Support for COLRv1 fonts

[unnamedname]

Is your feature request related to a problem? Please describe.
Kitty won't display COLRv1 (emoji) fonts.

Describe the solution you'd like

Describe alternatives you've considered

Additional context

[kovidgoyal]

Works fine for me with Segoe UI Emoji which uses COLRv1

kitty --config=None --debug-font-fallback -o 'symbol_map U+1f63a Segoe UI Emoji' sh -c 'printf "\U1f63a"; read'

[unnamedname]

Thanks for so prompt reply!

It indeed works with Segoe UI Emoji, but no luck with COLRv1 variant Noto Color Emoji:

kitty --config=None --debug-font-fallback -o 'symbol_map U+1f63a Noto Color Emoji' sh -c 'printf "\U1f63a"; read'

Also I noticed that the Segoe UI Emoji displayed by Kitty (left) is different from the one in GTK (right):

This is most likely because the Segoe UI Emoji is a hybrid font that supports both COLRv1 and COLRv0, and Kitty actual displayed COLRv0 glyphs.

[kovidgoyal]

Works fine with Noto Color Emoji as well

kitty --config=None --debug-font-fallback -o 'symbol_map U+1f63a Noto Color Emoji' sh -c 'printf "\U1f63a"; read'

Presumably that has bitmap fallbacks too?

[unnamedname]

Maybe the COLRv1 version share the same font name Noto Color Emoji with the CDBT version packaged by most distros (assuming your system has the both installed).
Anyway, I suppose we could simply try to scale the interface large enough to determine if it is in CBDT or COLR format:
CBDT (packaged by Archlinux):

COLRv1 (packaged by the PKGBUILD):

pkgname=noto-color-emoji
pkgver=2.042
pkgrel=1
epoch=1
pkgdesc='Google Noto color emoji font using COLRv1'
arch=(any)
url='https://www.google.com/get/noto/'
license=(custom:OFL)
provides=(noto-fonts-emoji emoji-font)
conflicts=(noto-fonts-emoji)
source=(
    NotoColorEmoji.ttf::https://github.com/googlefonts/noto-emoji/raw/v$pkgver/fonts/Noto-COLRv1.ttf
    LICENSE::https://github.com/googlefonts/noto-emoji/raw/v$pkgver/LICENSE
)
sha256sums=('d5ce808f06f4776023202c22441d03d7e57f8980091d2107404438592f2b62f6'
            'c71d239df91726fc519c6eb72d318ec65820627232b2f796219e87dcf35d0ab4')

package() {
  mkdir -p "$pkgdir"/usr/share/fonts/noto
  install -m644 NotoColorEmoji.ttf "$pkgdir"/usr/share/fonts/noto/
  install -Dm644 LICENSE "$pkgdir"/usr/share/licenses/$pkgname/LICENSE
}

[kovidgoyal]

Well given that the distro packaged fonts include color bitmaps, whats the motivation for supporting rendering of SVG? It's much slower/wasteful of CPU cycles.

[unnamedname]

Well, I would have thought that with FreeType supporting COLRv1, Kitty could easily support it like COLRv0.
Since the harms outweigh the benefits, it doesn't count as an enhancement and duplicates #2691.

Thanks for your hard work!

[kovidgoyal]

You are welcome. If there is ever a reasonably widely used font that supports only COLRv1 without COLRv0 fallbacks we can reconsider, but as it stands I dont think the performance hit is worth it. Using pre-rendered bitmaps for emoji works fine.

[kovidgoyal]

And just for the record, it takes approx 1200 lines of C to render a COLRv1 glyph and thats using the facilities already provided by the cairo library. See for example https://fossies.org/linux/firefox/gfx/cairo/cairo/src/cairo-colr-glyph-render.c

Definitely not worth the effort.

[kovidgoyal]

I have enabled support for COLRv1 fonts by depending on the cairo library for rendering them (was about 300 lines of code in kitty). There is now a usecase for these as kitty supports multiple text sizes, see #8226. The support is in the cell branch will be merged to master eventually.