Pura-image: pure-Ruby image processing running on ruby.wasm (browser playground)

[komagata]

Hi ruby.wasm community,

Wanted to share a project that ended up being a fairly involved ruby.wasm use case: a pure-Ruby image processing library that runs unchanged in a wasm sandbox.

What it is

pura-image — pure-Ruby decoders/encoders for JPEG, PNG, BMP, GIF, TIFF, ICO, and WebP. Originally built so Rails Active Storage could work without libvips / ImageMagick (the typical install-pain killer for Rails beginners), but the "no C extensions" property turned out to mean it works in ruby.wasm too.

Browser playground

komagata.github.io/pura-image

The page boots ruby.wasm and runs ImageProcessing::Pura chains on a generated 400×300 image (or one you drop in). The exact code shown is what you'd put in a Rails Active Storage variant definition — same chain, executed in the browser tab.

What might be interesting for this community

• Real Ruby code path, not a demo with a tiny manual script. The chain ImageProcessing::Pura.source(...).resize_to_limit(...).convert(...).call runs identically in CRuby and in ruby.wasm. No conditional code paths.
• Bundled with rbwasm. The build scripts live under demo/build/ if you want to see how the gems get pulled into the wasm bundle.
• Image encoding/decoding in pure Ruby is non-trivial. Pixels-in-Ruby-arrays performance is rough but works; YJIT helps. The journey from "this won't work on wasm" to "shipping in production-ready Rails apps" is interesting if you're thinking about pure-Ruby alternatives to other C-extension gems.
• WebP works too (after a v0.2 fix), so you can try VP8 decoding in your browser via Ruby code, which still feels strange to type.

Honest notes

• JPEG / WebP encoding throughput is slower than libvips. Fine for thumbnail-class operations, not a fit for high-volume transcoding.
• The library is young; benchmark against your real workload before depending on it.
• The wasm bundle is not tiny (this is unavoidable for image processing). I haven't optimized hard yet.

Asking for

• Feedback on the demo experience
• Suggestions for ruby.wasm-specific optimizations I'm missing
• If anyone has integrated rbwasm-bundled gems into a larger wasm app, I'd love to hear what worked

If this fits a "Showcase" or example list anywhere in ruby.wasm docs, happy to contribute a PR pointing to it.

— @komagata