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.