web功能实例 - Canvas裁剪工具

嗯,手撸官方文档2天，发现没啥用，尤其是动画,那种计算出来的，根本想不到。因此学着学了抱着要做个东西的想法,去网上找相关案例,最终做出了这个裁剪工具。

PS :先说一下思路:

核心实现有3个canvas图层, 其中一个负责图片的预览。
另外2个叠加到一起,底层canvas负责图片的渲染; 上层的canvas负责蒙版的绘制,和选择框(挖空)区域的绘制。
我们将,移动选择框,记录的坐标和宽高,一个是同步到蒙版canvas里面,实现挖空。
第二个同步到图片渲染canvas,通过getImageData()方法,拿取选择框挖空的区域的像素数据,渲染到图片预览canvas里面,然后通过canvas的toBlob()方法将预览canvas,导出成blobUrl,实现图片下载。

针对部分核心功能进行思路讲解。

1.上传图片

FileReader.readAsDataURL() - Web API 接口参考 | MDN (mozilla.org)

在 web 应用程序中使用文件 - Web API 接口参考 | MDN (mozilla.org)

下面,我们点击upBut,从而触发upInp的change事件,从而触发文件上传。我们对文件上传的类型进行是不是图片判断。然后,将图片File ,通过FileReader对象的readAsDataURL(File)方法,将图片File转换为dataUrl,并封装成Img对象。进而绘制到图片渲染canvas里面。

除此以外涉及到的函数,下文都会讲到。

	<input type="file" id="up-inp" name="文件上传" style="display: none" /><button type="button" id="up-but">上传</button>let upBut = document.getElementById('up-but')let upInp = document.getElementById('up-inp')

	upBut.addEventListener('click', (e) => { // 给按钮绑定事件,点击 input type='file',从而弹出文件上传框upInp.click()})// 上传图片const updateFile =  (e) =>{let file = e.target.files[0]if (!file.type.startsWith('image')) {alert('只允许上传图片')return}const reader = new FileReader()reader.onload = (e) => {  // 利用fileReader将file文件转换成dataUrllet img1 = new Image() // 转换成img对象,进而绘画到canvas里面img1.src = e.target.resultimg = img1img1.onload = (e) => { // 读取完毕之后selectCropObj = computeImage({imgWidth: img.width,imgHeight: img.height,width: cropCardbg.width,height: cropCardbg.height,})initImgObj = JSON.parse(JSON.stringify(selectCropObj))drawImage(initImgObj)drawModal()drawClip()drawClipDiv()imgPreview()cropModal.style.display = 'none'clip.style.display = 'block'}}reader.readAsDataURL(file)}upInp.addEventListener('change', updateFile)

2. 蒙版绘制

蒙版说白了，就是占据canvas画布全部的半透明矩形。

	// 画模版const drawModal =  () =>{ctxCardbg.clearRect(0, 0, cropCardbg.width, cropCardbg.height)ctxCardbg.fillStyle = 'rgba(0,0,0,0.5)'ctxCardbg.fillRect(0, 0, cropCardbg.width, cropCardbg.height)}

3.挖空

挖空:蒙版随选择框的移动要扣除的透明区域。我们先绘制蒙版，然后用clearRect()方法清除指定区域( 随着选择框移动,对应的在蒙版canvas里面的坐标和宽高围绕的区域,清除这个区域),达到挖空的效果。

        // 挖空const drawClip = () => {ctxCardbg.clearRect(selectCropObj.x, selectCropObj.y, selectCropObj.w, selectCropObj.h)}

4.选择框的绘制

我们先封装一个函数,用来注册拖拽选择框(中心，和8个点)的鼠标按下、移动、抬起事件。

// 注册选择框拖拽事件const registerEvents = () => {// 注册那8个拖拽点事件const register = (_class) => {let node = document.querySelector(`.${_class}`)node.addEventListener('mousedown', (e) => {down = true})node.addEventListener('mousemove', carMouseMove)node.addEventListener('mouseup', function (e) {down = false})}register('top-center')register('bottom-center')register('left-center')register('right-center')register('bottom-right')register('bottom-left')register('top-right')register('top-left')// 注册拖拽中央移动的事件let clip = document.getElementById('crop-clip')clip.addEventListener('mousedown', (e) => {down = true})clip.addEventListener('mousemove', carMouseMove)clip.addEventListener('mouseup', (e) => {down = false})}

这是拖拽选择框的html 结构。我们的选择框采用html绘制,由于它和另外2个叠加的canvas,由于父元素相对定位,子绝对定位叠在一起,因此选择框的left和top值,就相当于canvas里面的x、y坐标,我们同样将选择框宽高映射到,canvas的挖空区域,从而在拖拽的时候实现实时挖空的效果。

	<div id="crop-clip" style="display: none"><div class="dot top-left"></div><div class="dot top-right"></div><div class="dot top-center"></div><div class="dot bottom-left"></div><div class="dot bottom-center"></div><div class="dot bottom-right"></div><div class="dot left-center"></div><div class="dot right-center"></div></div>

下面通过selectCropObj对象,记录选择框在移动期间的x,y坐标,以及选择框变化的宽高。

	// 在canvas的裁剪框尺寸和坐标let selectCropObj = {x: 0,y: 0,w: 0,h: 0,}

这是我们处理选择框移动的函数。根据拖拽的元素(选择框中央、其余8个点)的携带的class不同,从而调用不同的移动处理方法。

	// 通过拖拽事件,调用的方法(将选择框中央拖拽 ,和点拖拽通过一个函数处理)const carMouseMove = (e) => {// 两个都是false,证明我们一个没按下if (img === null) {return}if (!down) {return}let ele = e.targetlet { movementX, movementY } = econst isExistsCls = (_cls) => {for (let i = 0; i < ele.classList.length; i++) {const val = ele.classList[i]if (val === _cls) {return true}}return false}// 中央拖拽if (ele.id == 'crop-clip') {selectCropObj.x += movementXselectCropObj.y += movementY// 点拖拽} else if (isExistsCls('top-left')) {// 坐标和宽高的都要变selectCropObj.x += movementXselectCropObj.y += movementYselectCropObj.w += -movementXselectCropObj.h += -movementY} else if (isExistsCls('top-right')) {selectCropObj.y += movementYselectCropObj.w += movementXselectCropObj.h += -movementY} else if (isExistsCls('top-center')) {selectCropObj.y += movementYselectCropObj.h += -movementY} else if (isExistsCls('bottom-left')) {selectCropObj.x += movementXselectCropObj.y += movementYselectCropObj.w += -movementXselectCropObj.h += movementY} else if (isExistsCls('bottom-right')) {selectCropObj.w += movementXselectCropObj.h += movementY} else if (isExistsCls('bottom-center')) {selectCropObj.h += movementY} else if (isExistsCls('left-center')) {selectCropObj.x += movementXselectCropObj.w += -movementX} else if (isExistsCls('right-center')) {selectCropObj.w += movementX}drawClipDiv()drawModal()drawClip()imgPreview()}

对应选择框的具体绘制方法。

		const drawClipDiv = () => {let cropClip = document.getElementById('crop-clip')cropClip.style.width = `${selectCropObj.w}px`cropClip.style.height = `${selectCropObj.h}px`cropClip.style.left = `${selectCropObj.x}px`cropClip.style.top = `${selectCropObj.y}px`}

5.画图像

就是清除之前渲染到图片canvas里面的图像,并将新的图片绘制到上面。

	// 画图像const drawImage = ({ x, y, w, h }) => {ctxImg.clearRect(0, 0, cropImg.width, cropImg.height)ctxImg.drawImage(img, x, y, w, h)}

6.图片旋转

CanvasRenderingContext2D.rotate() - Web API 接口参考 | MDN (mozilla.org)

关于图片旋转这里,就是现将原点移动canvas画布中央(图像中央),然后定义旋转角度,然后又将原点移动回去,然后画的图像就是围绕中心旋转的。

主要是围绕图像的中心原点旋转。

大家可以自行去网上找关于rotate让图像围绕中心原点旋转的问题。因为作者也没能明白。

// 主要是这段代码:

            ctxImg.translate(cropImg.width / 2, cropImg.height / 2)
           ctxImg.rotate(angle)
           ctxImg.translate(-cropImg.width / 2, -cropImg.height / 2)

	// 向右、左转(每次向左30度、向右30度)图片const imgRotate = (e, t = 1) => {if (img == null) {alert('请先上传图片')return}if (t == 1) {// 向右转，逆时针angle += -30 * (Math.PI / 180)} else {// 向左转，顺时针angle += 30 * (Math.PI / 180)}ctxImg.clearRect(0, 0, cropImg.width, cropImg.height)ctxImg.translate(cropImg.width / 2, cropImg.height / 2)ctxImg.rotate(angle)ctxImg.translate(-cropImg.width / 2, -cropImg.height / 2)drawImage(initImgObj)imgPreview()}

7.图片扩大和缩小

通过initImgObj对象，记录图像渲染到图片渲染canvas的尺寸和坐标。

// 图像渲染到到canvas的坐标尺寸和坐标let initImgObj = {// 主要是为了缩放/扩大图片用x: 0,y: 0,w: 0,h: 0,}

通过computeImage()函数,计算出图像要在canvas画布中央实际渲染的坐标和尺寸。

// 计算图像在canvas实际渲染的图片坐标和宽高。(居于canvas中央,尺寸小于画布尺寸)const computeImage = function ({ imgWidth, imgHeight, width, height, base = 1 }) {if (imgWidth / base < width && imgHeight / base < height) {return {x: (width - imgWidth) / 2,y: (height - imgHeight) / 2,w: imgWidth / base,h: imgHeight / base,}}return computeImage({imgWidth,imgHeight,width,height,base: base + 0.1,})}

通过计算 initImgObj对象的放大、缩小后的宽高,然后通过computeImage()方法计算出canvas实际渲染的图像坐标和尺寸,并渲染。

// 缩小/扩大图片const imgScale = (e, t) => {if (img == null) {alert('请先上传图片')return}let { w, h } = initImgObjif (t == 1) {// 缩小w = w / 2h = h / 2} else {// 扩大w = w * 2h = h * 2}// 重新求出，缩小、放大之后的图片宽高和坐标。initImgObj = computeImage({imgWidth: w,imgHeight: h,width: cropImg.width,height: cropImg.height,})drawImage(initImgObj)imgPreview()}

8.图片预览

ImageData - Web API 接口参考 | MDN (mozilla.org)

通过选择框拖拽,记录的 selectCropObj对象的坐标和宽高信息。

通过getImageData()方法获取图片渲染canvas里面的ImageData对象,并通过putImageData()方法将这个对象渲染到图片预览canvas里面。

// 图片预览const imgPreview = () => {ctxPreview.clearRect(0, 0, cropPreview.width, cropPreview.height)let { x, y, w, h } = selectCropObjlet imgData = ctxImg.getImageData(x, y, w, h)ctxPreview.putImageData(imgData, x, y)}

9.保存预览图片

HTMLCanvasElement.toBlob() - Web API 接口参考 | MDN (mozilla.org)

HTMLCanvasElement.toDataURL() - Web API 接口参考 | MDN (mozilla.org)

我们通过,canvas的toBlob()方法将canvas画布转换为blobUrl,然后就是创建一个a 元素,并将这个url封装成a的href属性,用日期作为下载的图片名字，用js模拟点击实现下载。

// 保存裁剪图片const saveImg = (e) => {if (img == null) {alert('请先上传图片')return}cropPreview.toBlob((blob) => {// 下载图片let a = document.createElement('a')a.href = window.URL.createObjectURL(blob)a.download = `${getDateStr(new Date())}.png` a.dispatchEvent(new MouseEvent('click'))})}

完整代码

<!DOCTYPE html>
<html lang="en"><head><meta charset="UTF-8" /><meta name="viewport" content="width=device-width, initial-scale=1.0" /><title>图片裁剪工具</title><style>* {padding: 0;margin: 0;}#crop-tool {position: relative;overflow: hidden;width: 500px;height: 300px;margin: 100px auto;border: 1px solid black;border-radius: 15px;}#crop-img {z-index: -1;top: 0;left: 0;background: url('./img/mosaic.jpg');}#crop-cardbg {position: absolute;top: 0;left: 0;}#crop-modal {position: absolute;top: 0;left: 0;}#crop-clip {position: absolute;left: 25px;top: 37.1364px;cursor: all-scroll;width: 308.091px;height: 197.864px;border: 1px solid rgb(30, 158, 251);}#crop-modal {position: absolute;left: 0;top: 0;width: 500px;height: 300px;line-height: 300px;font-size: 30px;font-weight: 700;text-align: center;color: white;background-color: rgba(0, 0, 0, 0.5);}#crop-preview {border: 1px solid black;border-radius: 15px;width: 500px;height: 300px;margin: 0 700px;}#crop-but {width: 400px;margin: 0 auto;}button {width: 50px;}#crop-clip .dot {position: absolute;width: 20px;height: 20px;border-radius: 50%;background: #1e9efb;}#crop-clip .top-left {top: -10px;left: -10px;cursor: nwse-resize;}#crop-clip .top-right {top: -10px;right: -10px;cursor: nesw-resize;}#crop-clip .top-center {top: -10px;left: 50%;transform: translate(-50%);cursor: ns-resize;}#crop-clip .bottom-left {bottom: -10px;left: -10px;cursor: nesw-resize;}#crop-clip .bottom-center {bottom: -10px;left: 50%;transform: translate(-50%);cursor: ns-resize;}#crop-clip .bottom-right {bottom: -10px;right: -10px;cursor: nwse-resize;}#crop-clip .left-center {top: 50%;transform: translateY(-50%);left: -10px;cursor: ew-resize;}#crop-clip .right-center {top: 50%;transform: translateY(-50%);right: -10px;cursor: ew-resize;}</style><!-- <link rel="stylesheet" href="./css/iconfont.css" /> --></head><body><div id="crop-tool"><canvas id="crop-img" width="500" height="300"></canvas><canvas id="crop-cardbg" width="500" height="300"></canvas><div id="crop-clip" style="display: none"><div class="dot top-left"></div><div class="dot top-right"></div><div class="dot top-center"></div><div class="dot bottom-left"></div><div class="dot bottom-center"></div><div class="dot bottom-right"></div><div class="dot left-center"></div><div class="dot right-center"></div></div><div id="crop-modal">请先上传图片</div></div><canvas id="crop-preview" width="500" height="300"> </canvas><div id="crop-but"><input type="file" id="up-inp" name="文件上传" style="display: none" /><button type="button" id="up-but">上传</button><button type="button" id="add-but">+</button><button type="button" id="del-but">-</button><button type="button" id="xzz-but">left<!-- <span class="iconfont icon-xiangzuoxuanzhuan"></span> --></button><button type="button" id="xyz-but">right<!-- <span class="iconfont icon-xiangyouxuanzhuan"></span> --></button><button type="button" id="save-but">截图</button></div></body><script src="./js/Index.js"></script><script>///variable///// 按钮组let upBut = document.getElementById('up-but')let saveBut = document.getElementById('save-but')let addBut = document.getElementById('add-but')let delBut = document.getElementById('del-but')let xzzBut = document.getElementById('xzz-but')let xyzBut = document.getElementById('xyz-but')let upInp = document.getElementById('up-inp')let cropImg = document.querySelector('#crop-img') // 背景层let cropCardbg = document.querySelector('#crop-cardbg') // 裁剪层let cropPreview = document.querySelector('#crop-preview') // 裁剪层let cropModal = document.querySelector('#crop-modal') // 遮罩层let clip = document.getElementById('crop-clip') // 选择框let ctxCardbg = cropCardbg.getContext('2d')let ctxImg = cropImg.getContext('2d')let ctxPreview = cropPreview.getContext('2d')// 在canvas的裁剪框尺寸和坐标let selectCropObj = {x: 0,y: 0,w: 0,h: 0,}// 一开始存储到canvas的坐标尺寸和坐标let initImgObj = {// 主要是为了缩放/扩大图片用x: 0,y: 0,w: 0,h: 0,}let img = nulllet down = false // 中心拖拽或者按钮拖拽let angle = 0 // 旋转角度///method///upBut.addEventListener('click', (e) => {upInp.click()})// 上传图片const updateFile = function (e) {let file = e.target.files[0]if (!file.type.startsWith('image')) {alert('只允许上传图片')return}const reader = new FileReader()reader.onload = (e) => {let img1 = new Image()img1.src = e.target.resultimg = img1img1.onload = (e) => {// 读取完毕之后selectCropObj = computeImage({imgWidth: img.width,imgHeight: img.height,width: cropCardbg.width,height: cropCardbg.height,})initImgObj = JSON.parse(JSON.stringify(selectCropObj))drawImage(initImgObj)drawModal()drawClip()drawClipDiv()imgPreview()cropModal.style.display = 'none'clip.style.display = 'block'}}reader.readAsDataURL(file)}upInp.addEventListener('change', updateFile)// 保存裁剪图片const saveImg = (e) => {if (img == null) {alert('请先上传图片')return}cropPreview.toBlob((blob) => {let a = document.createElement('a')a.href = window.URL.createObjectURL(blob)a.download = `${getDateStr(new Date())}.png`a.dispatchEvent(new MouseEvent('click'))})}saveBut.addEventListener('click', saveImg)// 图片预览const imgPreview = () => {ctxPreview.clearRect(0, 0, cropPreview.width, cropPreview.height)let { x, y, w, h } = selectCropObjlet imgData = ctxImg.getImageData(x, y, w, h)ctxPreview.putImageData(imgData, x, y)}// 缩小/扩大图片const imgScale = (e, t) => {if (img == null) {alert('请先上传图片')return}let { w, h } = initImgObjif (t == 1) {// 缩小w = w / 2h = h / 2} else {// 扩大w = w * 2h = h * 2}// 重新求出，缩小，方法的图片坐标initImgObj = computeImage({imgWidth: w,imgHeight: h,width: cropImg.width,height: cropImg.height,})drawImage(initImgObj)imgPreview()}// 向右、左转(每次向左90度、向右90度)图片const imgRotate = (e, t = 1) => {if (img == null) {alert('请先上传图片')return}if (t == 1) {// 向右转，逆时针angle += -30 * (Math.PI / 180)} else {// 向左转，顺时针angle += 30 * (Math.PI / 180)}ctxImg.clearRect(0, 0, cropImg.width, cropImg.height)ctxImg.translate(cropImg.width / 2, cropImg.height / 2)ctxImg.rotate(angle)ctxImg.translate(-cropImg.width / 2, -cropImg.height / 2)drawImage(initImgObj)imgPreview()}delBut.addEventListener('click', (e) => {imgScale(e, 1)})addBut.addEventListener('click', (e) => {imgScale(e, 2)})xzzBut.addEventListener('click', (e) => {imgRotate(e, 1)})xyzBut.addEventListener('click', (e) => {imgRotate(e, 2)})// 画模版const drawModal = function () {ctxCardbg.clearRect(0, 0, cropCardbg.width, cropCardbg.height)ctxCardbg.fillStyle = 'rgba(0,0,0,0.5)'ctxCardbg.fillRect(0, 0, cropCardbg.width, cropCardbg.height)}// 计算图片在canvas实际坐标和尺寸const computeImage = function ({ imgWidth, imgHeight, width, height, base = 1 }) {if (imgWidth / base < width && imgHeight / base < height) {return {x: (width - imgWidth) / 2,y: (height - imgHeight) / 2,w: imgWidth / base,h: imgHeight / base,}}return computeImage({imgWidth,imgHeight,width,height,base: base + 0.1,})}// 画图像const drawImage = function ({ x, y, w, h }) {ctxImg.clearRect(0, 0, cropImg.width, cropImg.height)ctxImg.drawImage(img, x, y, w, h)}// 挖空const drawClip = function () {ctxCardbg.save()ctxCardbg.clearRect(selectCropObj.x, selectCropObj.y, selectCropObj.w, selectCropObj.h)ctxCardbg.restore()}// 挖空时,选择框的变化const drawClipDiv = () => {let cropClip = document.getElementById('crop-clip')cropClip.style.width = `${selectCropObj.w}px`cropClip.style.height = `${selectCropObj.h}px`cropClip.style.left = `${selectCropObj.x}px`cropClip.style.top = `${selectCropObj.y}px`}// 卡片拖拽事件,调用的方法(将选择框中央拖拽 ,和点拖拽一个方法处理)const carMouseMove = (e) => {// 两个都是false,证明我们一个没按下if (img === null) {return}if (!down) {return}let ele = e.targetlet { movementX, movementY } = econst isExistsCls = (_cls) => {for (let i = 0; i < ele.classList.length; i++) {const val = ele.classList[i]if (val === _cls) {return true}}return false}// 中央拖拽if (ele.id == 'crop-clip') {selectCropObj.x += movementXselectCropObj.y += movementY// 点拖拽} else if (isExistsCls('top-left')) {// 坐标和宽高的都要变selectCropObj.x += movementXselectCropObj.y += movementYselectCropObj.w += -movementXselectCropObj.h += -movementY} else if (isExistsCls('top-right')) {selectCropObj.y += movementYselectCropObj.w += movementXselectCropObj.h += -movementY} else if (isExistsCls('top-center')) {selectCropObj.y += movementYselectCropObj.h += -movementY} else if (isExistsCls('bottom-left')) {selectCropObj.x += movementXselectCropObj.y += movementYselectCropObj.w += -movementXselectCropObj.h += movementY} else if (isExistsCls('bottom-right')) {selectCropObj.w += movementXselectCropObj.h += movementY} else if (isExistsCls('bottom-center')) {selectCropObj.h += movementY} else if (isExistsCls('left-center')) {selectCropObj.x += movementXselectCropObj.w += -movementX} else if (isExistsCls('right-center')) {selectCropObj.w += movementX}drawClipDiv()drawModal()drawClip()imgPreview()}// 注册选择框拖拽事件const registerEvents = () => {// 注册那8个拖拽点事件const register = (_class) => {let node = document.querySelector(`.${_class}`)node.addEventListener('mousedown', (e) => {down = true})node.addEventListener('mousemove', carMouseMove)node.addEventListener('mouseup', function (e) {down = false})}register('top-center')register('bottom-center')register('left-center')register('right-center')register('bottom-right')register('bottom-left')register('top-right')register('top-left')// 注册拖拽中央移动的事件let clip = document.getElementById('crop-clip')clip.addEventListener('mousedown', (e) => {down = true})clip.addEventListener('mousemove', carMouseMove)clip.addEventListener('mouseup', (e) => {down = false})}registerEvents()</script>
</html>

const getDateStr = (time, tag1 = '-', tag2 = ':') => {const date = new Date(time)let y = date.getFullYear()let M = date.getMonth() + 1let d = date.getDate()let h = date.getHours()let m = date.getMinutes()let s = date.getSeconds()return `${y}${tag1}${M}${tag1}${d} ${h}${tag2}${m}${tag2}${s}`
}