100% Client Side Image Resizing

... I know, I have said "Happy Holidays" already, but yesterday, after a (annoying) picture upload in Facebook, I had a an idea ... why on earth I should have a Java plugin to perform images resizes on Facebook? Why on earth if I don't have such plugin I have to wait the possibly extremely long upload, up to 10x slower for high quality images, stressing Facebook servers for such "simple" operation as an image resize/resample could be?

The FileReader Interface

In the W3C File API, I guess part of the HTML5 buzzword, we can find all we need to perform the operation we want totally on client side. The interface is called FileReader, and it provides functionalities to read chosen files from an input node with type file, and that's it: we can even disconnect from the network and keep resizing and saving images without problems.

The Canvas Trick

Still into HTML5 buzzword world, the canvas element and it's 2dContext.drawImage method is the key to perform a resample/resize operation. It's not about changing a DOM Image node size and show it, it's about creating a totally fresh new image with exactly desired pixels size.
Once this is done, it is possible to send via Ajax the bas64 encoded image or it is possible to simply save the created image or reuse it, or resize it again ...

The Demo Page

This is the demo page I gonna show you soon, the code is hopefully self explanatory:

<!doctype html>
<html>
 <head>
  <title>JavaScript Image Resample :: WebReflection</title>
 </head>
 <body>
  <input id="width" type="text" value="320" />
  <input id="height" type="text" />
  <input id="file" type="file" />
  <br /><span id="message"></span><br />
  <div id="img"></div>
 </body>
 <script src="resample.js"></script>
 <script>
 (function (global, $width, $height, $file, $message, $img) {
  
  // (C) WebReflection Mit Style License
  
  // simple FileReader detection
  if (!global.FileReader)
   // no way to do what we are trying to do ...
   return $message.innerHTML = "FileReader API not supported"
  ;
  
  // async callback, received the
  // base 64 encoded resampled image
  function resampled(data) {
   $message.innerHTML = "done";
   ($img.lastChild || $img.appendChild(new Image)
   ).src = data;
  }
  
  // async callback, fired when the image
  // file has been loaded
  function load(e) {
   $message.innerHTML = "resampling ...";
   // see resample.js
   Resample(
     this.result,
     this._width || null,
     this._height || null,
     resampled
   );
   
  }
  
  // async callback, fired if the operation
  // is aborted ( for whatever reason )
  function abort(e) {
   $message.innerHTML = "operation aborted";
  }
  
  // async callback, fired
  // if an error occur (i.e. security)
  function error(e) {
   $message.innerHTML = "Error: " + (this.result || e);
  }
  
  // listener for the input@file onchange
  $file.addEventListener("change", function change() {
   var
    // retrieve the width in pixel
    width = parseInt($width.value, 10),
    // retrieve the height in pixels
    height = parseInt($height.value, 10),
    // temporary variable, different purposes
    file
   ;
   // no width and height specified
   // or both are NaN
   if (!width && !height) {
    // reset the input simply swapping it
    $file.parentNode.replaceChild(
     file = $file.cloneNode(false),
     $file
    );
    // remove the listener to avoid leaks, if any
    $file.removeEventListener("change", change, false);
    // reassign the $file DOM pointer
    // with the new input text and
    // add the change listener
    ($file = file).addEventListener("change", change, false);
    // notify user there was something wrong
    $message.innerHTML = "please specify width or height";
   } else if(
    // there is a files property
    // and this has a length greater than 0
    ($file.files || []).length &&
    // the first file in this list 
    // has an image type, hopefully
    // compatible with canvas and drawImage
    // not strictly filtered in this example
    /^image\//.test((file = $file.files[0]).type)
   ) {
    // reading action notification
    $message.innerHTML = "reading ...";
    // create a new object
    file = new FileReader;
    // assign directly events
    // as example, Chrome does not
    // inherit EventTarget yet
    // so addEventListener won't
    // work as expected
    file.onload = load;
    file.onabort = abort;
    file.onerror = error;
    // cheap and easy place to store
    // desired width and/or height
    file._width = width;
    file._height = height;
    // time to read as base 64 encoded
    // data te selected image
    file.readAsDataURL($file.files[0]);
    // it will notify onload when finished
    // An onprogress listener could be added
    // as well, not in this demo tho (I am lazy)
   } else if (file) {
    // if file variable has been created
    // during precedent checks, there is a file
    // but the type is not the expected one
    // wrong file type notification
    $message.innerHTML = "please chose an image";
   } else {
    // no file selected ... or no files at all
    // there is really nothing to do here ...
    $message.innerHTML = "nothing to do";
   }
  }, false);
 }(
  // the global object
  this,
  // all required fields ...
  document.getElementById("width"),
  document.getElementById("height"),
  document.getElementById("file"),
  document.getElementById("message"),
  document.getElementById("img")
 ));
 </script>
</html>


The resample.js File



var Resample = (function (canvas) {

 // (C) WebReflection Mit Style License

 // Resample function, accepts an image
 // as url, base64 string, or Image/HTMLImgElement
 // optional width or height, and a callback
 // to invoke on operation complete
 function Resample(img, width, height, onresample) {
  var
   // check the image type
   load = typeof img == "string",
   // Image pointer
   i = load || img
  ;
  // if string, a new Image is needed
  if (load) {
   i = new Image;
   // with propers callbacks
   i.onload = onload;
   i.onerror = onerror;
  }
  // easy/cheap way to store info
  i._onresample = onresample;
  i._width = width;
  i._height = height;
  // if string, we trust the onload event
  // otherwise we call onload directly
  // with the image as callback context
  load ? (i.src = img) : onload.call(img);
 }
 
 // just in case something goes wrong
 function onerror() {
  throw ("not found: " + this.src);
 }
 
 // called when the Image is ready
 function onload() {
  var
   // minifier friendly
   img = this,
   // the desired width, if any
   width = img._width,
   // the desired height, if any
   height = img._height,
   // the callback
   onresample = img._onresample
  ;
  // if width and height are both specified
  // the resample uses these pixels
  // if width is specified but not the height
  // the resample respects proportions
  // accordingly with orginal size
  // same is if there is a height, but no width
  width == null && (width = round(img.width * height / img.height));
  height == null && (height = round(img.height * width / img.width));
  // remove (hopefully) stored info
  delete img._onresample;
  delete img._width;
  delete img._height;
  // when we reassign a canvas size
  // this clears automatically
  // the size should be exactly the same
  // of the final image
  // so that toDataURL ctx method
  // will return the whole canvas as png
  // without empty spaces or lines
  canvas.width = width;
  canvas.height = height;
  // drawImage has different overloads
  // in this case we need the following one ...
  context.drawImage(
   // original image
   img,
   // starting x point
   0,
   // starting y point
   0,
   // image width
   img.width,
   // image height
   img.height,
   // destination x point
   0,
   // destination y point
   0,
   // destination width
   width,
   // destination height
   height
  );
  // retrieve the canvas content as
  // base4 encoded PNG image
  // and pass the result to the callback
  onresample(canvas.toDataURL("image/png"));
 }
 
 var
  // point one, use every time ...
  context = canvas.getContext("2d"),
  // local scope shortcut
  round = Math.round
 ;
 
 return Resample;
 
}(
 // lucky us we don't even need to append
 // and render anything on the screen
 // let's keep this DOM node in RAM
 // for all resizes we want
 this.document.createElement("canvas"))
);


The Resample Demo In Action

First input for the width, second input for the height, if one out of 2 is defined, the resize maintain the aspect ratio.
You can even disconnect your machine from the network, since nothing is absolutely stored or saved in my website, everything simply runs in your machine.
Compatibility? Minefield and latest Chrome work pretty well. I don't have my MacMini with me right now but I will test eventually WebKit nightly later.
Happy end of 2010

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