slint-ui/slint · error

more than 65535 glyphs are not supported

Error message

more than 65535 glyphs are not supported

What it means

This panic fires in the glyph-embedding pass (internal/compiler/passes/embed_glyphs.rs:400) while building the character map for embedded fonts. Glyph indices are stored as `u16` (matching font formats' 16-bit glyph ids), so if the accumulated `glyph_index` exceeds 65535 the `u16::try_from` fails and the `expect` panics. It means the font set being embedded produces more glyphs than the 16-bit glyph id space supports.

Solutions

1. Subset the font before embedding: use tools like pyftsubset/fonttools to reduce the glyph count below 65535 and limit to needed code points.
2. Use `EmbedResources::EmbedTextures` (SDF textures) or `DoNotEmbed` instead of glyph embedding for very large fonts.
3. Remove unnecessary fallback fonts from the font chain so the combined glyph count stays within 16 bits.
4. Split text across multiple smaller fonts if coverage requires more than 65535 glyphs.

Example fix

// before (shell)
pyftsubset big.ttf # keeps too many glyphs
// after
pyftsubset big.ttf --unicodes=U+0020-007E,U+4E00-9FFF --glyph-names --layout-features='*'

#!/bin/sh
# check glyph count of fonts before enabling embed_glyphs
python3 -c "from fontTools.ttLib import TTFont; import sys; f=TTFont(sys.argv[1]); n=f['maxp'].numGlyphs; sys.exit(1 if n>65535 else 0)" myfont.ttf || echo "subset the font first"

When it happens

Trigger: Compiling with `embed_glyphs` (font glyph embedding enabled in `CompilerConfiguration`) on a font (or font + fallback fonts chain) where the enumerated glyph index passed 65535 — very large fonts, many merged fallback fonts, or fonts with malformed/oversized glyph tables.

Common situations: Embedded firmware builds with big CJK/emoji fonts embedded as glyphs; accidentally including multiple huge fallback fonts; using a broken/subsetted font with an inflated glyph table.

Understand the failure class

Background: "value must be between 0 and 1" / "out of range" / "must not be negative" errors: fixing range-validation failures across open-source libraries — this error's family across 42 libraries.

Related errors

• large glyph x coordinate
• large glyph y coordinate
• large width
• EmbedTextures builds the shared font collection
• a setter belongs to a field

AI-assisted analysis of slint-ui/slint@bb937076de (2026-09-16). Data as JSON: /api/errors/eb9450e4562a744d. Report an issue: GitHub.

Appendix: source

Thrown at internal/compiler/passes/embed_glyphs.rs:400

    fallback_fonts: &[Font],
    _compiler_config: &CompilerConfiguration,
    normalized_coords: &[skrifa::instance::NormalizedCoord],
    _variations: &[(skrifa::Tag, f32)],
    override_weight: Option<u16>,
) -> BitmapFont {
    let coords_i16: Vec<i16> = normalized_coords.iter().map(|c| c.to_bits()).collect();

    let mut character_map: Vec<CharacterMapEntry> = character_coverage
        .filter(|code_point| {
            core::iter::once(&font)
                .chain(fallback_fonts.iter())
                .any(|font| swash_font_ref(font).charmap().map(*code_point) != 0)
        })
        .enumerate()
        .map(|(glyph_index, code_point)| CharacterMapEntry {
            code_point,
            glyph_index: u16::try_from(glyph_index)
                .expect("more than 65535 glyphs are not supported"),
        })
        .collect();

    #[cfg(feature = "sdf-fonts")]
    let glyphs = if _compiler_config.use_sdf_fonts {
        embed_sdf_glyphs(pixel_sizes, &character_map, &font, fallback_fonts, _variations)
    } else {
        embed_alpha_map_glyphs(pixel_sizes, &character_map, &font, fallback_fonts, &coords_i16)
    };
    #[cfg(not(feature = "sdf-fonts"))]
    let glyphs =
        embed_alpha_map_glyphs(pixel_sizes, &character_map, &font, fallback_fonts, &coords_i16);

    character_map.sort_by_key(|entry| entry.code_point);

    let font_ref = skrifa::FontRef::from_index(font.font.blob.data(), font.font.index).unwrap();
    let location = skrifa::instance::LocationRef::new(normalized_coords);
    let metrics =

View on GitHub (pinned to bb937076de)