Introduce the concept of a shared glyph cache

[codymullins]

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• I have provided test coverage for my change (where applicable)

Description

tl;dr: Adds an opt-in per-canvas glyph-outline cache (Configuration.SetSharedGlyphCache(true)) that reuses glyph outlines across DrawText calls on a canvas, making text-heavy rendering up to ~3× faster and cutting allocations ~2.6× (404 MB → 155 MB at 1000 text runs).

Draft pull request for a possible shared glyph cache. Shows promising performance benchmarks in Starling, especially when rendering an entire screen of text.

Hoping to see if you had appetite for merging in something like this -- it would be great to have. Or if there's anything I'm missing please LMK!

Notes:

• I tried to find the best way to make it "backwards compatible" since it did seem to slip tolerance a bit for one of the cache tests.
• The "configuration" route matched an existing pattern and seemed to cause the least downstream effects and not make any breaking changes.

Early benchmarks here, the full suite is still running:

EDIT - benchmarks from full suite finished:

  ┌──────────┬─────────────────────────┬───────────────────┬───────┬──────────────────────────────┐
  │ RunCount │   Default (per-call)    │ Shared per-canvas │ Ratio │ Allocated (default → shared) │
  ├──────────┼─────────────────────────┼───────────────────┼───────┼──────────────────────────────┤
  │ 50       │ 6.56 ms                 │ 3.25 ms           │ 0.50  │ 20 MB → 10 MB                │
  ├──────────┼─────────────────────────┼───────────────────┼───────┼──────────────────────────────┤
  │ 250      │ 29.74 ms                │ 12.95 ms          │ 0.44  │ 103 MB → 40 MB               │
  ├──────────┼─────────────────────────┼───────────────────┼───────┼──────────────────────────────┤
  │ 1000     │ 139.4 ms (median 117.5) │ 45.04 ms          │ 0.34  │ 404 MB → 155 MB              │
  └──────────┴─────────────────────────┴───────────────────┴───────┴──────────────────────────────┘

[CLAassistant]

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[JimBobSquarePants]

This looks fantastic. I wish I had thought of it!

I think we can drop the configuration and use truthy default behaviour. The current differences are imperceptible to the human eye.

For the updated reference image just reuse your new output. I use pinga with lossless optimization to optimize them for storage. https://css-ig.net/pinga

[Copilot]

Pull request overview

Note

Copilot was unable to run its full agentic suite in this review.

Introduces an opt-in per-canvas shared glyph-outline cache for DrawText calls, allowing glyph outlines built once to be reused across multiple text draws on the same canvas.

Changes:

• Adds a new GlyphCacheDefaultsExtensions API to enable/disable the shared glyph cache via Configuration.
• Threads an optional shared Dictionary<CacheKey, List<GlyphRenderData>> from DrawingCanvas<TPixel> into RichTextGlyphRenderer, with ownership tracking so injected caches aren't cleared on dispose.
• Adds a benchmark comparing the per-call cache against the per-canvas shared cache.

Reviewed changes

Copilot reviewed 4 out of 4 changed files in this pull request and generated 4 comments.

File	Description
src/ImageSharp.Drawing/Processing/GlyphCacheDefaultsExtensions.cs	New public extensions to set/get a shared-glyph-cache flag on Configuration.
src/ImageSharp.Drawing/Processing/RichTextGlyphRenderer.cs	Accepts an optional shared cache; conditionally clears it on dispose; promotes CacheKey/GlyphRenderData visibility to internal.
src/ImageSharp.Drawing/Processing/DrawingCanvas{TPixel}.cs	Lazily creates and passes a per-canvas shared cache to each renderer instance when enabled.
tests/ImageSharp.Drawing.Benchmarks/Drawing/DrawTextSharedGlyphCache.cs	New benchmark exercising shared vs. per-call glyph cache across many runs.

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[codymullins]

Thanks @JimBobSquarePants -- I'll work on updating. Just to be clear, use truthy default behaviour - you mean just use the shared glyph cache by canvas by default?