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[Vray] - Exposure Color Correction physical cam
show user profile  9krausec
Got a question for some of you experience Vray users...

Why keep Exposure color correction on if you are looking for a balanced, "perfectly exposed" image? Is there a good reason to keep it on 100% of the time?

I understand that with it off, you are in essence removing the physical part of the physical camera, but you in turn eliminate the need to balance f-stop, iso, shutter speed to reach perfect exposure...

Just wondering as in my current situation I'm overriding the environment BG to be 100% white and I find myself modifying the ISO until the BG hits 255,255,255...

I'm going to post this in the Choasgroup forum too. Thanks guys.

- Portfolio-

read 488 times
2/22/2016 8:08:35 PM (last edit: 2/22/2016 8:08:35 PM)
show user profile  donvella
using the exposure control gives u the ability to render the 'perspective' view as per the camera specs (iso, shutter etc). It has other uses but for what you are trying to achieve by overriding the background to white and keeping your settings set the environment value to 0.0 (this means it will use the white color but not affect the GI)

This should give u the affect you are after

read 479 times
2/22/2016 11:05:10 PM (last edit: 2/22/2016 11:05:10 PM)
show user profile  9krausec
Thanks for the reply. Maya/vray is pretty straight forward about affecting GI vs BG. I'm doing this without affecting the GI. However still when only affecting the Background and not the GI, the background changes with the camera settings as long as Exposure color correction is turned on.

- Portfolio-

read 475 times
2/22/2016 11:20:42 PM (last edit: 2/22/2016 11:20:42 PM)
show user profile  donvella
if you want this to remain the same press 8 - use a background color. while having the environment set to 0.0 the background should remain the same colour despite the camera settings

its not so straight forward but the background affects the GI... unless you set the env override to 0.0

same with the environment color, that affects the GI... unless you set it to 0.0

Its a bit wierd but this enables you to have a background plate, and a different environment color/hdri, and enable each one not to affect or to affect the GI depending on what ur trying to do. It makes sense once u get yur head around it but it would be good if it were like maya and had a checkbox to say - do not affect gi or something similar. The logic is basically: use background to light GI, if not use ENV, if not use value 1.0 or 0.0... to this affect

read 465 times
2/23/2016 12:37:23 AM (last edit: 2/23/2016 12:43:50 AM)
show user profile  9krausec
Interesting little tid-bit from Vlado back in 2007-

"The problem:

We want to obtain an exact RGB value for some (small) surface in our image. Let's say the RGB value is xr, xg, xb which range from 0 to 255.

The solution:

1) Render the scene in any way you want with any settings you want, but make sure you use Linear color mapping with 1.0 for both Bright and Dark multipliers and the "Clamp output" option is off, and you are using the V-Ray camera "Exposure" setting. It would also be handy if you render to the V-Ray VFB.

2) Measure the RGB value at the pixel you need to match. If the values are, let's say (pr, pg, pb) again from 0 to 255, adjust the V-Ray camera "White balance" to be (pr*255/xr, pg*255/xg, pb*255/xb) and multiply the camera ISO setting by (xr+xg+xb)/(pr+pg+pb).

Note that this works only for Linear color mapping. In principle it can be done for any color mapping, but the numbers are hard to find by hand.

Best regards,

- Portfolio-

read 442 times
2/23/2016 2:29:56 PM (last edit: 2/23/2016 2:29:56 PM)
show user profile  Bolteon
"Note that this works only for Linear color mapping"

That's the only way you should ever be doing 3D in. Everyone included.

That's a nice and quick equation to get the base gist of what is essentially digital light metering.

-Marko Mandaric

read 436 times
2/23/2016 4:14:53 PM (last edit: 2/23/2016 4:14:53 PM)
show user profile  9krausec
I've got the itch to python a more automated solution for that equation. Mmm.

- Portfolio-

read 432 times
2/23/2016 5:25:19 PM (last edit: 2/23/2016 5:25:19 PM)
show user profile  donvella
which is why I find it strange the latest version of vray comes with reinhard as default

read 420 times
2/24/2016 12:29:42 AM (last edit: 2/24/2016 12:29:42 AM)
show user profile  Bolteon
Thinking about this some more... One of the things that we did on the MS work, was the adjustment that was made at the start of a shot.

We'd take the RGB of a material; and dial it down about 20% via a gamma adjustment node so that a light set to a power of 1 (while the PhysicalCam settings were disabled) rendered out a surface that matched what the initial goal RGB value was.

Seeing that without the adjustment to the material, a surface would always read 20% brighter in the middle of the tonal range we were working in.

This leads me to believe that materials by definition aren't in linear space to begin with but rather (like textures being brought in) need to be linearized prior to getting a proper surface.

Mathematically, this makes sense since the surface of an object absorbs and reflects photons in a certain way regardless of what kind of light is being used to illuminate it (a brighter light will of course bring up the mid of the range but it'll also bring everything else with it).

That being said, Clayton, if you could do me a favor and test out whether changing the white balance to the image produces an identical image as adjusting the individual material diffuse. I currently don't have (and can't have) V-Ray installed on this machine (MR for the un-win).

Further down the line, ideally, you should be able (programatically) to put a grey sphere in the center of a camera's viewport and evaluate the RGB of the center area of it; and "balance" the existing lighting through adjusting the White Balance of the camera and compensating ISO for exposure.


V-Ray also comes with a digital light meter (at least in Max it does). These efforts might go well in advancing the capabilities of that light meter if we find there is some serious use here.

-Marko Mandaric

read 392 times
2/25/2016 8:53:21 PM (last edit: 2/25/2016 8:56:51 PM)
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