Node gm composition

gm-composite.js

//To use this, in another file call
//require("./gm-composite.js")(gm.prototype);
//You can then do gm("baseImage.png").composite(gm("watermark.png"), "SouthEast", function(err, composite_gm_obj){...});

var gm = require('gm');

var randInt = function(min, max) {
    return Math.floor(min + (Math.random() * (max-min)));
};

var randString = function(len) {
    len = len || 16;

    var chars = "0123456789ABCDEFGHIJKLMNOPQRSTUVWXTZabcdefghiklmnopqrstuvwxyz";
    var ret = "";
    while(ret.length < len) {
        ret += chars[randInt(0, chars.length)];
    }
    return ret;
};

module.exports = function(proto) {
    proto.compose = function(other, gravity, callback) {
        // We have potentially two streams to deal with, so we first write the watermark to a file,
        // then pull the base image from stdin and write to stdout
        // gm composite -resize 400x400 -gravity center Watermark.png test_image.png test_output.png 
        var path = "/tmp/watermark_"+randString();
        var prev = {
            _subCommand: this._subCommand,
            _out: this._out.slice(), //copy
            _in: this._in.slice()
        };
        var curr = this;
        other.write(path, function(err) {
            curr.subCommand("composite");
            curr.gravity(gravity);
            curr.in(path);

            var composite = gm(curr.stream());

            //re-applying other properties
            composite._subCommand = prev._subCommand;
            composite._out = prev._out;
            composite._in = prev._in;
            callback(null, composite);
        });
        return this;
    };
};

You should use a miff file for temporary output. It's fast and lossless