Module glyph_cache 

Glyph vertex cache for the GL rendering path.

Problem

fill_text called shape_and_flatten_text_via_agg every frame, running rustybuzz shaping + AGG ConvCurve Bézier flattening + tess2 tessellation for every visible text string. On a frame with ~20 text strings this dominated render time (~60 ms/frame).

Solution

GlyphCache tessellates each (font, glyph_id, size) triple once and stores the resulting triangle mesh in glyph-local pixel coordinates (origin 0, 0; scaled by size / units_per_em). On subsequent frames the caller offsets those vertices by the glyph’s screen position and uploads directly to the GPU — no Bézier evaluation or tessellation.

Key design choices

• One entry per (font_ptr, glyph_id, size_bits) — different sizes produce different tessellations; identical sizes share a single entry.
• Glyph-local coordinates — the CTM is applied at draw time (transform_pt(pen_x + vx, y + vy)), which is correct for any affine transform including rotation.
• None entries are cached too — so glyphs without outlines (space, tab) never re-enter the shaper.
• The cache is never cleared between frames (reset() must NOT call glyph_cache.clear()); it grows until the widget tree changes fonts or sizes, then entries for the old parameters simply become dead weight (acceptable for typical UI workloads).

Structs

CachedGlyph

Pre-tessellated triangle mesh for one glyph at a specific pixel size.

GlyphCache

Per-frame glyph vertex cache shared by one [GlGfxCtx] instance.