Question

I want to merge 2 images using node.js. Or rather, i want to place one smaller image on cordinates x,y on a larger image.
Even more precise: I have an image of glasses, and an image of a face and i want to put the glasses on the face.
I did some googling, and found some image manipulating libraries, but none seem to be able to merge images.

Answer 1

I’ve used:

https://github.com/learnboost/node-canvas

to do something similar (build a composite image from components on the fly).

It works great.

Here’s some example code:

var Canvas = require('canvas'),
  fs = require('fs'),
  Image = Canvas.Image;



var _components = [{prefix:'f', count:12},
                   {prefix:'h', count:12},
                   {prefix:'i', count:12},
                   {prefix:'m', count:12}];


var _total = 1;
for (var i=_components.length - 1; i>=0; i--){
  _components[i].mult = _total;
  _total *= _components[i].count;
}


module.exports.ensureImageExists = function(img, cb){
  fs.stat(__dirname + '/../public/images/rb/' + img, function(err, stats){
    if (err){
      if (err.code == 'ENOENT')
        generateImage(img, cb);
      else
        cb(err);
    }
    else{
      cb();
    }
  });
}

function generateImage(name, cb){
  var re = /rb([0-9]*)\.png/

  var num = parseInt(re.exec(name)[1]) % _total;

  var parts = [];
  for (var i=0; i<_components.length; i++){
    var n = Math.floor(num / _components[i].mult);
    parts.push(_components[i].prefix + (n + 1));
    num -= n * _components[i].mult;
  }

  var canvas = new Canvas(45, 45),
   ctx = canvas.getContext('2d');

  drawParts();

  function drawParts(){
    var part = parts.shift();
    if (!part)
      saveCanvas();
    else {
      var img = new Image;
      img.onload = function(){
        ctx.drawImage(img, 0, 0, 45, 45);
        drawParts();
      };
     img.src = __dirname + '/components/' + part + '.png';
    }
  }

  function saveCanvas(){
    canvas.toBuffer(function(err, buf){
      if (err)
        cb(err);
      else
        fs.writeFile(__dirname + '/../public/images/rb/' + name, buf, function(){
          cb();
        });
    });
  }

}

In this case, the components are selected based upon the name of the image, but you clearly could do otherwise. Also, I imagine you could just stream the image out if you wanted — I write it to a file so it’s available the next time it’s requested.

I put a route like this in to handle the generation:

app.get('/images/rb/:img', requireLogin, function(req, res, next){
  //first make sure image exists, then pass along so it is handled
  //by the static router
  rbgen.ensureImageExists(req.params.img, function(err){
    next();
  })
});