如果你想绘制一个黑色的透明笔划，你可能需要:

If you want to draw a black transparent stroke, you probably want:

context.globalCompositeOperation = "destination-out";
context.strokeStyle = "rgba(0,0,0,1)";

记得保存之前的globalCompositeOperation，稍后再恢复，否则透明度将无法正常工作！

Remember to save the previous globalCompositeOperation and then restore it later or transparency won't work properly!