pura-jpeg

A pure Ruby JPEG decoder/encoder without additional image-processing libraries.

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 image-specific native extension or FFI dependency
• 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

These historical measurements include ffmpeg process startup. They do not compare against an in-process C codec or establish Rails pipeline throughput.

400×400 image, Ruby 4.0.2 + YJIT.

• gem install and go — no brew install, no apt install, no C compiler needed
• Perfect for dev/CI — no ImageMagick or libvips setup. rails new → image upload → it just works
• Unix philosophy — one format, one gem, composable

Images contain 8-bit RGB pixels. Baseline JPEG only; progressive JPEG is unsupported. EXIF orientation and color profiles are not applied or retained. decode accepts a file path or a binary JPEG string.

crop(x, y, width, height) requires integer coordinates, positive dimensions, and a region entirely inside the image; invalid regions raise ArgumentError.

MIT