// forked from akm2's "Canvas clearArc()" http://jsdo.it/akm2/e8CK CanvasRenderingContext2D.prototype.clear = function() { this.save(); this.globalCompositeOperation = 'destination-out'; this.fillStyle = 'black'; this.fill(); this.restore(); }; CanvasRenderingContext2D.prototype.clearArc = function(x, y, radius, startAngle, endAngle, anticlockwise) { this.beginPath(); this.arc(x, y, radius, startA
![Canvas 描画内容を薄く消す - jsdo.it - Share JavaScript, HTML5 and CSS](https://cdn-ak-scissors.b.st-hatena.com/image/square/a69df117d44d20ecfaffedb43b0e7a49d623e1d4/height=288;version=1;width=512/http%3A%2F%2Fjsdo-static-contents.s3.amazonaws.com%2Fimages%2Fcapture%2Fm%2F6%2FX%2Fm6Xs.jpg%3Ft%3D1353897345)