<canvas id="mc" width="600" height="600" style="border: 1px dotted black"></canvas>
var canvas = document.getElementById('mc');
var ctx = canvas.getContext('2d');
width = canvas.width;
height = canvas.height;
var len = 600;
var step = 10;
for(var i = len; i >= step; i = i-step)
{
ctx.moveTo(0,i);
ctx.lineTo(len+step-i,0);
ctx.lineTo(len,len+step-i);
ctx.moveTo(i-step,len);
ctx.lineTo(len,len-i);
ctx.moveTo(0,len-i);
ctx.lineTo(len-i+step,len);
ctx.strokeStyle = 'gray';
ctx.stroke();
}
结果: