Fabricjs画布任何形状

Question

我如何实现挂钟定制？我需要更改画布形状并限制画布形状内的对象。可能吗？帮助我。例如，

var canvas = new fabric.Canvas('canvas');
var w, h;
var ctx = canvas.getContext('2d');
w = canvas.width / 2;
h = canvas.height / 2;
ctx.save();
ctx.arc(w, h, 230, 0, Math.PI * 2,true);