Basic text shaping: kern table and horizontal advance #26

Phase 2: Pure Rust Font Engine#

Implement basic text shaping: position glyphs horizontally using advance widths (already available from hmtx) and kern pair adjustments.

kern table parsing#

• Parse the kern table (both format 0 and format 3 subtables)
• Format 0: array of kern pairs sorted by left/right glyph ID pair, binary-searchable
• Extract kerning values (FUnits) for glyph pairs
• Support both horizontal and cross-stream kerning
• Handle multiple subtables (coverage field)

Text shaping pipeline#

• Input: string of Unicode code points + font + size
• Map each code point to a glyph ID via cmap
• Look up advance width for each glyph from hmtx
• Apply kern pair adjustments between consecutive glyphs
• Output: Vec<ShapedGlyph> with glyph IDs and x/y positions

Data structures#

pub struct ShapedGlyph {
    pub glyph_id: u16,
    pub x_offset: f32,  // horizontal position in pixels
    pub y_offset: f32,  // vertical offset (usually 0 for basic shaping)
    pub x_advance: f32, // horizontal advance in pixels
}

Public API#

• Font::shape_text(text: &str, size_px: f32) -> Vec<ShapedGlyph>
• Font::kern_pair(left: u16, right: u16) -> i16 (raw kern value in font units)

Acceptance criteria#

• kern table is parsed for fonts that contain it
• Glyph advances are correctly read from hmtx
• Kern pair adjustments are applied between glyph pairs
• shape_text produces correctly positioned glyph runs
• Fonts without kern tables work (zero kerning)
• All existing tests continue to pass
• cargo clippy --workspace -- -D warnings passes
• cargo fmt --all --check passes