dither.js

(function(global) {
    
    function Dither(container, width, height) {
        this.container = container;
        this.width     = width;
        this.height    = height;
        this.canvas    = null;
        this.image     = null;

        this.draw               = this.draw.bind(this);
        this.handleSliderChange = this.handleSliderChange.bind(this);
        this.handleDragOver     = this.handleDragOver.bind(this);
        this.handleDrop         = this.handleDrop.bind(this);
    }

    Dither.prototype.init = function() {
        this.canvas = this.container.ownerDocument.createElement('canvas');
        this.canvas.width = this.width;
        this.canvas.height = this.height;
        this.canvas.addEventListener('dragover', this.handleDragOver, true);
        this.canvas.addEventListener('drop', this.handleDrop, true);
        this.canvas.style.outline = '1px outset #ccc';
        
        this.slider = this.container.ownerDocument.createElement('input');
        this.slider.type = 'range';
        this.slider.min = 2;
        this.slider.max = 16;
        this.slider.value = 2;
        this.slider.addEventListener('change', this.handleSliderChange);

        this.container.appendChild(this.canvas);
        this.container.appendChild(this.slider);
    };

    Dither.prototype.loadImage = function(file) {
        this.image = new Image;
        this.image.src = URL.createObjectURL(file);
        this.image.onload = this.draw;
    }

    Dither.prototype.draw = function() {
        var ctx = this.canvas.getContext('2d');
        ctx.drawImage(this.image, 0, 0, this.width, this.height);
        var pixels = ctx.getImageData(0, 0, this.width, this.height);
        for (var j = 0; j < this.height - 1; j++) {
            for (var i = 0; i < this.width - 1; i++) {
                var idx = 4 * (j * this.height + i),
                    r = pixels[idx],
                    g = pixels[idx+1],
                    b = pixels[idx+2],
                    l = this.constructor.luma(r, g, b),
                    v = Math.round(Math.round((l / 255) * this.slider.value) / this.slider.value * 255);
                pixels[idx] = pixels[idx + 1] = pixels[idx + 2] = v;
            }
        }
        ctx.putImageData(pixels, 0, 0);
    };



    Dither.prototype.handleSliderChange = function(ev) {
        this.draw();
    };

    Dither.prototype.handleDragOver = function(ev) {
        ev.preventDefault();
    };

    Dither.prototype.handleDrop = function(ev){
        ev.preventDefault();
        this.loadImage(ev.dataTransfer.files[0]);
    };

    Dither.luma = function(r, g, b) {
        return Math.round(0.2126 * r + 0.7152 * g + 0.0722 * b);
    };

    global.Dither = Dither;

})(this);