Glyph rasterizer: scanline fill to grayscale bitmaps #25

Phase 2: Pure Rust Font Engine#

Implement a scanline rasterizer that converts glyph outlines (from the glyf table) into grayscale bitmaps suitable for rendering.

Depends on: Parse glyf and OS/2 tables (glyph outline extraction)

Outline processing#

• Convert TrueType quadratic bezier curves to monotonic segments
• Flatten bezier curves into line segments at a given resolution (adaptive or fixed subdivision)
• Scale outline coordinates from font units (units per em from head table) to target pixel size

Scanline rasterizer#

• Use a coverage-based approach: for each pixel row, compute horizontal intersections with the outline
• Apply the even-odd or non-zero winding rule to determine inside/outside
• Produce anti-aliased output: compute sub-pixel coverage as a grayscale value (0-255)
• Handle overlapping contours correctly

Output format#

• GlyphBitmap { width: u32, height: u32, bearing_x: i32, bearing_y: i32, data: Vec<u8> }
• Each byte is a grayscale alpha value (0 = transparent, 255 = fully opaque)
• bearing_x/bearing_y specify the offset from the glyph origin to the bitmap top-left

Public API#

• Font::rasterize_glyph(glyph_id: u16, size_px: f32) -> Option<GlyphBitmap>
• Scale factor: size_px / units_per_em

Acceptance criteria#

• Rasterize basic Latin glyphs (A-Z, a-z, 0-9) at multiple sizes
• Anti-aliased output (not just binary black/white)
• Correct bearing/positioning values
• Compound glyphs rasterize correctly
• All existing tests continue to pass
• cargo clippy --workspace -- -D warnings passes
• cargo fmt --all --check passes