HDRI RGB Level vs. VFB Exposure

[cjwidd]

*The following discussion assumes Gamma / LUT correction (2.2)
 
Every HDR appears differently when rendered based on differences in capture technique, color space, color range, etc. Of course, this also true even for HDR images from different spaces that are captured using the same production techniques. Consequently, the lighting for a scene must be balanced using a combination of [bitmap] RGB level and [VFB] exposure settings, a process that is mostly driven by trial-and-error and artistic taste.

My question is, 'Are there best practices for establishing a proper balance between these two parameters to achieve a physically accurate result?'

I understand that turning dials is a part of the artistic process in 3d, but surely there must be a way to constrain what is physically plausible in the renderer using some guidelines(?)

[PROH]

Hi. The main factor here is the HDRI, and how well it's made.

Regarding the "dialing in" I personally have used Dubcats tut-post about Calibrating HDRI's, to calibrate a lot of my HDRI's, so that I can swap them out without changing EV. This makes it easier to compare them in a scene, and to chose which one to use.

Can't remember the place of his post, but search this forum for "Dubcat", and/or "calibrating HDRI" and I'm sure you'll find it.

Hope it helps

[Njen]

For what it's worth, the at the VFX company I work at, I recently led the charge to completely overhaul our HDRI process, because up until recently it was all over the place for us and I was not happy with the existing system (even if you could call it as such). The company was using an assortment of random HDRI's that were all over the place, causing work to get redone.

So now all of our base facility HDRI's are ones that I have personally shot myself, capturing 22 stops. But then, we do some very specific post processing to make sure that each of these HDRI's live in the same 'world'. One of the main steps we do is to calibrate all of the HDRI's to an EV of 13, we then faithfully paint in correct values of all clipped light sources, and it all goes through a macbeth colour match.

The end result is that now our facility HDRI's are of a better quality than anything else found on the internet, even the paid ones, and the quality of our look dev has increased markedly.

[cjwidd]

Haha, well I certainly appreciate the story, I'm glad to know the process I was referring to is valuable in and of itself. If you have any notes you'd like to share about how to help HDRIs 'live in the same world', I'd be eager to learn.

[cjwidd]

PROH,

I assume you are referring to this thread

[PROH]

Yes, that's the one. Especially the first page :)

I took the time to calibrate a bunch of my HDRIs, and then I rendered out a couple of pics of each with the same fixed cameras and models, so I could go through my testrenders to see which ones could potentially be of interest in the current project. I saved them as matlib and wrote down the values as well (takes a **** of time to load a matlib with many HDRIs).

Sometimes I use a Corona Multimap as a container/switcher for a number of calibrated HDRIs.

Hope it helps

[cjwidd]

PROH,

I guess the next obvious question is whether you feel that the quality of your renders benefited from using calibrated HDRIS?

NJEN,

If you are using Corona for personal work, do you apply the same methods, as it pertains to HDRI calibration?

[PROH]

I do a lot with Corona Sun & Sky, but I would say yes. Besides the the already mentioned benefit (easy swapping HDRIs), there is less unexpected behaviors when it comes to materials.

But I guess it's all about how it fit into your preferred workflow :)

[cjwidd]

To be clear, you are saying that you tend to light your scenes with a sun rather than an HDRI? I suppose you prefer the predictability (control), or perhaps the authenticity, of using a single sun light source?

[PROH]

I do mostly exteriors. But besides that, I chose what fits the project. When location and time of day is important, the daylight system is often easier to use, than to find an useable HDRI.

[cjwidd]

Ah, I see, that's a good point. Thank you for clarifying :)

[Njen]

I am happy to go in to more detail on our process soon, but just quickly, what I have decided that our studio has now opted for is our HDRI's specifically for look dev are as physically based as possible in an EV 13 world with a colour grade to macbeth reference colours in sRGB space. This means that for our look dev turntables, we do not use extra lights, all illumination comes from the HDRI only, including the sun.

Our lighters however need the flexibility for art direction, so while photoreal is the aim, if a director wants the sun at double the strength, than that's what they get! So the lighters will take the physically based look dev HDRI's, and then paint out the "light sources" and recreate them with lights for more control.

I must add to that I've been a huge fan of Terragen for a long time now, and at two studios I've successfully introduced many fully digital HDRI's to both facilities. The great thing about Terragen, is that the sun values are already close to photoreal when output at 32bit, no guess work. I've done tests where I've captured HDRI's near the equator at 12pm noon on a clear day, and it is almost a one to one match to similar Terragen setup.

So for my own personal work I generally follow the same system. My look dev HDRI's have all of the correct illumination in the image only because I don't want to cheat the physical nature of the shaders. But my lighting scenes have painted out light sources and then recreated actual lights for greater control.

[cjwidd]

Very helpful description Njen, thank you very much!