Dropzone.js + Client Side Image Resizing

Question

I want to integrate Dropzone.js with a Client Side Image Resizing. I know that there is a function to resize the thumbnail, but I would like to create a functio

Answer 1

The Dropzone documentation on the pre-upload resize feature is confusing. The way it reads, you can either limit the width, limit the height, or limit both and sacrifice Aspect Ratio, distorting the image. This is not the case. This:

resizeWidth: 1000, resizeHeight: 1000,
resizeMethod: 'contain', resizeQuality: 1.0,

Limits either width or height to a max of 1000px - whichever is larger. The other will get reduced in accord with the Aspect Ratio, without distorting the image. So for example, in my test I uploaded a 2688x1512 image. Dropzone cropped and resized the Thumbnail to its default 120x120, but the file sent to the server was resized separately by Dropzone, to 1000x562, then sent to the server.

There is an interesting caveat here. JPEGs are going to be recompressed, lossy, so even a resizeQuality of 1.0 is going to result in loss. I see this feature as a method of preventing insanely large files, but you should be careful of resizing twice if you can avoid it (once on server once on client).

If this isn't enough for you - and you really wanted to overload the transform method - it's worth noting that following the code path inside Dropzone is a little confusing, because the createThumbnail codepath is used twice every upload, once to create a thumbnail like you'd expect, and again to pre-resize the image here before passing it to the server. Likewise, the resize method is confusingly named; while resizeWidth etc refer to prepping the image for upload, resize refers to resizing for the thumbnail, and does nothing to the image sent to the server.

Answer 2

Here's how to do it without uploading file from addedfile.

Important thing is to set autoQueue option to false, that way dropzone will not auto upload files selected by a user.

var dropzone = new Dropzone (".dropzone", {
  ...
  autoQueue: false,
  ...
});

Next step is to do resizing and enqueue resized versions in addedfile event.

dropzone.on("addedfile", function(origFile) {
  var MAX_WIDTH  = 800;
  var MAX_HEIGHT = 600;

  var reader = new FileReader();

  // Convert file to img

  reader.addEventListener("load", function(event) {

    var origImg = new Image();
    origImg.src = event.target.result;

    origImg.addEventListener("load", function(event) {

      var width  = event.target.width;
      var height = event.target.height;


      // Don't resize if it's small enough

      if (width <= MAX_WIDTH && height <= MAX_HEIGHT) {
        dropzone.enqueueFile(origFile);
        return;
      }


      // Calc new dims otherwise

      if (width > height) {
        if (width > MAX_WIDTH) {
          height *= MAX_WIDTH / width;
          width = MAX_WIDTH;
        }
      } else {
        if (height > MAX_HEIGHT) {
          width *= MAX_HEIGHT / height;
          height = MAX_HEIGHT;
        }
      }


      // Resize

      var canvas = document.createElement('canvas');
      canvas.width = width;
      canvas.height = height;

      var ctx = canvas.getContext("2d");
      ctx.drawImage(origImg, 0, 0, width, height);

      var resizedFile = base64ToFile(canvas.toDataURL(), origFile);


      // Replace original with resized

      var origFileIndex = dropzone.files.indexOf(origFile);
      dropzone.files[origFileIndex] = resizedFile;


      // Enqueue added file manually making it available for
      // further processing by dropzone

      dropzone.enqueueFile(resizedFile);
    });
  });

  reader.readAsDataURL(origFile);
});

Here's a function for converting dataURL to a dropzone file. Process could be simpler if you use canvas.toBlob() instead of canvas.toDataURL(), to get the content of resized file, but latter isn't supported well by all browsers.

It is just modified version of this function.

function base64ToFile(dataURI, origFile) {
  var byteString, mimestring;

  if(dataURI.split(',')[0].indexOf('base64') !== -1 ) {
    byteString = atob(dataURI.split(',')[1]);
  } else {
    byteString = decodeURI(dataURI.split(',')[1]);
  }

  mimestring = dataURI.split(',')[0].split(':')[1].split(';')[0];

  var content = new Array();
  for (var i = 0; i < byteString.length; i++) {
    content[i] = byteString.charCodeAt(i);
  }

  var newFile = new File(
    [new Uint8Array(content)], origFile.name, {type: mimestring}
  );


  // Copy props set by the dropzone in the original file

  var origProps = [ 
    "upload", "status", "previewElement", "previewTemplate", "accepted" 
  ];

  $.each(origProps, function(i, p) {
    newFile[p] = origFile[p];
  });

  return newFile;
}

Answer 3

It can be done by bypassing the upload done by Dropzone.js. For that, you have to set autoQueue: false in the options (as in the Bootstrap example).

Then you can just send the thumbnail, or you can do it the hard way.

---

Solution 1: Send the thumbnail

myDropzone.on('thumbnail', function(file, dataURL) {
   $.post('http://example.com', {img: imgToSend})
})

You can define the way you will resize/crop by adding the resize function in the options.

---

Solution 2: Resize by yourself

There was an other issue: images can need to be rotated (it happens really often). For that, the EXIF orientation tag has to be used. It is apparently lost when you resize, so you have to fetch this information in the frontend, before you resize. For that, I used the Exif.js library.

Here is a working example on JSFiddle. You can see the resized image's data URI in your console.

Answer 4

Dropzone version 5 is recently released, so if you upgrade to dropzone 5, then you can simply use resizeWidth and resizeHeigh to compress your image on the client side.

If you provide only one of them, then dropzone will respect the original aspect ratio, for example if you just add the option: resizeWidth: 800 then your image will be compressed to width=800 pixels and your original image's aspect ration will be respected.

Answer 5

Ok. It took me some and then some more time to find how to implement the resize.

Here is final solution that works for me:

Import css and dropzone.js

<div class="row">
    <div class="col-sm-20 well">
        <form action="@Url.Action("_UploadImages", "Notebooks", new { unique = Model.UniqueId })" 
              method="post" 
              enctype="multipart/form-data"
              class="dropzone"
              id="my-dropzone"></form>
    </div>
</div>

<script type="text/javascript">
    Dropzone.options.myDropzone = {
        url: '@Url.Action("_UploadImages", "Notebooks", new { unique = Model.UniqueId })',
        autoProcessQueue: true,
        uploadMultiple: true,
        parallelUploads: 3,
        maxFilesize: 10,
        resizeWidth: 2048,
        addRemoveLinks: false,
        init: function () {
        }
    }
</script>