HDRI Community

I did some more tests and upload the results:
In this test, a chrome ball instaed was used for acquiring HDR environment map.


A sunset scene. DSLR + Sigma 8mm for HDR environment map. This is the closest CG gray ball render I ever got with the default Final Gather value.


Outdoor afternoon. Final gather value has to be increased 6 times.


I expect a much closer CG gray ball render without artificially increase the illumination like crazy in 3D. Any thoughts?

cheers,
Jason

Thank you Dschaga.

Regarding the white balance, I am not quite getting what you mean.
If you were referring to the WB in brackets, although I didn't shoot RAWs but I am sure that all JPEG brackets and the gray ball image were shot with the same WB setting (sun light).
If you were referring to white-balance the HDR environment map as mentioned in Bloch book P.137, then I didn't do this because I was not trying to create a tone-mapped BG plate as in sIBL. I graded the HDR environment map to match my BG plate - the gray ball image.

About the final gather issue, what I can figure is that I will not be able to fully capture the sun intensity and hence the FG value has to be raised to add illumination. In the sunset scene above, the CG gray ball rendered with default FG value matches OK to the photographed one. However, what I wonder is that in the interior scene, where I (should) capture the full DR of the scene, my CG gray ball is still rendered much darker than the photographed one.

The BRDF of the gray ball is a big unknown in this test and pure diffusive CG gray ball is also in incorrect, but I wonder if this is the reason causing all these difference or there is something more to it.

Bt the way, to create a mid-gray texture, should I feed (R128, G128, B128) in a 8-bit color picker or (R187, R187, R187)?

Thanks,
Jason

Hi, Kel,
Thank you for the gretagMacbeth card tip. I have a gray card for white balance and it may be of use.

Although I did try to search some information regarding absolute calibration via some apparatus, I would love to know if I did something obviously wrong on certain component during the whole workflow before I lose my mind and invest on those hardware.

In my calibration process, the actual BRDF of the real-world gray ball is a big unknown. Also, how I should define the material for the CG gray ball is another unknown to me. But I was thinking (maybe totally wrong) if I can keep using the same real-world gray ball, lock down the CG material, and make sure scene full DR is stored (not talking about direct sun-lit scene) in my HDRI, then maybe I can indirectly derive a setting that links between real-world luminance and scene luminance. It will be great to know exactly how professional VFX artist set up the scene for calibrating the scene luminance.

Don't get me wrong. Bloch will probably tell me: it is art. If it is all that rocket science, where is the fun? I totally buy that. But, again, I don't want to miss an obvious point and then just "art" it out. That would be so wrong....

Kel Solaar wrote on 01/28/10 at 00:13:54:


I would love to know the reason... I found that cranking the default exposure of HDRI 1 stop up will usually bring my CG gray ball into ballpark, and I cannot figure out what is causing this gap??

BTW, I have tried to evaluate several sIBL sets and find no consistent relationships on the mid-tone values between the tone-mapped BG and the HDRI. Bloch ever said that back in days, some stock HDRI baked gamma into their HDRI so the render looks brighter. I find no this situation in the sIBL sets.

Back to experiments.....

- Jason

Yes, color (hues) relationships is a key aspect. This is why it's advisable a similarity between the working color space gamut and the output medium gamut. When their gamuts are very different in shape, size and center, it's very difficult to avoid clipping and shifts in the hues relationships later in post.  This is why some CG elements don't match with the BG plate in some cases, even when the lighting artists and colorist are excellent and the color grading system is high-end. And in those cases, it's better to complete the CG work with some kind of 3DLUT or color profiles conversions.  But when these gamuts are similar, the hues/colors relationships are kept and this is easily solved in post as Blochi has already mentioned. 
 
Besides the diffuse/reflection shading and lighting acquisition/reconstruction aspects (and probably  the need of a kind of spatial IBL in some cases), something that is worth to consider about the 18% middle-grey is that some digital cameras are calibrated to a middle-grey reflectivity of about 12.5% - not 18% (!) as old analog cameras. I'm posting this here too, because it's something commonly overlooked. Some people deny this, some others claims that this is an inaccuracy of digital cameras hystogram or their builtin lightmeters.  However even Kodak recommends now to increase the exposure by a half f-stop ( 0.5 compensation factor) when using the 18% middle-grey card. The percentage is not also a consensus, some people says 12%, some others 13% and there's also people reporting 10% for some digital cameras (which would increase the compensation factor). In HDRI photography, we would need to re-adjust our film speed accordingly - let's say from ISO 100 to ISO 80 - or even less - to try to compensate Kodak's recommendation.     
 
After this compensation, in the cheater's workflow (or inverse linear workflow), the middle-grey would be RGB 128 again (unless you compensate in the lighting reconstruction process), and in the linear workflow the linearization value depends on the working color space gamma. 



Gerardo

Hi Jason,

I'm wondering where did you purchase the gray ball? Did you paint it by yourself? If yes, is there any standard or method to match the color of paint? Thanks.

Quote:


White Balance might be the hardest thing to learn in photography, because you start off thinking it is so basic, and it is in reality so very technical. This thread is a great example. Recently I was working on a design for a professional still motion video and I had to start over because of my misjudgment on the white balance. It was very frustrating seeing what happened when we put the image files on to a dvd with dvd burning software. It really proves how sure you need to be ahead of time.

if you have the cash: http://akromatic.com/gadgets-shop/