const RED_PREFERENCE = 0.1;

const WEIGHTS = {

r: 0.299 + RED_PREFERENCE,

g: 0.587 + RED_PREFERENCE * -0.5,

b: 0.114 + RED_PREFERENCE * -0.5,

};

export function canvasDataToGrayscale(canvasData: ImageData): {

canvasData: ImageData;

averageLightness: number;

} {

let pixels = canvasData.data;

let minLightness = Infinity;

let maxLightness = 0;

let averageLightness = 0;

for (let i = 0; i < pixels.length; i += 4) {

let lightness =

pixels[i] * WEIGHTS.r +

pixels[i + 1] * WEIGHTS.g +

pixels[i + 2] * WEIGHTS.b;

if (pixels[i + 3] < 128) {

lightness = 255;

}

minLightness = Math.min(minLightness, lightness);

maxLightness = Math.max(maxLightness, lightness);

averageLightness += lightness;

}

averageLightness = Math.round(averageLightness / (pixels.length / 4));

minLightness += 32;

maxLightness -= 32;

const contrast = 255 / (maxLightness - minLightness);

for (let i = 0; i < pixels.length; i += 4) {

let lightness =

pixels[i] * WEIGHTS.r +

pixels[i + 1] * WEIGHTS.g +

pixels[i + 2] * WEIGHTS.b;

pixels[i] = Math.round(lightness * contrast) - minLightness;

pixels[i + 1] = Math.round(lightness * contrast) - minLightness;

pixels[i + 2] = Math.round(lightness * contrast) - minLightness;

}

return { canvasData, averageLightness };

}