Calibrate HDRI (Dubcat)

[cjwidd]

Following Kim Amland's (Dubcat) HOW TO: Make a Calibrated Material Scene using only physical lights, I arrived at a dissociation between light intensity and tonemapping exposure in two different scenes, which I do not understand.

I understand, however, that exposure is logarithmic, so an exposure of 3 is 2^3=8.

In 'test_dubcat_3' there are three physical lights, each with an intensity of 2.2, tonemapping exposure of 3, and Kim_Amland_Photographic_01 (Opacity: 0.5) enabled.
In 'test_dubcat_4' there are three physical lights, each with an intensity of 18, tonemapping exposure of 0, and Kim_Amland_Photographic_01 (Opacity: 0.5) enabled.

You can see the images are identical. Why is that?

[cjwidd]

I did some experiments based on suggestions from Betrand Benoit, a tip about reducing gamma in the HDRI to punch contrast / shadows. You can see the effects of reducing the gamma to 0.5, 0.6, 0.7, 0.8, and 1.0 (default). This was done in combination with the calibration procedure described by Dubcat, linked above.

First, I *really* appreciate Dubcat sharing this procedure because it is completely invaluable, but this creates a situation in which the optimal decision would be to calibrate my entire HDRI library.

Is there a way to automate this procedure?

[cjwidd]

The answer to the second question seems to be, "No, you must calibrate HDRIs by hand for now". That is to say, there doesn't seem to be a publicly available software that can automate this process.

[romullus]

I'm not sure what you mean by calibration of HDRI. If you think, that altering HDRI's gamma, is calibrating, then i'm affraid you got wrong idea.

[burnin]

For calibration sake, one of basic fundamentals is to have references (i.e. something like xrite color checker passport)
Check Paul Debevec's presentation from Total Chaos 2019 @30:50)

[cjwidd]

I'm not sure what you mean by calibration of HDRI. If you think, that altering HDRI's gamma, is calibrating, then i'm affraid you got wrong idea.

In this case I am referring to the process by which an HDRI is calibrated such that the brightest value on a diffuse grey ball is equal to 18% grey (0.459 sRGB float / 117 sRGB). Adjusting HDRI gamma is another option to influence scene lighting, but is separate from the calibration process.