Glyph cache: HashMap of rasterized bitmaps #27

Phase 2: Pure Rust Font Engine#

Implement a glyph cache that stores rasterized glyph bitmaps to avoid re-rasterizing the same glyphs repeatedly.

Depends on: Glyph rasterizer

Cache design#

• Key: (glyph_id: u16, size_px: u16) — size quantized to integer pixels to bound cache size
• Value: GlyphBitmap (the rasterized bitmap from the rasterizer)
• Storage: HashMap<(u16, u16), GlyphBitmap>
• Keep it simple — no eviction policy for now (Phase 12 will add a texture atlas with LRU)

Integration with Font#

• Font::get_glyph_bitmap(glyph_id: u16, size_px: f32) -> Option<&GlyphBitmap>
  • Check cache first
  • On miss: rasterize, insert into cache, return reference
• Cache is per-font (stored in the Font struct, behind RefCell or similar for interior mutability)

Text rendering helper#

• Font::render_text(text: &str, size_px: f32) -> Vec<PositionedGlyph>
  • Combines shaping (from kern/shaping issue) with cached bitmap lookup
  • Each PositionedGlyph has: x/y position, reference to bitmap data
  • This is the main entry point for the rendering pipeline

Acceptance criteria#

• Cache hit avoids re-rasterization (test by checking bitmap pointer equality or call count)
• Multiple sizes of the same glyph are cached independently
• Cache integrates cleanly with Font struct
• render_text produces a complete positioned+rasterized glyph run
• All existing tests continue to pass
• cargo clippy --workspace -- -D warnings passes
• cargo fmt --all --check passes