Glyph texture atlas for GPU text rendering #117

Phase 12: Metal GPU Compositor (4/7)#

Goal: Pack rasterized glyph bitmaps into a GPU texture atlas for efficient text rendering.

Depends on: Metal texture management and image upload

Requirements#

• Implement a texture atlas allocator:

  • Shelf-packing algorithm: maintain rows ("shelves") of glyphs sorted by height
  • Start with a reasonable atlas size (e.g., 1024×1024 R8 texture)
  • Grow or create additional atlas pages when full
  • Track allocated regions: (glyph_id, font_size) → AtlasRegion { x, y, width, height, atlas_page }

• Integrate with existing glyph cache in text crate:

  • When a glyph is first rasterized (CPU scanline rasterizer), upload its grayscale bitmap to the atlas
  • Return UV coordinates for the glyph's region in the atlas
  • Existing CPU glyph cache can remain for software fallback

• Provide an API for the Metal renderer:

  • Given a TextLine (list of positioned glyphs), produce a list of textured quads with atlas UV coordinates
  • Each quad: position from glyph metrics, UVs from atlas region, color from text color

• Handle atlas invalidation:

  • Clear atlas when font size distribution changes significantly (rare)
  • Track atlas utilization metrics

Acceptance Criteria#

• Glyph bitmaps are packed into a GPU texture atlas
• Atlas lookup returns correct UV coordinates for each glyph
• Text renders correctly using atlas quads (visual parity with software renderer)
• Atlas grows or pages as needed when many unique glyphs are used
• No external crate dependencies
• cargo clippy --workspace -- -D warnings passes
• cargo test --workspace passes