1. 绘制多边形

在画布中以鼠标按下的点为圆心，以鼠标划过的距离为半径绘制多边形：

var Polygon = function(){
    this.mouseIn = false
    this.startPoints = {} //鼠标在画布中按下的点左边
    this.line_list = {}
    this.imageData = null

    var canvas = document.getElementById("myCanvas");
    var ctx = canvas.getContext("2d");
    canvas.onmousedown = (event) => {
        this.saveImage(ctx, canvas)
        let some = this.pointsOnCanvas(event, canvas)
        this.startPoints.x = some.x;
        this.startPoints.y = some.y
        this.mouseIn = true
    }
    canvas.onmousemove = (event) => {
        if(this.mouseIn){   //如果按下鼠标并拖行
            this.loadingImage(ctx)
            this.line_list = this.pointsOnCanvas(event, canvas)
            ctx.beginPath()
            let radius = this.getRadius()
            let startAngel = 0 //开始角度
            let sides = 8 //边数
            let startX = this.mousedown.x + radius * Math.cos(Math.PI*startAngel/180)  //这里其实是角度转为弧度的算法：2*Math.PI/360 * startAngel(2π弧度=360度，所以1度=2π/360)
            let startY = this.mousedown.y + radius * Math.sin(Math.PI*startAngel/180)
            ctx.moveTo(startX, startY)
            for(let i=1;i<sides;i++){
                let nextX = this.mousedown.x + radius * Math.cos(Math.PI/180 * (i*360/sides)+startAngel)
                let nextY = this.mousedown.y + radius * Math.sin(Math.PI/180 * (i*360/sides)+startAngel)
                ctx.lineTo(nextX, nextY);
            }
            ctx.closePath();
            ctx.lineWidth = 2;
            ctx.stroke();
        }
    }
    canvas.onmouseup = () => {
        this.mouseIn = false //鼠标抬起来了
    }
     //算出你点击在canvas画布的坐标 并返回
    Polygon.prototype.pointsOnCanvas = function(event, canvas){
        var rect = canvas.getBoundingClientRect()
        return {
            x: event.clientX - rect.left,
            y: event.clientY - rect.top
        } 
    }
    // 获取半径
    Polygon.prototype.getRadius = function(){
        var x_width = Math.abs(line_list.x - this.mousedown.x)
        var y_width = Math.abs(line_list.y - this.mousedown.y) 
        return Math.sqrt(x_width*x_width + y_width*y_width) 
    }
    Polygon.prototype.saveImage = function(ctx, canvas){  //储存此刻画布数据
        this.imageData = ctx.getImageData(0,0,canvas.width,canvas.height)
    }
    Polygon.prototype.loadingImage = function(ctx){ //导入画布数据
        ctx.putImageData(this.imageData,0,0)
    }
}

效果如下：

Polygon.gif

2. 实现图片反转，显示为底片效果

<input type="file" onChange="chooseFile" />
<img :src="src"> //正常图片
<img :src="revertSrc"> //反转后的图片

function chooseFile(e){
    let imgFile = e.target.files[0]
    let reader = new FileReader()
    let img = new Image()
    reader.onload = () => {
        src = reader.result
        img.src = reader.result
    }
    img.onload = () => {
        revertSrc = createRevertPic(img)
    }
    reader.readAsDataURL(imgFile)
}
//图片反转
function createRevertPic(img){
    var canvas = document.createElement("canvas");
    canvas.width = img.width;   
    canvas.height = img.height;
    var ctx = canvas.getContext("2d");
    ctx.drawImage(img,0,0);  
    var c = ctx.getImageData(0, 0, img.width, img.height);
    for(var i = 0; i < c.height; ++i){
        for(var j = 0; j < c.width; ++j){
            var x = i*4*c.width + 4*j,  //imagedata读取的像素数据存储在data属性里，是从上到下，从左到右的，每个像素需要占用4位数据，分别是r,g,b,alpha透明通道
            r = c.data[x],
            g = c.data[x+1],
            b = c.data[x+2];
            c.data[x+3] = 150;    //透明度设置为150,0表示完全透明
            //图片反相：
            c.data[x] = 255-r;
            c.data[x+1] = 255-g;
            c.data[x+2] = 255-b; 
        }
    }
    ctx.putImageData(c,0,0);
    return canvas.toDataURL();  //返回canvas图片数据为base64的url
}

效果如下：

dipian.gif