图片的拖拽，缩放，截图功能--自写版

    <div class="bg">
        <img src="2.jpg" alt="头像" id="pic" class="drag">
        <div class="mask">
        </div>
    </div>
    <canvas id=‘c3‘ width="200" height="200"></canvas>
    <img src="2.jpg"  class="preview"  >
    <p style="width:100%;">
        <button type="button" id="btn1" onclick="ImageChange(true)">放大</button>
        <button type="button" id="btn2" onclick="ImageChange(false)">缩小</button>
        <button type="button" id="btn3" onclick="RotateChange()">旋转</button>
        <button type="button" id="btn4" onclick="Screenshot()">截图</button>
    </p>

 .bg {
            border: 1px solid #ddd;
            width: 300px;
            height: 300px;
            overflow: hidden;
            position: relative;
            cursor: move;
            margin: 0 auto;
            display: inline-block;
        }

       .bg>img {   
            background-position: center center;
            background-size: cover;
            background-repeat: no-repeat;
            position: absolute;
            vertical-align: middle;         
            width: 100%;
            height: 100%;   
        }

        .mask {
            width: 100%;
            height: 100%;
            border-radius: 50%;
            position: absolute;
            box-shadow: 0px 0px 0px 110px rgba(255, 255, 255, 0.5);
            z-index: 100;
        }
        #c3{
             border:1px solid #ddd;
             display:inline-block; 
             /* position: absolute; */
             margin-left:20px ;

        }
        .preview{
            width:200px;
            height: 200px;
            margin-left:20px;
        }

  //放大缩小
        var zoom = 1;
        var left = $("#pic").width(), tops = $("#pic").height();
        function ImageChange(args) {
            var oimg = $("#pic"), imgW = oimg.width(), imgH = oimg.height();
            if (args) {
                if (zoom < 2) {
                    zoom += 0.1;
                    oimg.css({ ‘width‘: imgW * 1.1, ‘left‘: -(imgW * 1.1 - left) / 2 });  用计算宽高，还是transform ,zoom都是可以的。不限制
                    oimg.css({ ‘height‘: imgH * 1.1, ‘top‘: -(imgH * 1.1 - tops) / 2 });
                    // oimg.css({ ‘transform‘:‘scale(‘+zoom+‘)‘});
                }
            } else {
                if (zoom > 1) {
                    zoom -= 0.1
                    // oimg.css({ ‘transform‘:‘scale(‘+zoom+‘)‘});
                    oimg.css({ ‘width‘: imgW / 1.1, ‘left‘: -(imgW / 1.1 - left) / 2 });
                    oimg.css({ ‘height‘: imgH / 1.1, ‘top‘: - (imgH / 1.1 - tops) / 2 });
                }
            }
            return zoom;
        }

 //鼠标移上去滚动放大缩小
        document.getElementsByClassName(‘mask‘)[0].addEventListener("mousewheel", function (e) {
            var e = e || window.event, v = e.wheelDelta || e.detail;
            if (v > 0) { //这里主要明白是像上滚动还是下滚动
                ImageChange(true);
                window.event.returnValue = false;
                return false;
            } else {
                ImageChange(false);
                window.event.returnValue = false;
                return false;
            }
        })

 //拖拽
        var oDiv = document.getElementById(‘pic‘);
        var flag = false;
        var clientX, newX;
        $(".mask").on(‘mousedown‘, function (e) {
            var disX = e.clientX - oDiv.offsetLeft;
            var disY = e.clientY - oDiv.offsetTop;
            flag = true;
            clientX = e.clientX;
            clientY = e.clientY;
            document.onmousemove = function (ev) {
                newX = clientX - ev.clientX;
                newY = clientY - ev.clientY;
                if (flag && zoom != 1) { //这里限制了当zoom等于1的时候不允许拖拽
                    //获取当前移动的距离
                    var a = $("#pic").css(‘left‘).replace(/px/, ‘‘);
                    var b = $("#pic").css(‘top‘).replace(/px/, ‘‘);
                    var c = $("#pic").css(‘right‘).replace(/px/, ‘‘);
                    var d = $("#pic").css(‘bottom‘).replace(/px/, ‘‘);
                    if (newX < 0) { //向右拖动     //限制拖拽的边距，不能旁边留空
                        if (Math.abs(a) > Math.abs(ev.clientX - disX)) {  //据左边的距离大于拖动的
                            oDiv.style.left = ev.clientX - disX > 0 ? 0 : ev.clientX - disX + "px";
                        }
                    } else {
                        if (Math.abs(c) > Math.abs(ev.clientX - disX)) {  //据右边的距离大于拖动的
                            oDiv.style.left = ev.clientX - disX > 0 ? 0 : ev.clientX - disX + "px";
                        }
                    }
                    if (newY < 0) { //向上拖动
                        if (Math.abs(b) > Math.abs(ev.clientY - disY)) {  //据上边的距离大于拖动的
                            oDiv.style.top = ev.clientY - disY > 0 ? 0 : ev.clientY - disY + "px";
                        }
                    } else {
                        if (Math.abs(c) > Math.abs(ev.clientY - disY)) {  //据下边的距离大于拖动的
                            oDiv.style.top = ev.clientY - disY > 0 ? 0 : ev.clientY - disY + "px";
                        }
                    }
                }
            }
            document.onmouseup = function () {
                document.onmousemove = null;
                document.onmouseup = null;
                flag = false;
            }
        })

 //旋转
        var i = 1;
        function RotateChange() {
            $("#pic").css(‘transform‘, ‘rotate(‘ + 90 * i + ‘deg)‘);
            i++;
        }

  //截图功能
        function Screenshot() {
            var canvas = document.getElementById(‘c3‘);
            var pic = document.getElementById(‘pic‘);
            var mask = document.getElementsByClassName(‘mask‘)[0];
            var preview = document.getElementsByClassName(‘preview‘);
            var img = new Image();
            img.src = pic.src;
            var c = img.width / $("#pic").width();//计算出原图和缩放图的比例
            var b = img.height / $("#pic").height();
            if (canvas.getContext) {
                    var ctx = canvas.getContext(‘2d‘);
                    ctx.clearRect(0, 0, 300, 300); //清楚上一次的画布  
                    ctx.drawImage(pic, Math.abs(pic.offsetLeft) * c, Math.abs(pic.offsetTop) * b, mask.offsetWidth * c, mask.offsetHeight * b, 0, 0, canvas.offsetWidth, canvas.offsetWidth);  //一定要乘以原图和缩放图的比例
                    //转成base64 
                   var dataUrl = canvas.toDataURL(‘image/jpeg‘, 0.6);
                   preview[0].src = dataUrl;         
            }
        }