我現在只能讓物件的移動限制在canvas方形內
請問要怎麼讓物件移動限制在我設定的圓形canvas邊界內?

我把canvas用style="border-radius:999em"弄出一個圓形

js的部分:
const canvas = new fabric.Canvas('canvas', {
width: 650,
height: 650,
})

canvas.on('object:moving', function (e) {
var obj = e.target;

   if(obj.getScaledHeight() > obj.canvas.height || obj.getScaledWidth() > obj.canvas.width){
       return;
   }        
   obj.setCoords();        
   // top-left  corner
   if(obj.getBoundingRect().top < 0 || obj.getBoundingRect().left < 0){
       obj.top = Math.max(obj.top, obj.top-obj.getBoundingRect().top);
       obj.left = Math.max(obj.left, obj.left-obj.getBoundingRect().left);
   }
   // bot-right corner
   if(obj.getBoundingRect().top+obj.getBoundingRect().height  > obj.canvas.height || obj.getBoundingRect().left+obj.getBoundingRect().width  > obj.canvas.width){
       obj.top = Math.min(obj.top, obj.canvas.height-obj.getBoundingRect().height+obj.top-obj.getBoundingRect().top);
       obj.left = Math.min(obj.left, obj.canvas.width-obj.getBoundingRect().width+obj.left-obj.getBoundingRect().left);
   }});