Step-down downsample + sharpen convolution

HTML

<canvas id="canvas" width=379 height=100></canvas><br>
Sharpen: <input type=range id=mix min=0 max=100 value=20 onchange="update()">

JavaScript

var canvas = document.getElementById("canvas"),
    ctx = canvas.getContext("2d"),
    offScreen = document.createElement('canvas'),
    offctx = offScreen.getContext('2d'),
    img = new Image();

/// as we need pixel access to apply convolution we
/// need to get around CORS:
img.crossOrigin = 'anonymous';

/// when image is loaded step-down the downscaling
img.onload = resize;
img.src = "http://cdn2.hubspot.net/hub/20286/file-13400665-jpg/images/coloured_pencils.jpg";

/// sharpen image:
/// USAGE:
///    sharpen(context, width, height, mixFactor)
///  mixFactor: [0.0, 1.0]
function sharpen(ctx, w, h, mix) {

    var weights = [0, -1, 0, -1, 5, -1, 0, -1, 0],
        katet = Math.round(Math.sqrt(weights.length)),
        half = (katet * 0.5) | 0,
        dstData = ctx.createImageData(w, h),
        dstBuff = dstData.data,
        srcBuff = ctx.getImageData(0, 0, w, h).data,
        y = h;

    while (y--) {

        x = w;

        while (x--) {

            var sy = y,
                sx = x,
                dstOff = (y * w + x) * 4,
                r = 0,
                g = 0,
                b = 0,
                a = 0;

            for (var cy = 0; cy < katet; cy++) {
                for (var cx = 0; cx < katet; cx++) {

                    var scy = sy + cy - half;
                    var scx = sx + cx - half;

                    if (scy >= 0 && scy < h && scx >= 0 && scx < w) {

                        var srcOff = (scy * w + scx) * 4;
                        var wt = weights[cy * katet + cx];

                        r += srcBuff[srcOff] * wt;
                        g += srcBuff[srcOff + 1] * wt;
                        b += srcBuff[srcOff + 2] * wt;
                        a += srcBuff[srcOff + 3] * wt;
                    }
                }
            }

            dstBuff[dstOff] = r * mix + srcBuff[dstOff] * (1 - mix);
            dstBuff[dstOff + 1] = g * mix + srcBuff[dstOff + 1] * (1 - mix);
            dstBuff[dstOff + 2] = b * mix +...