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。所以就有了这篇文章。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,这里取2字节,只考虑height为正数。(为负数时图像倒置) 
        let height = readUint16LE(bytes, 22); 
        let width = readUint16LE(bytes, 18); 
        return { width, height } 
    } 
}


NPM

npm i image-dimensionjs

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