Html5 Canvas一个简单的画笔例子

相比了下Qt quick的canvas和HTML5的canvas，发现HTML5 Canvas在同样绘制绘制操作下性能比Qt的canvas强很多，附上一个HTML5 canvas画笔一例子

var DW  = function( canvasid){
   this._points = [];
   this._canvas = document.getElementById( canvasid );
   this._ctx = this._canvas.getContext("2d");
   this._isPressed = false;
   this._color  = "#223344";
   this._widht = 8;

   var self = this;

   function __init(){
      self._ctx.lineCap = "round";
      self._ctx.lineJoin="round";
      self._ctx.strokeStyle = self._color;
      self._ctx.lineWidth = self._widht;
   };

   this._addPoints = function( x, y ){
    this._points.push({x: x , y : y });
   };

   this._capturePath = function( x , y ){
    this._addPoints( x,  y );
   };

   this._prepareDrawing = function( x, y ){
    this._points.length = 0 ;
    this._addPoints( x, y );
    this._ctx.moveTo( x, y );
   };

   this._render = function(){
   	console.log("renderrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrr");
    var p1 = this._points[0] , p2 =this._points[1];
    this._ctx.save();
    this._ctx.beginPath();

    if(this._points.length === 2 && p1.x === p2.x && p1.y===p2.y ){
      p1.x -= 0.5 ;
      p1.y -= 0.5 ;
    }

    this._ctx.moveTo( p1.x , p1.y );

    for( var i = 1 , len = this._points.length ; i < len ; i++ ){
      var mp = {x : (p1.x + (p2.x - p1.x ) /2) , y : (p1.y + (p2.y - p1.y)/2)};
      this._ctx.quadraticCurveTo( p1.x , p1.y , mp.x , mp.y );
      p1 = this._points[i] ; p2 = this._points[i+1];
    }

    this._ctx.lineTo( p1.x , p1.y );
    this._ctx.stroke();
    this._ctx.restore();
   }

   this._clearContext = function(){
      this._ctx.clearRect(0,0, this._canvas.width , this._canvas.height);
   }
   __init();

};

DW.prototype.setColor = function(color){ 
  this._color = color;
  this._ctx.strokeStyle = color;
};

DW.prototype.setWidth = function( w ){
  this._widht = w;
  this._ctx.lineWidth = w ;
}

应用

<!doctype html>
  <html>
    <head>
     <title> Canvas </title>
     <style type="text/css">
       canvas{
       	 border-radius: 8;
       	 border-style: solid;
       	 border-color: 'gray'
       }
     </style>
     <script ==> 引用上头JS文件 =========/script>
    </head>
    <body>
      <canvas id="_canvas" width='500' height="400" ></canvas>
      <script type="text/javascript">
       var $ = function( id ){ return document.getElementById(id);}
       var c = $('_canvas');
       var dObject = new DW("_canvas");
       function on_mouse_press(e){
                dObject.setColor("#778899");
                dObject._prepareDrawing(e.offsetX,e.offsetY);
                dObject._capturePath(e.offsetX,e.offsetY);
                dObject._render();

                dObject._isPressed = true;
        };

        function on_mouse_move( e) {
                 if( dObject._isPressed === true ){
                      dObject._capturePath(e.offsetX,e.offsetY);
                      dObject._clearContext();
                     dObject._render(); //redraw
         }
      };

       function on_mouse_up(e){
           dObject._isPressed = false;
       }

       c.addEventListener('mousedown',function(e){
       	on_mouse_press(e);
       },false);

       c.addEventListener('mousemove' , function(e){
       	 on_mouse_move(e);
       },false);

       c.addEventListener('mouseup',function(e){
       	  on_mouse_up(e);
       },false);
      </script>
    </body>
  </html>