mcanthony/jpg-stream

A streaming JPEG encoder and decoder

jpg-stream

A streaming JPEG encoder and decoder for Node and the browser. It is a direct compilation of libjpeg to JavaScript using Emscripten.

Installation

npm install jpg-stream

For the browser, you can build using Browserify.

Decoding

This example uses the concat-frames module to collect the output of the JPEG decoder into a single buffer. It also shows how to get EXIF metadata contained in the JPEG file.

var JPEGDecoder = require('jpg-stream/decoder');
var concat = require('concat-frames');

// decode a JPEG file to RGB pixels
fs.createReadStream('in.jpg')
  .pipe(new JPEGDecoder)
  .on('meta', function(meta) {
    // meta contains an exif object as decoded by
    // https://github.com/devongovett/exif-reader
  })
  .pipe(concat(function(frames) {
    // frames is an array of frame objects (one for JPEGs)
    // each element has a `pixels` property containing
    // the raw RGB pixel data for that frame, as
    // well as the width, height, etc.
  }));

Scaling

Large JPEGs from DSLRs can be somewhat slow to decode. If you don't need the image at its full size for preview, or will be resizing the image anyway, there is an option to perform scaling at decode time. This improves performance dramatically since only the DCT coefficients necessary for the desired size are decoded.

To specify decode scaling, provide width and height options to the decoder. This represents the minimum size you want, and the decoder will output an image of at least this size, but likely not exactly that size. For exact resizing, provide your minimum allowed size to the decoder and use the resize-pixels module to resize the JPEG decoder's output to the exact size.

fs.createReadStream('large.jpg')
  .pipe(new JPEGDecoder({ width: 600, height: 400 }))
  .pipe(concat(function(frames) {
    // frames[0].width >= 600 and frames[0].height >= 400
  }));

Encoding

You can encode a JPEG by writing or piping pixel data to a JPEGEncoder stream. You can set the quality option to a number between 1 and 100 to control the size vs quality tradeoff made by the encoder.

The JPEG encoder supports writing data in the RGB, grayscale, or CMYK color spaces. If you need to convert from another unsupported color space, first pipe your data through the color-transform module.

var JPEGEncoder = require('jpg-stream/encoder');
var ColorTransform = require('color-transform');

// convert a PNG to a JPEG
fs.createReadStream('in.png')
  .pipe(new PNGDecoder)
  .pipe(new JPEGEncoder({ quality: 80 }))
  .pipe(fs.createWriteStream('out.jpg'));
  
// colorspace conversion to convert from RGBA to RGB
fs.createReadStream('rgba.png')
  .pipe(new PNGDecoder)
  .pipe(new ColorTransform('rgb'))
  .pipe(new JPEGEncoder)
  .pipe(fs.createWriteStream('rgb.jpg'));

License

MIT