JS 獲取 Image 圖像 寬高 尺寸

簡介
項目中用到獲取圖片的原始尺寸，然后適配寬高；網上的大部分前端解決方案，都是new Image()后,在onload事件中獲取image的尺寸。
在圖片數量較多的時候，這樣的獲取效率實在是低下。所以就有了這篇文章。通過直接讀取解析文件的字節碼來獲取圖片的尺寸。

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，寬在后。
                            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()) {
        //雖然格式為4字節，這里取2字節，只考慮height為正數。（為負數時圖像倒置）
        let height = readUint16LE(bytes, 22);
        let width = readUint16LE(bytes, 18);
        return { width, height }
    }
}

 

NPM

npm i image-dimensionjs