Did a few more experiments to drill down on the blue shift value more. As I still wasn’t happy with the 5.4% figure, even though it looked good in most cases. We can see from my last post’s histogram that it was just a hair too blue.
So I went back to the baseline and tested a bunch of levels. 4.5% works great at most points as Ibex predicted, but it specifically falls apart when the albedo gets very high (above 0.95) and begins to turn yellow again. I didn’t notice anywhere else that it didn’t perform well, as you’ll see below.
Maybe this doesn’t matter, as Krzyztof said, as no materials reflect this much light except mirrors. But I wanted to push on anyway. I found a value of 6.8% corrected the yellowing at a value of 1.
So I decided to color correct using a remapped range. Anything below 0.95 would use a 4.5% adjustment, and anything above 0.95 would begin to lerp towards 6.8%.
Here is an image and histogram of every 0.05 increment from 0.6-1.0 showing proper white balance across the full range of values.
Its not perfect, but it seems nearly so across the full range. At low albedo levels I would disable the effect, as the reflected light no longer contributes enough to be noticeably colored, but the slight tint in reflections would be.
I don’t expect this will come up often or ever realistic scenes. Maybe in a bright, white marble room.
Here’s an example node group. From here on out I propose this technique be nicknamed Blumen.
Unless you actually need to use the 0.95 range for some reason, I’d probably start with a flat figure like 4.5 and then adjust as needed.
Anyway, I think I’ve beat this horse past dead so I’ll move on from here and hopefully this proves useful to some of you.