下面介绍如何使用canvas像Paint一样进行绘画
如果您想要撤消功能，最好的选择是记录用户绘制的所有线段。
这是通过包含用户绘制的所有点（折线）的点数组来完成的。
要跟踪画笔大小和画笔颜色，您还需要将此信息包含在数组中。
因此数组的每个元素都将包含有关每个线段的信息：
- x：该线段的结束x坐标
- y：结束 y 坐标
- size：画笔大小（线宽）
- color：画笔颜色（笔触样式）
- mode：“begin”表示新行的开始，“end”表示本行的结束。
它是如何工作的？
当用户拖动绘制线段时，每个 mousemove 事件都会扩展当前线段context.lineTo and context.stroke.
当用户选择新的 BrushSize 或 BrushColor 时，context.beginPath 启动context.beginPath.
当用户按住“撤消”按钮时，最后一条线段中的最后一个点将从点数组中弹出。然后重新绘制所有幸存的线段。按住撤消按钮时，每 1/10 秒触发一次。
这是代码和小提琴：http://jsfiddle.net/m1erickson/AEYYq/
<!doctype html>
<html>
<head>
<link rel="stylesheet" type="text/css" media="all" href="css/reset.css" /> <!-- reset css -->
<script type="text/javascript" src="http://code.jquery.com/jquery.min.js"></script>
<!--[if lt IE 9]><script type="text/javascript" src="../excanvas.js"></script><![endif]-->
<style>
body{ background-color: ivory; }
canvas{border:1px solid red;}
</style>
<script>
$(function(){
var canvas=document.getElementById("canvas");
var ctx=canvas.getContext("2d");
var lastX;
var lastY;
var mouseX;
var mouseY;
var canvasOffset=$("#canvas").offset();
var offsetX=canvasOffset.left;
var offsetY=canvasOffset.top;
var isMouseDown=false;
var brushSize=20;
var brushColor="#ff0000";
var points=[];
function handleMouseDown(e){
mouseX=parseInt(e.clientX-offsetX);
mouseY=parseInt(e.clientY-offsetY);
// Put your mousedown stuff here
ctx.beginPath();
if(ctx.lineWidth!=brushSize){ctx.lineWidth=brushSize;}
if(ctx.strokeStyle!=brushColor){ctx.strokeStyle=brushColor;}
ctx.moveTo(mouseX,mouseY);
points.push({x:mouseX,y:mouseY,size:brushSize,color:brushColor,mode:"begin"});
lastX=mouseX;
lastY=mouseY;
isMouseDown=true;
}
function handleMouseUp(e){
mouseX=parseInt(e.clientX-offsetX);
mouseY=parseInt(e.clientY-offsetY);
// Put your mouseup stuff here
isMouseDown=false;
points.push({x:mouseX,y:mouseY,size:brushSize,color:brushColor,mode:"end"});
}
function handleMouseMove(e){
mouseX=parseInt(e.clientX-offsetX);
mouseY=parseInt(e.clientY-offsetY);
// Put your mousemove stuff here
if(isMouseDown){
ctx.lineTo(mouseX,mouseY);
ctx.stroke();
lastX=mouseX;
lastY=mouseY;
// command pattern stuff
points.push({x:mouseX,y:mouseY,size:brushSize,color:brushColor,mode:"draw"});
}
}
function redrawAll(){
if(points.length==0){return;}
ctx.clearRect(0,0,canvas.width,canvas.height);
for(var i=0;i<points.length;i++){
var pt=points[i];
var begin=false;
if(ctx.lineWidth!=pt.size){
ctx.lineWidth=pt.size;
begin=true;
}
if(ctx.strokeStyle!=pt.color){
ctx.strokeStyle=pt.color;
begin=true;
}
if(pt.mode=="begin" || begin){
ctx.beginPath();
ctx.moveTo(pt.x,pt.y);
}
ctx.lineTo(pt.x,pt.y);
if(pt.mode=="end" || (i==points.length-1)){
ctx.stroke();
}
}
ctx.stroke();
}
function undoLast(){
points.pop();
redrawAll();
}
ctx.lineJoin = "round";
ctx.fillStyle=brushColor;
ctx.lineWidth=brushSize;
$("#brush5").click(function(){ brushSize=5; });
$("#brush10").click(function(){ brushSize=10; });
// Important! Brush colors must be defined in 6-digit hex format only
$("#brushRed").click(function(){ brushColor="#ff0000"; });
$("#brushBlue").click(function(){ brushColor="#0000ff"; });
$("#canvas").mousedown(function(e){handleMouseDown(e);});
$("#canvas").mousemove(function(e){handleMouseMove(e);});
$("#canvas").mouseup(function(e){handleMouseUp(e);});
// hold down the undo button to erase the last line segment
var interval;
$("#undo").mousedown(function() {
interval = setInterval(undoLast, 100);
}).mouseup(function() {
clearInterval(interval);
});
}); // end $(function(){});
</script>
</head>
<body>
<p>Drag to draw. Use buttons to change lineWidth/color</p>
<canvas id="canvas" width=300 height=300></canvas><br>
<button id="undo">Hold this button down to Undo</button><br><br>
<button id="brush5">5px Brush</button>
<button id="brush10">10px Brush</button>
<button id="brushRed">Red Brush</button>
<button id="brushBlue">Blue Brush</button>
</body>
</html>
