ycbcr.js

const sharp = require("sharp");

(async () => {

const img = sharp("test.png");
const meta = await img.metadata();
const raw = await img.raw().toBuffer();

const out = Buffer.alloc(meta.width * meta.height * 3);
for(let x = 0; x < meta.width; x++) {
    for(let y = 0; y < meta.height; y++) {
        
        const idx = (y * meta.width + x) * 3;
        const r = raw[idx], g = raw[idx + 1], b = raw[idx + 2];
        
        //const Y = 0.299*r + 0.587*g + 0.114*b;
        Y = 128;
        //const cb = -0.1687*r - 0.3313*g + 0.5*b + 128;
        cb = 128;
        const cr = 0.5*r - 0.4187*g - 0.0813*b + 128;
        

        //out[idx] = out[idx + 1] = out[idx + 2] = Y;
        out[idx] = Y + 1.402*(cr-128);
        out[idx + 1] = Y - 0.34414 * (cb-128) - 0.71414 * (cr-128);
        out[idx + 2] = Y + 1.772*(cb - 128);

    }
}

await sharp(out, {raw: {width: meta.width, height: meta.height, channels: 3}}).png().toFile("y.png");

})();