[RESOLVED] Calibrate Corona Sun and Sky Enviro?

[cjwidd]

I have seen plenty of articles about calibrating HDRIs, including Dubcat's original post on this forum, but is there a way to calibrate the Corona Sun and Sky in a similar way?

[romullus]

What exactly do you want to calibrate? Corona sun&sky is already set to physically plausible brightness and colour temperature, there's no need to calibrate anything - just plug and play.

[cjwidd]

Whenever I add a Corona Sun /Sky to the scene, the entire scene is blown out and requires adjustment.

With HDRI there is a way to calibrate the map to a grey ball for accurate representation.

Corona Sun / Sky cannot really be treated the same way(?)

[PROH]

That is done by exposurecontrol :)
I believe Dubcat did mention a "calibrated" value somewhere in his thread (somewhere around -5,xxx?)

Hope it helps

[Juraj]

Dubcat calibrates the HDRi to CoronaSun :- )

The light values of CoronaSun/Sky are physically accurate. The reason default 'exposure' is overblown by about 2 stops is because the default exposure doesn't mean anything at all, it's very much arbitrary (though Ondra does have a very specific term for it, it's basically that Light with value of 1 corresponds to material albedo looking 1:1).

You can use Physical Camera exposure with some exact values (for example f8, 1/256 shutter, ISO 100) and you will see you will get the same results +/- as you would with reality.

[cjwidd]

Juraj, that's interesting, thank you for sharing!

To be sure, I am referring to the fact that at default post-processing settings (exposure: 0) the Sun / Sky is blown out, so the Sun / Sky needs to be dialed down to maybe 0.02(?) for an 18% grey ball to register 0.459 sRGB float / 117 sRGB.\

If the Sun / Sky is overblown by 2 stops, then I could just dial down the Sun / Sky by that amount - I'll give that a try.

[Juraj]

You cannot dial down Sun...you would have to dial down Sky as well...and why would you do it ? (Unless for artistic control..that's good).

EV 0 in Corona doesn't mean anything. Back in time, long time ago, there was such thing as EV tables ( http://www.photographers-resource.co.uk/photography/exposure/EV_table%20.htm ), and EV 1 corresponded to f8, 30 seconds, ISO 100.

MentalRay Physical Camera control uses those tables, it's inverted measure, i.e EV 1 is LOT MORE light than maximum EV 15.

Corona is not using those tables. EV 0 is just EV 0, random exposure. It's exactly what it is on modern digital cameras, exposure offset. If you want more light, moving it into positive direction, if you need less, move it into negative direction.
You don't have to recalibrate Corona Sun at all.

It's already physically accurate at default value.

[cjwidd]

I see, thanks for clarifying, that helps  a lot.

I think the issue arose for me because my predilection is to not adjust the camera exposure and instead adjust the light sources, similar to what Dubcat described.

If the exposure 0 is arbitrary than I suppose it doesn't matter.

[Juraj]

Yes, but Dubcat calibrated arbitrary intensity of HDRi so they correlate to actual physical luminosity :- ). The CoronaSun/Sky already is.

If you create a 900 Lumen lightbulb in Corona, and CoronaSun, the ratio between them is going to be identical to real-world 900 Lumen lightbulb and "RealWorld" Sun.

Calibration is for uncalibrated stuff, to make everything consistent. Corona Sun/Sky already is. The default exposure being overblown with Sun/Sky outdoor is just what throws people off.

[romullus]

In real life, most of the time you're adjusting camera exposure, not the light source, especially if that light source is the sun :] The same principle is true in Corona.

[cjwidd]

Right, and that makes sense, although I really enjoy working with calibrated HDRI libraries because I don't have to constantly adjust the camera based on which HDRI is selected; I can hook up a bunch of calibrated HDRI into a Corona Select and flick through them and get correctly exposed lighting with no fuss, enable Dubcat LUT, and ready to go.

[romullus]

So you "calibrated" your HDRIs that all of them would produce similar brightness at 0EV in Corona VFB? Sure, that's convenient to use (if your light source is HDRI only), but that also means that all of them most likely has physically incorrect brightness and your calibration will be worthless if you'll need to combine HDRI with physically plausible light sources. On the other hand, if you only render cars on the backplates, that's not a problem.

[cjwidd]

That's right, the calibrated HDRIs can have physically incorrect brightness, and that is a drawback, and yes, adding any additional lights will "break" the calibration, but altogether it has been useful for experimenting with different lighting scenarios during look development.

Actually, it was somewhere in Juraj's thread where it was mentioned that he often uses Sun / Sky that I really felt inspired to work with the sun / sky system at all.

[cjwidd]

Just to arrive at the bottom line, my reason for asking questions like this is ultimately driven by a desire to have consistent, accurate, lighting, which is something that seems to elude me. After reading Dubcat's post I thought certainly this was the answer; after all, this is the point of grey ball lighting calibration(?)

Then, as I ask around or review other artist's projects, it just seems like the Wild West from project to project - everyone seems to just arrive at a pleasing result through some arbitrary sequence of adjustments of HDRI, physical lights, post processing, etc.

[cjwidd]

Well, I reset the sun / sky to default and set camera exposure to -5.25 and the image is almost exactly the same (!)

Thanks for the tips everyone +1