as, evt) {
                const rect = canvas.getBoundingClientRect();
                return { 
                    x: evt.clientX - rect.left, 
                    y: evt.clientY - rect.top 
                };
            }
            
            _getPointType(i, j) {
                if ((i === 0 || i === 3) && (j ===0 || j === 3))
                    return 'corner';
                if (i === 0 || i === 3 || j ===0 || j === 3)
                    return 'handle';
                return 'interior';
   