how to convert image into base64 string?

hello I am trying to make simple application in ionic using camera or select file from gallery or file system and send to / or upload to a server. I found one plugin which capture one image and send base64 string here is plugin http://ngcordova.com/docs/plugins/camera/ using this I am able to get base64 string 

$cordovaCamera.getPicture(options).then(function(imageData) {
      var image = document.getElementById('myImage');
      image.src = "data:image/jpeg;base64," + imageData;
    }, function(err) {
      // error
    });

that base64 string I used to send to server

But my problem is that when I use this plugin image gallary plugin

http://ngcordova.com/docs/plugins/imagePicker/ it send me only image url (where is in memory) .can we get base64 string after selecting the image from image picker.

$cordovaImagePicker.getPictures(options)
    .then(function (results) {
      for (var i = 0; i < results.length; i++) {
        console.log('Image URI: ' + results[i]);
      }
    }, function(error) {
      // error getting photos
    });

In other words when I using camera I am getting base64 string as shown above.But when I use image picker plugin I am getting image url.can I get base64 string .so that I am able to send or upload on server . how to convert image url to base64 string ?

To convert image to base64 you can use HTML5 canvans as mention in

How to convert image into base64 string using javascript

Please refer above question

Use this code

/**
 * Convert an image 
 * to a base64 url
 * @param  {String}   url         
 * @param  {Function} callback    
 * @param  {String}   [outputFormat=image/png]           
 */
function convertImgToBase64URL(url, callback, outputFormat){
    var img = new Image();
    img.crossOrigin = 'Anonymous';
    img.onload = function(){
        var canvas = document.createElement('CANVAS'),
        ctx = canvas.getContext('2d'), dataURL;
        canvas.height = this.height;
        canvas.width = this.width;
        ctx.drawImage(this, 0, 0);
        dataURL = canvas.toDataURL(outputFormat);
        callback(dataURL);
        canvas = null; 
    };
    img.src = url;
}