jpg-stream

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

This fork:

• makes the build repeatable using docker, nix, and emscripten v1
• exposes the libjpeg optimize_coding option on JPEGEncoder as optimizeCoding
• is compiled with emscripten's ALLOW_MEMORY_GROWTH=1

Original embind work done by @devongovett in jpg-stream

Installation

npm install @stealthybox/jpg-stream

Building

Compiling this package currently depends on Emscripten v1. This fork publishes a public docker image that can be used like so:

make docker-compile

You can also rebuild the image for yourself:

ORG=$USER make docker-image
ORG=$USER make docker-push
ORG=$USER make docker-compile

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 optimizeCoding option defaults to false and will use the JPEG standard huffman tables to produce output data as input pixels are processed.

You can set the optimizeCoding option to true to tell the encoder optimize the huffman tables used to code/compress the image data. This requires a second pass over all of 8x8 MCU's (after DCT and quantization has been performed). This can result in significantly smaller jpeg output, but at the cost of blocking the output stream until the input has been fully processed. Memory usage with optimizeCoding enabled is much higher. This fork has been compiled with ALLOW_MEMORY_GROWTH=1 to support the internal buffer needed for libjpeg to compute the tables.

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 PNGDecoder = require('png-stream/decoder');
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'));

// produce a smaller, optimized JPEG
fs.createReadStream('in.png')
  .pipe(new PNGDecoder)
  .pipe(new JPEGEncoder({ quality: 80, optimizeCoding: true }))
  .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