apply a grayscale filter with a pre-build contrast brightness option. (ES6)

grayscale.js

grayscale(pixels, contrast, brightness) {
    const d = pixels.data;
    for (let i = 0; i < d.length; i += 4) {
        const r = d[i + 0];
        const g = d[i + 1];
        const b = d[i + 2];
        const a = d[i + 3];
        let v = 0.2126 * r + 0.7152 * g + 0.0722 * b;

        if (contrast && brightness) {
            v /= a;

            // Apply contrast.
            v = ((v - 0.5) * Math.max(contrast, 0)) + 0.5;

            // Apply brightness.
            v += brightness;

            // Return final pixel color.
            v *= a;
        }

        d[i + 0] = d[i + 1] = d[i + 2] = v;
    }
    return pixels;
}