"For colour images the first step would be to convert to grayscale"

That's what was in my mind over night. When I used that formular, I kept thinking that I am calculating luminous.

My major in university is related to this, and I kept thinking what's the difference now and then. One thing I remembered is that, the equipment we used only take black and white pictures, so I started to think that first thing today is to convert picture to black and white.

Now your post is a great remind to me, and confirmed my thought. Thank you!