canvasFilters.js

const Filters = {
    filterImage (filter, image, var_args) {
        var args = [this.getPixels(image)];
        for (var i=2; i<arguments.length; i++) {
            args.push(arguments[i]);
        }
        return filter.apply(null, args);
    },
    brightness (pixels, adjustment) {
        var d = pixels.data;
        for (var i=0; i<d.length; i+=4) {
            d[i] += adjustment;
            d[i+1] += adjustment;
            d[i+2] += adjustment;
        }
        return pixels;
    },
    threshold (pixels, threshold) {
        var d = pixels.data;
        for (var i=0; i<d.length; i+=4) {
            var r = d[i];
            var g = d[i+1];
            var b = d[i+2];
            var v = (0.2126*r + 0.7152*g + 0.0722*b >= threshold) ? 255 : 0;
            d[i] = d[i+1] = d[i+2] = v
        }
        return pixels;
    },
    convolute (pixels, weights, opaque) {
        var side = Math.round(Math.sqrt(weights.length));
        var halfSide = Math.floor(side/2);
        var src = pixels.data;
        var sw = pixels.width;
        var sh = pixels.height;
        // pad output by the convolution matrix
        var w = sw;
        var h = sh;
        var output = Filters.createImageData(w, h);
        var dst = output.data;
        // go through the destination image pixels
        var alphaFac = opaque ? 1 : 0;
        for (var y=0; y<h; y++) {
            for (var x=0; x<w; x++) {
                var sy = y;
                var sx = x;
                var dstOff = (y*w+x)*4;
                // calculate the weighed sum of the source image pixels that
                // fall under the convolution matrix
                var r=0, g=0, b=0, a=0;
                for (var cy=0; cy<side; cy++) {
                    for (var cx=0; cx<side; cx++) {
                        var scy = sy + cy - halfSide;
                        var scx = sx + cx - halfSide;
                        if (scy >= 0 && scy < sh && scx >= 0 && scx < sw) {
                            var srcOff = (scy*sw+scx)*4;
                            var wt = weights[cy*side+cx];
                            r += src[srcOff] * wt;
                            g += src[srcOff+1] * wt;
                            b += src[srcOff+2] * wt;
                            a += src[srcOff+3] * wt;
                        }
                    }
                }
                dst[dstOff] = r;
                dst[dstOff+1] = g;
                dst[dstOff+2] = b;
                dst[dstOff+3] = a + alphaFac*(255-a);
            }
        }
        return output;
    }
}