3. Grayscale Conversion

Before dither can be applied, the RGB colorspace needs to be converted to grayscale. This pixel2grayscale function will take in an RGB pixel and return an integer between 0 and 225.

A standard linear conversion is used (no gamma correction). Not great, but it's fast and good enough.

static int pixel2grayscale(monolith_pixel *p)
{
    double oned255;
    double r, g, b;
    double gray;

    oned255 = 1.0 / 255;

    r = p->r * oned255;
    g = p->g * oned255;
    b = p->b * oned255;

    gray = 0.2126 * r + 0.715 * g + 0.0722 * b;

    return floor(gray * 255);
}

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