18% whaat?

Hello world.

Almost half a year without a single post. Just as I feared. This sad story ends now with some warmed up information about colour science that I tend to forget all the time. So yes, I am basically blogging to myself for now but I hope this will be help for people who stumble upon it.

The topic is the magical „18% Grey“ and what it is used for. It can be found on „Grey-Cards“ used mainly by photographers (to get the correct exposure and whitebalance) or on „Grey-Balls“ used in post production of movies and commercials and also on the Macbeth Color Checker (bottom row, fourth from the left).

Another word for it is „middle-grey“ and that is where the confusion starts because the phrase rather hints at 50% Grey, right? So to get this out of the way you have to know that human vision works non-linear which means that the brightness of the things all around us is not perceived linear. We are much more sensitive to darker shades then to brighter ones.
A physical brightness of 18% is perceived as about half way from white to black. This is why the terms „18% Grey“ and „middle-grey“ are used for the same thing.

Why do I need to know this? Well, if you are in a full CG situation, it’s not that critical because you don’t need to match colours to a plate. Nevertheless it comes in handy if you are working with HDRI lighting. Middle-grey happens to be about the average brightness of the real world, too. So if you expose your HDRI to that value you therefore calibrate the scene. I do this by rendering a diffuse middle-grey-ball and a middle-grey emitting material side by side and tweak the exposure of the HDRI until the two grey materials match in the brightest parts.

When shooting HDRI you need to know about middle-grey at the grading and balancing step. Usually you have a Macbeth Checker in the imagery which helps to find the neutral exposure. Basically you only need to sample the middle-grey spot and grade your image until the value matches.
It gets more complicated if you got gamma or log encoded footage from the camera like JPGs. You have to figure out how to transform the colours back to linear first before you can grade truthfully.

The most crucial case in which you need to know about the term middle-grey is when you are merging live-action plates with CG. I try to explain that (to me) in the next post which I hope to write still in 2016.

Cheers, Michael.