HTML5 canvas学习小例代码

window.onload = function(){
  createCanvas();
 }
  function createCanvas(){
   var canvas_width= 200, canvas_height = 200;
   document.body.innerHTML = "<canvas id=\"canvas\" width=\""+canvas_width+"\" height=\""+canvas_height+"\"></canvas>";
  }

var canvas_width= 500, canvas_height = 500;
var mycanvas, context;

window.onload = function(){
  createCanvas();
  drawRect();
 }
  function createCanvas(){
   
   document.body.innerHTML = "<canvas id=\"mycanvas\" width=\""+canvas_width+"\" height=\""+canvas_height+"\"></canvas>";
   mycanvas = document.getElementById("mycanvas");
   context = mycanvas.getContext("2d");
  }
 
  function drawRect(){
 context.fillStyle ="#FF0000";
 //context.rotate(45);//旋转45度
 //context.translate(200,200);//移动
 //context.scale(2,0.5);//缩放
 context.fillRect(0,0,200,200);
  }

var canvas_width= 500, canvas_height = 500;
var mycanvas, context;

window.onload = function(){
  createCanvas();
  drawImage();
 }
  function createCanvas(){
   
   document.body.innerHTML = "<canvas id=\"mycanvas\" width=\""+canvas_width+"\" height=\""+canvas_height+"\"></canvas>";
   mycanvas = document.getElementById("mycanvas");
   context = mycanvas.getContext("2d");
  }
 
  function drawImage(){
 var img = new Image();
 img.onload = function(){
  context.drawImage(img,0,0);
 }
 img.src = "1.png";
  }