canvas 实现自由画线，变换颜色、画笔大小，撤销上一步等简单功能
<!DOCTYPE html>
<html lang="en">
<head>
<meta charset="UTF-8">
<meta name="viewport" content="width=device-width, initial-scale=1.0">
<title>画板</title>
<script src="https://apps.bdimg.com/libs/jquery/2.1.4/jquery.min.js"></script>
<style>
body,div,canvas,h5,input,ul,li,p,button{
margin: 0px;
padding: 0px;
position: relative;
}
#mycanvas{
margin: 5px;
}
#content{
margin: 5px auto;
700px;
height: 510px;
border: 1px solid gray;
}
#canvas_selector{
position: absolute;
margin-left: 505px;
margin-top: -512px;
height: 500px;
190px;
border:1px solid black;
}
.title{
text-align: center;
margin-bottom: 10px;
}
ul li{
list-style-type: none;
margin: 10px 30px 10px 20px;
display: block;
float: left;
40px;
height: 20px;
background:greenyellow;
cursor: pointer;
border: 1px solid gray;
}
#canvas_color,#canvas_brush,#canvas_control,#canvasImage{
margin:50px 0 50px 0;
}
#canvas_brush{
height: 80px;
margin:10px 10px 0px 20px;
background:greenyellow;
text-align:center;
}
#lineT{
150px;
height: 30px;
background:bisque;
}
#canvas_control{
margin:10px 10px 20px 30px;
text-align:center;
}
#canvasImage{
text-align: center;
}
#imgDiv{
margin: 0 auto;
}
#line{
40px;
height: 20px;
cursor: pointer;
}
</style>
</head>
<body>
<div id="content">
<canvas id="mycanvas" width="500" height="500" style="border: 1px solid red;"></canvas>
<div id="canvas_selector">
<div id="canvas_color">
<h5 class="title">颜色<input type="color" name="color" id="changeColor" /></h5>
</div>
<div id="canvas_brush">
<h5 class="title">画笔大小</h5>
<input type="range" id="lineT" min="1" max="100" value="2">
</div>
<div id="canvas_control">
<h5 class="title">操作</h5>
<span><button style="background:greenyellow" id="prev">上一步</button></span>
<span><button style="background:greenyellow" id="cloth">橡皮擦</button></span>
<span><button style="background:#ffc200" id="clear">清除</button></span>
</div>
<div id="canvasImage">
<button id="createImg">生成图像</button>
</div>
</div>
</div>
<div id="imgDiv"></div>
</body>
</html>
<script>
var c=$("#mycanvas")[0];
var canvas= $("#mycanvas");
console.log(c);
var ctx=c.getContext("2d");//创建画布对象
var bool=false;
var left=$("#mycanvas").offset().left;//获取画布的left值
console.log("left",left);
var top=$("#mycanvas").offset().top;//获取画布的top值
console.log("top",top);
var canvasW=$("#mycanvas").width();//获取画布的宽度
console.log("canvasW",canvasW);
var canvasH=$("#mycanvas").height();//获取画布的高度
console.log("canvasH",canvasH);
var img = []; //用于存放画布图片截图的数组
var imgDiv=document.getElementById("imgDiv");
var content=document.getElementById("content")
var color="#000";
ctx.lineCap="round";// 设置线条的结束端点样式
ctx.lineJion="round";//设置两条线相交时，所创建的拐角类型
//鼠标点击设置画布起点
/* $("#mycanvas") */canvas.mousedown(function(e){
bool=true;
console.log("mousedown",bool);
ctx.beginPath();//起始/重置一条路径
ctx.moveTo(e.clientX-left,e.clientY-top); //把路径移动到画布中的指定点，不创建线条
var pic=ctx.getImageData(0,0,canvasW,canvasH);//获取当前画布的图像
img.push(pic);//将当前图像存入数组
});
//当bool=ture时鼠标移动画线
/* $("#mycanvas") */canvas.mousemove(function(e){
console.log("mousemove",bool);
if(bool){ //通过bool值控制画线的连续性,如果bool=true,画线
console.log("if(bool)",bool);
ctx.lineTo(e.clientX-left,e.clientY-10);//添加一个新点，在画布中创建从该点到最后指定点的线条
ctx.stroke();//画线
}
});
//鼠标移出画布或者抬起时，退出当前画线，并新建画线，实现画线断续
/* $("#mycanvas") */canvas.mouseout(function(e){
ctx.closePath();//当鼠标移出画布区域时,创建从当前点回到起始点的路径
bool=false;
console.log("mouseout",bool);
});
/* $("#mycanvas") */canvas.mouseup(function(e){
ctx.closePath();//当鼠标抬起时,创建从当前点回到起始点的路径
bool=false;
console.log("mouseup",bool);
});
//清除画布
$("#clear").click(function(){
//alert("Are you sure clear the canvas?");
ctx.clearRect(0,0,canvasW, canvasH);//创建矩形清空
});
//擦除画布
$("#cloth").click(function(){
ctx.strokeStyle="#fff";//利用画线为白色实现橡皮擦功能
});
//上一步
$("#prev").click(function(){
if(img.length>=0){
console.log("img.length",img.length);
var newImgLength=img.length;
console.log("newImgLength",newImgLength);
ctx.putImageData(img.pop(),0,0);
}
});
//改变颜色
$("#changeColor").change(function(){
ctx.strokeStyle=this.value;//改变颜色
});
//改变画笔大小
$("#lineT").change(function(){
ctx.lineWidth=this.value;
});
//生成图片
$("#createImg").click(function(){
var url=c.toDataURL('image/png');
var newImg=new Image();//创建一个Image对象
newImg.src=url;
imgDiv.appendChild(newImg);
imgDiv.style.width="500px";
imgDiv.style.height="500px";
imgDiv.style.background="#ccc";
});
</script>