app.js

var http = require('http'),
    url = require('url'),
    child_process = require('child_process'),
    cluster = require('cluster'),
    fs = require('fs'),
    numCPUs = require('os').cpus().length;

var config = {
    fontPath: __dirname + '/aller-light.ttf'
};

function createImage(_config, callback) {
    var args = [];
    args.push('-background');
    args.push('"' + _config.background + '"');
    args.push('-fill');
    args.push('"' + _config.fill + '"');
    args.push('-font');
    args.push(config.fontPath);
    args.push('-size');
    args.push(_config.width + 'x' + _config.height);
    args.push('-pointsize');
    args.push('22');
    args.push('-gravity');
    args.push('center');
    args.push('label:' + _config.width + 'x' + _config.height);
    args.push('png:-'); //use stdout instead of a temp file
    console.log(args.join(' '));
    //noinspection JSCheckFunctionSignatures
    child_process.exec('convert ' + args.join(' '), {encoding:'binary'}, callback);
}

function parseParam(_url) {
    var arr = url.parse(_url).pathname.replace(/^\//, '').split('/'),
        ret;
    if (arr.length > 1) {
        ret = {
            width: parseInt(arr[0], 10),
            height: parseInt(arr[1], 10),
            fill: '#' + (color3to6(arr[2]) || 'dddddd'),
            background: '#' + (color3to6(arr[3]) || 'efefef')
        };
        ret.width = ret.width > 1000 ? 1000 : ret.width;
        ret.height = ret.height > 1000 ? 1000 : ret.height;
    }
    return ret;
}

function color3to6(color) {
    if (!color) {
        return '';
    }
    if (color.length == 6) {
        return color;
    }
    var str = '';
    [].forEach.call(color, function (s) {
        str += s;
        str += s;
    });
    return str;
}

//noinspection JSUnresolvedFunction
function spawnWebService(){
    return http.createServer(function (request, response) {
        var params = parseParam(request.url);
        if (!params || !params.width || !params.height) {
            //noinspection JSUnresolvedFunction
            response.writeHead(200, {
                'Content-Type': 'text/plain'
            });
            //noinspection JSValidateTypes
            response.end('Field Error');
            return;
        }
        createImage(params, function (error, imgStream) {
            if(error === null) {
                //noinspection JSUnresolvedFunction
                response.writeHead(200, {
                    'Content-Type': 'image/png',
                    'Content-Length': imgStream.length,
                    'cache-control': 'max-age=' + 1000 * 3600 * 24 * 30,
                    'Expires': new Date(Date.now() + 1000 * 3600 * 24 * 30)
                });

                //noinspection JSCheckFunctionSignatures
                response.end(imgStream, 'binary');
            }else{
                response.end('Error. internal error.');
                console.log(error);
            }
        });
    });
}

var port = 9527;
//Fork workers for multiple core CPU
if (cluster.isMaster) {
    // Fork workers.
    for (var i = 0; i < numCPUs; i++) {
        cluster.fork();
    }
    cluster.on('death', function(worker) {
        console.log('worker ' + worker.pid + ' died');
        //if died spwan again
        cluster.fork();
    });
    console.log( 'Master process running at http://127.0.0.1:'+port+' with pid ' + process.pid );
} else {
    //do work!
    spawnWebService().listen(port);
}