Transform a blurhash to grayscale

Does anyone have an idea of how we can implement an efficient algorithm (js/ts) that given a blurhash string returns the equivalent (even of reduced quality) but in grayscale?


I need it because I use blurhashes as image holders, but in a part of my application I need to display images in black and white. Thanks in advance

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EDIT


I managed to arrive at a code like this. the problem is that the blurhashes seem stretched and deformed compared to the originals
const WIDTH = 30;
const ASPECT_RATIO = validationConfig.media.profilePicture.aspectRatio;
const HEIGHT = Math.floor(WIDTH / ASPECT_RATIO);

export function convertBlurhashToGreyscale(blurhash: string) {

const pixels = decode(blurhash, WIDTH, HEIGHT);

for (let i = 0; i < pixels.length; i += 4) {
const grayValue = Math.round(
(pixels[i] + pixels[i + 1] + pixels[i + 2]) / 3
);

pixels[i] = grayValue;
pixels[i + 1] = grayValue;
pixels[i + 2] = grayValue;
pixels[i + 3] = 255; // Imposta l'opacità a 255
}

const newHeight = Math.floor(WIDTH / ASPECT_RATIO);
return encode(pixels, WIDTH, newHeight, 4, 3);
}