crop.js

var spawn = require('child_process').spawn;
var Stream = require('stream');

/**
 * crops and resizes images to our desired size
 * @param {Stream} streamIn in stream containing the raw image
 * @return {Stream}
 */
exports.cropImage = function(streamIn){

	var command = 'convert';

	// http://www.imagemagick.org/Usage/resize/#space_fill
	var args = [
		"-",                     // use stdin
		"-resize", "640x",       // resize width to 640
		"-resize", "x360<",      // resize height if it's smaller than 360
		"-gravity", "center",    // sets the offset to the center
		"-crop", "640x360+0+0",  // crop
		"+repage",               // reset the virtual canvas meta-data from the images.
		"-"                      // output to stdout
	];

	var proc = spawn(command, args);

	var stream = new Stream();

	proc.stderr.on('data', stream.emit.bind(stream, 'error'));
	proc.stdout.on('data', stream.emit.bind(stream, 'data'));
	proc.stdout.on('end', stream.emit.bind(stream, 'end'));
	proc.on('error', stream.emit.bind(stream, 'error'));

	streamIn.pipe(proc.stdin);

	return stream;
};

// usage
var fs = require('fs');
var streamIn = fs.createReadStream('./image.jpg');
var streamOut = fs.createWriteStream('./resized.jpg');
exports.cropImage(streamIn).pipe(streamOut);