How do I apply an ImageData pixel transformation in Darkroom.JS?

Question

I am attempting to write a Darkroom.JS plugin that will transform white space in images to transparency.

I have used this answer (solely canvas based) to write this code:

(function() {
'use strict';

var Transparency = Darkroom.Transformation.extend({
  applyTransformation: function(canvas, image, next) {
    console.log(canvas);
    console.log(image);
    var context = canvas.getContext("2d");
    var upperContext = $('.upper-canvas').get(0).getContext("2d");

    var imageData = context.getImageData(0, 0, image.getWidth(), image.getHeight());

    //var upperImageData = upperContext.createImageData(image.getWidth(), image.getHeight());

    console.log("apply transformation called");

    for(var i = 0, n = imageData.data.length; i < n; i +=4){
      var r = imageData.data[i],
              g = imageData.data[i+1],
              b = imageData.data[i+2];

      if(r >= 230 && g >= 230 && b >= 230){
        imageData.data[i] = 0;
        imageData.data[i+1] = 0;
        imageData.data[i+2] = 0;
        imageData.data[i+3] = 1;
      }
    };

    context.putImageData(imageData, 0, 0);
    upperContext.putImageData(imageData, 0, 0);


    //canvas.renderAll();

    next();
  }
});

Darkroom.plugins['transparency'] = Darkroom.Plugin.extend({

  defaults: {
    clearWhiteSpace: function() {
      this.darkroom.applyTransformation(
        new Transparency()
      );
    }
  },

  initialize: function InitializeDarkroomTransparencyPlugin() {
    var buttonGroup = this.darkroom.toolbar.createButtonGroup();

    this.destroyButton = buttonGroup.createButton({
      image: 'wand' //Magic Wand by John O'Shea from the Noun Project
    });

    this.destroyButton.addEventListener('click', this.options.clearWhiteSpace.bind(this));
  },
});

})();

(I should also note I based the structure of the plugin off of the existing rotate plugin )

The code does get called, and I do not currently have it in the code (for performance reasons) but a log statement indicated that the if block where the pixel editing is done also gets called.

To verify, I presently have the pixels set to fully opacity and black (instead of transparent so that I can see the effects of editing).

Also, I noticed that Darkroom.JS seems to generate two canvas objects, an upper canvas and lower canvas . The object passed to the transform function is the "lower canvas" object, so I even tried using jQuery to grab the "upper" one and set the image data on that, to no avail.

What am I missing?

Answer 1

I was focusing my search for an answer far too much on Darkroom.JS. Darkroom.JS is just a layer on top of Fabric.JS, and this answer holds the key: fabric js or imagick remove white from image

I actually used the second answer and it works perfectly:

So there is a filter in Fabric.js that does just that.

http://fabricjs.com/docs/fabric.Image.filters.RemoveWhite.html

var filter = new fabric.Image.filters.RemoveWhite({ threshold: 40,
distance: 140 }); image.filters.push(filter); image.applyFilters(canvas.renderAll.bind(canvas));

Here is my completed code (with some extraneous details removed to simplify):

       fabric.Image.fromURL(imgData.URL, function(logoImg){

            canvas.add(logoImg);

            var threshold = 40;

            var whitespace = function(){
                var filter = new fabric.Image.filters.RemoveWhite({
                  threshold: threshold,
                  distance: 140
                });

                threshold+=20;

                logoImg.filters.push(filter);
                logoImg.applyFilters(canvas.renderAll.bind(canvas));
            };
       });