cutting out rounded rectangle.

cutRoundedRect.js

var cutRoundedRect = function(ctx, radius, x, y, w, h) {
    var left = x;
    var top = y;
    var right = x + w;
    var bottom = y + h;

    ctx.globalCompositeOperation = 'destination-in';
    ctx.fillStyle = 'black';

    ctx.beginPath();
    ctx.moveTo(left + radius, top);
    ctx.lineTo(right - radius, top);
    ctx.quadraticCurveTo(right, top, right, top + radius);
    ctx.lineTo(right, bottom - radius);
    ctx.quadraticCurveTo(right, bottom, right - radius, bottom);
    ctx.lineTo(left + radius, bottom);
    ctx.quadraticCurveTo(left, bottom, left, bottom - radius);
    ctx.lineTo(left, top + radius);
    ctx.quadraticCurveTo(left, top, left + radius, top);
    ctx.fill();
};