18% Gray

[glorybound]

How do I make this box 18% gray in the viewport? I usually do it in photoshop, but I would like to do it with the cameras white balance. The box is 128, 128, 128.

[Morne]

from the values, you can see that the image has more red and green than blue. So you need to compensate for this by doing same with cam. Making the white balance more red, will introduce more blue in the image.
So,
take your "98, 89, 81" from your pixel reading there, and stick that into the cams white balance. Then its a case of adjusting exposure for the brightness

[piotrus3333]

128 rgb in max's color swatch for diffuse of VRayMtl is 50% gray.
trims around the windows should be white, right?

[Ihno]

Put a vray color in the material diffuse and set it to 0.18 x 0.18 x 0.18.
Then you get a representation of a real greycard with the advantage of not having reflections on it (unlike the real thing).
But arend 18% greycards used to get the exposure right in the real world?
They should also work for white balancing, though.
You can put such material in the material override and use the auto white balance to get the right number in vray.
That's even more accurate, since there will be no color bleeding from colored surfaces on your grey card/box.

[glorybound]

128 rgb in max's color swatch for diffuse of VRayMtl is 50% gray.
trims around the windows should be white, right?

No, trim is gray. The box is 128, 128, 128

[glorybound]

Put a vray color in the material diffuse and set it to 0.18 x 0.18 x 0.18.
Then you get a representation of a real greycard with the advantage of not having reflections on it (unlike the real thing).
But arend 18% greycards used to get the exposure right in the real world?
Both should work, though.
You can put such material in the material override and use the auto white balance to get the right number in vray.
That's even more accurate, since there will be no color bleeding from colored surfaces on your grey card/box.

I haven't had much luck with auto-exposure, but I'll try it with a gray override.