Javascript HTML5 Canvas实现的一个画板【javascript】

      浏览器不支持canvas  
YELLOWREDBLUEGREENWHITEBLACK
 4PX8PX16PX
 EXPORT 

   var canvas = document.getElementById('canvas'); //获取标签  var ctx = canvas.getContext("2d");    var fillStyle = "black";  ctx.fillRect(0,0,600,300);  var onoff = false; //按下标记  var oldx = -10;  var oldy = -10;  //设置颜色  var linecolor = "white";  var linw = 4;  canvas.addEventListener("mousemove",draw,true); //鼠标移动事件  canvas.addEventListener("mousedown",down,false); //鼠标按下事件  canvas.addEventListener("mouseup",up,false); //鼠标弹起事件  function down(event){    onoff = true;    oldx = event.pageX - 10;    oldy = event.pageY - 10;  }  function up(){    onoff = false;  }  function draw(event){    if (onoff==true) {      var newx = event.pageX - 10;      var newy = event.pageY - 10      ctx.beginPath();      ctx.moveTo(oldx,oldy);      ctx.lineTo(newx,newy);      ctx.strokeStyle = linecolor;      ctx.lineWidth = linw;      ctx.lineCap = "round";      ctx.stroke();       oldx = newx;      oldy = newy;    }  }  function copyimage(event)  {    var img_png_src = canvas.toDataURL("image/png"); //将画板保存为图片格式的函数    document.getElementById("image_png").src = img_png_src;  }