<canvas></canvas> 

var bodyStyle = document.body.style; 
bodyStyle.mozUserSelect = 'none'; 
bodyStyle.webkitUserSelect = 'none'; 

var img = new Image(); 
var canvas = document.querySelector('canvas'); 
canvas.style.backgroundColor='transparent'; 
canvas.style.position = 'absolute'; 
var imgs = ['p_0.jpg','p_1.jpg']; 
var num = Math.floor(Math.random()*2); 
img.src = imgs[num]; 

img.addEventListener('load', function(e) { 
 var ctx; 
 var w = img.width, 
  h = img.height; 
 var offsetX = canvas.offsetLeft, 
  offsetY = canvas.offsetTop; 
 var mousedown = false; 
 function layer(ctx) { 
  ctx.fillStyle = 'gray'; 
  ctx.fillRect(0, 0, w, h); 
 } 
 function eventDown(e){ 
  e.preventDefault(); 
  mousedown=true; 
 } 
 function eventUp(e){ 
  e.preventDefault(); 
  mousedown=false; 
 } 
 function eventMove(e){ 
  e.preventDefault(); 
  if(mousedown) { 
    if(e.changedTouches){ 
     e=e.changedTouches[e.changedTouches.length-1]; 
    } 
    var x = (e.clientX + document.body.scrollLeft || e.pageX) - offsetX || 0, 
     y = (e.clientY + document.body.scrollTop || e.pageY) - offsetY || 0; 
    with(ctx) { 
     beginPath() 
     arc(x, y, 10, 0, Math.PI * 2);//绘制圆点 
     fill(); 
    } 
  } 
 } 
 //... 
}); 

img.addEventListener('load', function(e) { 
 //..接上段代码 
 canvas.width=w; 
 canvas.height=h; 
 canvas.style.backgroundImage='url('+img.src+')'; 
 ctx=canvas.getContext('2d'); 
 ctx.fillStyle='transparent'; 
 ctx.fillRect(0, 0, w, h);//绘制矩形 
 layer(ctx); 
 ctx.globalCompositeOperation = 'destination-out'; 
 canvas.addEventListener('touchstart', eventDown); 
 canvas.addEventListener('touchend', eventUp); 
 canvas.addEventListener('touchmove', eventMove); 
 canvas.addEventListener('mousedown', eventDown); 
 canvas.addEventListener('mouseup', eventUp); 
 canvas.addEventListener('mousemove', eventMove);