前端 JS 获取 Image 图像 宽高 尺寸

简介

项目中用到获取图片的原始尺寸，然后适配宽高；网上的大部分前端解决方案，都是new Image()后,在onload事件中获取image的尺寸。

在图片数量较多的时候，这样的获取效率实在是低下。所有就有了这篇文章。通过直接读取解析文件的字节码来获取图片的尺寸。

IMAGE_HEAD_SIGS

var IMAGE_HEAD_SIGS = {

GIF: [0x47, 0x49, 0x46], //'G' 'I' 'F' ascii

PNG: [0x89, 0x50, 0x4e, 0x47, 0x0d, 0x0a, 0x1a, 0x0a],

JPG: [0xff, 0xd8, 0xff, 0xe0],

BMP: [0x42, 0x4d]

}

PNG

function ReadPNG(bytes) {

if (bytes.slice(0, 8).toString() === IMAGE_HEAD_SIGS.PNG.toString()) {

let width = readUint32BE(bytes, 16);

let height = readUint32BE(bytes, 20);

return { width, height }

}

}

JPG

function ReadJPG(bytes) {

if (bytes.slice(0, 4).toString() === IMAGE_HEAD_SIGS.JPG.toString()) {

const M_SOF0 = 0xC0; /* Start Of Frame N */

const M_SOF1 = 0xC1; /* N indicates which compression process */

const M_SOF2 = 0xC2; /* Only SOF0-SOF2 are now in common use */

const M_SOF3 = 0xC3;

const M_SOF5 = 0xC5; /* NB: codes C4 and CC are NOT SOF markers */

const M_SOF6 = 0xC6;

const M_SOF7 = 0xC7;

const M_SOF9 = 0xC9;

const M_SOF10 = 0xCA;

const M_SOF11 = 0xCB;

const M_SOF13 = 0xCD;

const M_SOF14 = 0xCE;

const M_SOF15 = 0xCF;

for (let i = 0; i < bytes.length; i++) {

if (bytes[i] === 0xFF) {

switch (bytes[i + 1]) {

case M_SOF0:

case M_SOF1:

case M_SOF2:

case M_SOF3:

case M_SOF5:

case M_SOF6:

case M_SOF7:

case M_SOF9:

case M_SOF10:

case M_SOF11:

case M_SOF13:

case M_SOF14:

case M_SOF15:

{

//高在前，宽在后。

let width = readUint16BE(bytes, i + 7)

let height = readUint16BE(bytes, i + 5)

return { width, height }

}

default:

break;

}

}

}

}

}

GIF

function ReadGIF(bytes) {

if (bytes.slice(0, 3).toString() === IMAGE_HEAD_SIGS.GIF.toString()) {

let width = readUint16LE(bytes, 6);

let height = readUint16LE(bytes, 8);

return { width, height }

}

}

BMP

function ReadBMP(bytes) {

if (bytes.slice(0, 2).toString() === IMAGE_HEAD_SIGS.BMP.toString()) {

//虽然格式为4字节，这里只取2字节，确保height为正数。为负数时，图像为倒置图像。

let height = readUint16LE(bytes, 22);

let width = readUint16LE(bytes, 18);

return { width, height }

}

}

NPM

npm i image-dimensionj