>

It works well in regular and bold fonts. But for CJK fonts, the font files rarely provide italic glyphs directly. Instead, they use fakeslant feature. Thus, I use this font config:

<?xml version="1.0"?>
<!DOCTYPE fontconfig SYSTEM "fonts.dtd">
<fontconfig>

  <!-- bold -->
  <match target="pattern">
    <test name="family"><string>Noto Sans CJK SC</string></test>
    <test name="weight" compare="more_eq"><const>bold</const></test>
    <edit name="family" mode="assign"><string>Noto Sans CJK SC</string></edit>
    <edit name="style" mode="assign"><string>Bold</string></edit>
  </match>

  <!-- italic (fake slant) -->
  <match target="font">
    <test name="family"><string>Noto Sans CJK SC</string></test>
    <test name="slant" compare="more_eq"><const>italic</const></test>
    <edit name="matrix" mode="assign">
      <matrix>
        <double>1</double><double>0.2</double>
        <double>0</double><double>1</double>
      </matrix>
    </edit>
    <edit name="slant" mode="assign"><const>italic</const></edit>
    <edit name="embeddedbitmap" mode="assign"><bool>false</bool></edit>
  </match>

</fontconfig>

$ fc-match "Noto Sans CJK SC:slant=italic" matrix
:matrix=1 0.2 0 1

I am sure that the configuration take effects, but it seems that kitty does not support this feature.

$ echo -e "\033[3m斜体中文测试 Italic\033[0m"
斜体中文测试 Italic

[43.511] U+659c italic using previous fallback font at index: 0
[43.513] U+4f53 italic using previous fallback font at index: 0
[43.515] U+4e2d italic using previous fallback font at index: 0
[43.516] U+6587 italic using previous fallback font at index: 0
[43.517] U+6d4b italic using previous fallback font at index: 0
[43.519] U+8bd5 italic using previous fallback font at index: 0

It fallbacks to Noto Sans CJK SC directly. I wonder if there is an approach to achieve my expected effects.

(Well, I know that fake slant is not a good feature in font designing, but it is special and important in CJK fonts.)

[itxashancode]

I recall kitty supports "font_features" to enable OpenType features, but fake slant is not an OpenType feature; it's a fontconfig transformation. Kitty uses harfbuzz and fontconfig directly? I think kitty respects fontconfig's matrix for slant if you set "italic_font" to the same family and enable "allow_bold"? Actually there is a known issue: kitty does not apply fontconfig's slant transform; you need to use the "italic_font" setting pointing to a font file that has italic glyphs, or use the "allow_bold" for bold. For fake slant, you can use the "italic_font" with same file and set "font_features" maybe? I think you need to set "italic_font" to the same font and then set "allow_bold"? Not sure.

[0And1Story]

It does not work if I set italic_font to the same family. I checked the source files and found that kitty probably does not care about the matrix config.

kitty/kitty/fontconfig.c

Lines 321 to 326 in 9354a33

	#define g(func, prop, output) if (func(match, prop, 0, &output) != FcResultMatch) { PyErr_SetString(PyExc_ValueError, "No " #prop " found in fontconfig match result"); goto end; }
	g(FcPatternGetString, FC_FILE, out);
	g(FcPatternGetInteger, FC_INDEX, ans->index);
	g(FcPatternGetInteger, FC_HINT_STYLE, ans->hintstyle);
	g(FcPatternGetBool, FC_HINTING, ans->hinting);
	#undef g

Since CJK fonts often do not contain italic glyphs, it is inconvenient to enable itatlic fonts. :(

[itxashancode]

Kitty explicitly ignores fontconfig's matrix transform in its font loading code, so fakeslant will never work there. Your only real options are: use a CJK font with actual italic glyphs (LXGW WenKai), or switch to WezTerm which handles this better.

[Strykar]

Your transform is silently dropped because kitty doesn't read FC_MATRIX from fontconfig. In kitty/fontconfig.c, pattern_as_dict() extracts only FC_FILE, FC_FAMILY, FC_STYLE, FC_WEIGHT, FC_SLANT, FC_INDEX, and a few hint properties. Whatever matrix fontconfig produces never reaches kitty, so fc-match reports it but rendering ignores it.

Kitty also has no italic synthesis path of its own. There is no FT_Set_Transform call anywhere in kitty/freetype.c, so it cannot oblique-shear an upright face at render time. Bold synthesis was removed years ago for the same reason.

The good news: fontconfig-based fallback does honour italic state. When your primary font lacks a CJK glyph, kitty queries fontconfig with slant=italic (see create_fallback_face() in kitty/fontconfig.c). So the route through fontconfig works, you just need it to land on a CJK font that ships actual italic glyphs in the file.

Two options that work without any kitty changes:

1. Switch the CJK family to one that has real italic styles. Sarasa Mono SC (https://github.com/be5invis/Sarasa-Gothic) is the cleanest match for a JetBrains Mono pairing. It builds CJK from Source Han Sans and bakes a sheared italic into the font file at build time, so the italic glyphs are real outlines from kitty's point of view. After installing, change your fontconfig prepend rule from Noto Sans CJK SC to Sarasa Mono SC and drop the <matrix> block entirely.

2. If you want to stay on Noto, use https://github.com/life888888/cjk-fonts-ttf which ships Noto CJK with italic and bold-italic added via a FontForge transform script. Same idea: real glyphs in the file, no runtime matrix needed.

Note that symbol_map is not the right tool here even though it might look like one. It binds a single face per family, ignoring italic state (create_symbol_map() in kitty/fonts/render.py). Stick with the fontconfig prepend rule.

There's also feature request #9857 asking for FC_MATRIX to be honoured; @kovidgoyal said "PR is welcome" so this may eventually work the way you originally tried.

[Strykar]

I opened #9990 which makes kitty honour FC_MATRIX. Two corrections to my answer above:

1. Your original setup was correct. fontconfig's stock 90-synthetic.conf (default-enabled in every distro) already produces the matrix (1, 0.2, 0, 1) for italic-context CJK queries on roman-only fonts. Your custom <edit name="matrix"> rule was redundant but harmless. The reason nothing rendered as italic was kitty silently dropping the matrix in pattern_as_dict(), not anything wrong on your side.

2. If Read FC_MATRIX from fontconfig #9990 lands you won't need to switch fonts. Stock Noto Sans CJK SC + stock fontconfig + your existing kitty.conf will fake-italic Chinese automatically. The Sarasa Mono SC and cjk-fonts-ttf options remain useful right now (no kitty rebuild needed) but become unnecessary the moment kitty starts reading FC_MATRIX.

[0And1Story]

Yes, that's what I want! Thanks.

[driuba]

Even after the 0.47.0 update I can't seem to get FiraCode rendering with fake slant. What could I be missing?