JS对图片尺寸和DPI进行编辑修改（1寸照修改为2寸照）

各种报名都对照片有大小限制，鉴于这种情况，网上搜了后拼凑出了如下代码，用于解决1寸照片修改为2寸照片，同时将DPI修改为300，当然也可以根据自己的情况修改代码：

HTML

<input type="file" id="input" accept="image/*">
<div style="display: grid;grid-template-columns: 1fr 5px 1fr;"><div><p>修改前：</p><img id="before-image" src="" style="display: block; max-width:100%;" ></div><div></div><div><p>修改后（点击图片下载）：</p><a href="javascript:;" download><img id="after-image" src="" style="display: block;"></a></div>
</div>

JS

<script type="text/javascript">const after_image = document.getElementById('before-image');document.getElementById('input').addEventListener('change', (e) => {const reader = new FileReader();reader.readAsDataURL(e.target.files[0]);reader.onload = (e) => {b64 = e.target.result;after_image.src = b64;//设置目标图片大小var target_width = 413;var target_height = 626;//计算目标图片宽高比例var target_wh_scale = target_width / target_height;//定义一个Image对象var bitmap = new Image();bitmap.src = b64;bitmap.onload = function () {//var cut_width = 0;var cut_height = 0;//var bitmap_wh_scale = bitmap.width / bitmap.height;if (bitmap_wh_scale > target_wh_scale) {cut_width = bitmap.width - bitmap.width / (bitmap_wh_scale / target_wh_scale);}//裁剪宽度else if (bitmap_wh_scale < target_wh_scale) {cut_height = bitmap.height - bitmap.height * (bitmap_wh_scale / target_wh_scale);} else {Console.log("比例一致无需裁剪");}console.log("图片裁剪宽度：" + cut_width + " px");console.log("图片裁剪高度：" + cut_height + " px");const canvas = document.createElement('canvas');const ctx = canvas.getContext('2d');canvas.width = target_width;canvas.height = target_height;ctx.drawImage(this, cut_width / 2, cut_height / 2, bitmap.width - cut_width, bitmap.height - cut_height, 0, 0, target_width, target_height);var after_img = document.getElementById('after-image');after_img.src = canvas.toDataURL('image/jpg');var dataUrl = canvas.toDataURL('image/jpeg', 0.9);//修改DPI为300downloadBase64Img(after_img.parentElement, changeDpiDataUrl(dataUrl, 300), "t.jpg");}};})function downloadBase64Img(a, base64URL, fileName) {// 将 a 标签的 download 属性设置为要下载的文件名a.download = fileName || 'image';// 创建 Blob 对象，并获取 base64 数据的 MIME 类型const mimeType = base64URL.match(/:(.*?);/)[1];// 将 base64 数据转换为字节数组const byteCharacters = atob(base64URL.split(',')[1]);const byteNumbers = new Array(byteCharacters.length);// 将字节数组填充到 Uint8Array 中for (let i = 0; i < byteCharacters.length; i++) {byteNumbers[i] = byteCharacters.charCodeAt(i);}const byteArray = new Uint8Array(byteNumbers);// 创建 Blob 对象const blob = new Blob([byteArray], { type: mimeType });// 将 Blob 对象的 URL 赋值给 a 标签的 href 属性a.href = URL.createObjectURL(blob);}function changeDpiDataUrl(base64Image, dpi) {const PNG = 'image/png';const JPEG = 'image/jpeg';const dataSplitted = base64Image.split(',');const format = dataSplitted[0];const body = dataSplitted[1];let type;let headerLength;let overwritepHYs = false;if (format.indexOf(PNG) !== -1) {type = PNG;const b64Index = detectPhysChunkFromDataUrl(body);// 28 bytes in dataUrl are 21bytes, length of phys chunk with everything inside.if (b64Index >= 0) {headerLength = Math.ceil((b64Index + 28) / 3) * 4;overwritepHYs = true;} else {headerLength = 33 / 3 * 4;}}if (format.indexOf(JPEG) !== -1) {type = JPEG;headerLength = 18 / 3 * 4;}// 33 bytes are ok for pngs and jpegs// to contain the information.const stringHeader = body.substring(0, headerLength);const restOfData = body.substring(headerLength);const headerBytes = atob(stringHeader);const dataArray = new Uint8Array(headerBytes.length);for (let i = 0; i < dataArray.length; i++) {dataArray[i] = headerBytes.charCodeAt(i);}const finalArray = changeDpiOnArray(dataArray, dpi, type, overwritepHYs);const base64Header = btoa(String.fromCharCode(...finalArray));return [format, ',', base64Header, restOfData].join('');}function changeDpiOnArray(dataArray, dpi, format, overwritepHYs) {const PNG = 'image/png';const JPEG = 'image/jpeg';if (format === JPEG) {dataArray[13] = 1; // 1 pixel per inch or 2 pixel per cmdataArray[14] = dpi >> 8; // dpiX high bytedataArray[15] = dpi & 0xff; // dpiX low bytedataArray[16] = dpi >> 8; // dpiY high bytedataArray[17] = dpi & 0xff; // dpiY low bytereturn dataArray;}if (format === PNG) {const physChunk = new Uint8Array(13);// chunk header pHYs// 9 bytes of data// 4 bytes of crc// this multiplication is because the standard is dpi per meter.const _P = 'p'.charCodeAt(0);const _H = 'H'.charCodeAt(0);const _Y = 'Y'.charCodeAt(0);const _S = 's'.charCodeAt(0);dpi *= 39.3701;physChunk[0] = _P;physChunk[1] = _H;physChunk[2] = _Y;physChunk[3] = _S;physChunk[4] = dpi >>> 24; // dpiX highest bytephysChunk[5] = dpi >>> 16; // dpiX veryhigh bytephysChunk[6] = dpi >>> 8; // dpiX high bytephysChunk[7] = dpi & 0xff; // dpiX low bytephysChunk[8] = physChunk[4]; // dpiY highest bytephysChunk[9] = physChunk[5]; // dpiY veryhigh bytephysChunk[10] = physChunk[6]; // dpiY high bytephysChunk[11] = physChunk[7]; // dpiY low bytephysChunk[12] = 1; // dot per meter....const crc = calcCrc(physChunk);const crcChunk = new Uint8Array(4);crcChunk[0] = crc >>> 24;crcChunk[1] = crc >>> 16;crcChunk[2] = crc >>> 8;crcChunk[3] = crc & 0xff;if (overwritepHYs) {const startingIndex = searchStartOfPhys(dataArray);dataArray.set(physChunk, startingIndex);dataArray.set(crcChunk, startingIndex + 13);return dataArray;} else {// i need to give back an array of data that is divisible by 3 so that// dataurl encoding gives me integers, for luck this chunk is 17 + 4 = 21// if it was we could add a text chunk contaning some info, untill desired// length is met.// chunk structur 4 bytes for length is 9const chunkLength = new Uint8Array(4);chunkLength[0] = 0;chunkLength[1] = 0;chunkLength[2] = 0;chunkLength[3] = 9;const finalHeader = new Uint8Array(54);finalHeader.set(dataArray, 0);finalHeader.set(chunkLength, 33);finalHeader.set(physChunk, 37);finalHeader.set(crcChunk, 50);return finalHeader;}}}
</script>

以上代码中changeDpiDataUrl()和changeDpiOnArray()两个函数来源于开源项目changeDPI。