Variable Font Support

[tsl0922]

My project uses fogleman/gg and CGO freetype binding for font renderring, migrated it to gogpu/gg recently.

The missing parts for me is BDF font and variable font support, I've wrote a custom BDF parser and replaced the variable font to multiple ttf files for different metrics, both works well now.

It would be good for me if gogpu/gg can support variable font.

Thank you for this great library.

[kolkov]

Thanks for the feature request, @tsl0922 — and for migrating to gogpu/gg! Great to have the author of ttyd in our community.

We've done a deep research and the good news is: go-text/typesetting (our shaping backend) already has full variable font support — fvar parsing, Face.SetVariations(), gvar/avar/HVAR/MVAR interpolation, HarfBuzz shaping with variation coordinates. All working and tested.

The gap is that gg doesn't expose it through its text API yet. The implementation is a thin pass-through — approximately 50-100 lines across the text package.

Planned API (follows our existing WithFeatures() pattern):

source, _ := text.NewFontSourceFromFile("Inter-Variable.ttf")

regular := source.Face(16)  // default axes
bold := source.Face(16, text.WithVariations(
    text.NewFontVariation("wght", 700),
))
condensed := source.Face(16, text.WithVariations(
    text.NewFontVariation("wght", 700),
    text.NewFontVariation("wdth", 75),
))

// Query what axes a font supports:
if source.IsVariable() {
    for _, axis := range source.VariationAxes() {
        fmt.Printf("%s: %.0f–%.0f (default %.0f)\n",
            string(axis.Tag[:]), axis.Minimum, axis.Maximum, axis.Default)
    }
}

Before we start implementing — does this API cover your use case? A few questions:

1. Would WithVariations() at Face creation time work for you, or do you need to change axis values on an existing Face dynamically?
2. Do you use named instances from the font (e.g., "Bold", "Light Italic"), or do you specify axis values directly (e.g., wght=700)?
3. For your e-paper rendering — do you typically create many Faces with different weights from the same variable font, or just a few fixed ones?
4. Any thoughts on the API shape? We want to get this right for the community.

We'll keep this issue open to track the implementation.

[tsl0922]

Yes, it's for e-paper rendering. The use case involves a canvas renderer that processes predefined JSON templates containing a list of draw operations, such as text, lines, shapes, and more.

1. Yes, I'm keeping a cache for used faces without making any dynamic changes to existing ones.
2. Both named instances and axis values are used (via the previous CGO freetype binding).
3. Yes, since a JSON template may use a font at different sizes and styles.
4. Your planned APIs look good to me.

The main motivation for this migration is that gogpu/gg supports aliased mode for both geometry and text rendering, which is a hard requirement for e-paper displays. Another advantage is that I can finally get rid of the CGO freetype binding.

[kolkov]

Thanks for the detailed feedback, @tsl0922! This is exactly what we needed.

Based on your input we've promoted named instances from "nice-to-have" to Phase 1. The implementation will include both APIs:

// Direct axis values:
bold := source.Face(16, text.WithVariations(
    text.NewFontVariation("wght", 700),
))

// Named instances:
instances := source.NamedInstances()  // [{Name:"Bold", Variations:[{wght,700}]}, ...]
bold := source.FaceNamed("Bold", 16)  // convenience shortcut

Good to know about the aliased mode motivation — that's a unique strength of gg compared to other Go 2D libraries and we'll keep investing in it.

The underlying shaping engine (go-text/typesetting) already has full fvar/gvar support including named instance parsing, so the implementation is primarily API plumbing on the gg side. We'll update this issue when work begins.

[kolkov]

Released in v0.49.0: https://github.com/gogpu/gg/releases/tag/v0.49.0

go get github.com/gogpu/gg@v0.49.0

See examples/variable_font/ for a working example with weight axis + named instances. Let us know if it works for your use case!

[tsl0922]

Thanks! I've tested the example code, but it does not seem to work.

Result of examples/variable_font/main.go:

go run main.go
Font: Bahnschrift (variable: true)
  wght: 300 – 700 (default 400)
  wdth: 75 – 100 (default 100)
Saved: variable_font.pn

Result of examples/variable_font/main.go (with a custom variable font):

go run main.go
Font: Noto Sans SC Thin (variable: true)
  wght: 100 – 900 (default 100)
Saved: variable_font.png

I've tried to set the text shaper manually with text.SetShaper(text.NewGoTextShaper()), not working too.