pura-jpeg

A pure Ruby JPEG decoder/encoder with zero C extension dependencies.

Part of the pura-* series — pure Ruby image codec gems.

• Baseline JPEG decoding and encoding (SOF0)
• Image resizing (bilinear / nearest-neighbor interpolation)
• Huffman coding, fast integer IDCT/FDCT, YCbCr ↔ RGB conversion
• 4:2:0, 4:2:2, and 4:4:4 chroma subsampling
• No native extensions, no FFI, no external dependencies
• CLI tool included

gem install pura-jpeg

require "pura-jpeg"

# Decode
image = Pura::Jpeg.decode("photo.jpg")
image.width      #=> 1920
image.height     #=> 1080
image.pixels     #=> Raw RGB byte string

# Resize
thumb = image.resize(200, 200)
fitted = image.resize_fit(800, 600)   # maintain aspect ratio

# Encode
Pura::Jpeg.encode(thumb, "thumb.jpg", quality: 80)

pura-jpeg decode input.jpg --info
pura-jpeg resize input.jpg --width 200 --height 200 --out thumb.jpg
pura-jpeg resize input.jpg --fit 800x600 --out fitted.jpg

400×400 image, Ruby 4.0.2 + YJIT.

pura-jpeg is 2× faster than ptjd (Tcl) and within 2× of jpeg-js running without JIT. These are the only three pure scripting-language JPEG implementations that exist — Python, Perl, PHP, and Lua all rely on C extensions.

• gem install and go — no brew install, no apt install, no C compiler needed
• Works everywhere Ruby works — CRuby, ruby.wasm, mruby, JRuby, TruffleRuby
• Edge/Wasm ready — runs in Cloudflare Workers, browsers (via ruby.wasm), sandboxed environments where you can't install system libraries
• Perfect for dev/CI — no ImageMagick or libvips setup. rails new → image upload → it just works
• Unix philosophy — one format, one gem, composable

MIT