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IBL and gray ball lighting calibration issue (Read 112914 times)
spittle
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Re: IBL and gray ball lighting calibration issue
Reply #15 - 05/17/14 at 13:51:41
 
EDIT (I messed up)...

I'm still trying to figure out how to calibrate my HDRIs,

I shot a grey ball, mirror ball, and colorchecker within my HDRI pano to simplify things, and I'm using one of the source images as my reference/backplate (one that includes the probes and checker).

Because I included the CC in the pano it was easy to scale the pano to match my reference exposure in HDRshop.

I've calculated the mirror ball reflectancy and plugged that into vray/Max (about 55%), I stuck a grey ball in too (I took the linearized value), which is (more or less) as per my color checker.

As per Jason, I'm experiencing a mismatch. My CG objects are rendering too bright. This requires me to adjust the multiplier on the HDRI, which to me makes the 'calibration' to the reference exposure (as mentioned in other forum posts on here) not worthwhile.

Also I've been reading HDR imaging, acquisition, display and IBL. This has a chapter on this process. However there was no mention of exposure or grading to match a reference image.

They were also using their grey balls to calculate the missing sun intensity of an exterior pano, but I can't understand how that could be possible without first aligning to real world values.

Am I missing a trick?

I could start calculating the difference between my render/real grey balls and adjust to suit, but as I mentioned before, the 'HDRI godfarthers' book didn't do this. And it is my understanding that it would be impossible to calculate the missing sun intensity of an exterior pano with an arbitrarily scaled HDRI.
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« Last Edit: 05/18/14 at 09:05:54 by N/A »  
 
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spittle
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Re: IBL and gray ball lighting calibration issue
Reply #16 - 05/17/14 at 23:27:33
 
Also - just to add - I've calibrated my response curves and everything has been done in linear space.
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Kel Solaar
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Re: IBL and gray ball lighting calibration issue
Reply #17 - 05/25/14 at 07:52:24
 
You shouldn't have any issues matching your reference color rendition chart provided you captured the whole dynamic range of your scene and worked linearly all along, from raw conversion to final merge. Now no DSLR will be able to capture the full dynamic range of a bright summer sun even shooting at 1/8000 f10 or f12 it won't happen. It means you have to use a ND filter, and a very big one. Problem using those is that you will hit some serious image color degradation.

The color rendition chart is the only thing you can trust, and should be your reference while calibrating everything: whitebalancing and setting your HDRI base exposure.

KS
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spittle
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Re: IBL and gray ball lighting calibration issue
Reply #18 - 05/25/14 at 12:23:53
 
Hi KS

I'm first trying to calibrate HDRIs with no clipped pixels. Once I've figured that out it's onto capturing the sun.


Based on what others have posted, I'm interested to know if this approach is working for some:

1) Grade the HDRI so that the color checker in the HDRI matches the color checker in the backplate.
2) place HDRI in environment slot in the renderer
3) Select camera exposure settings as per those used to shoot backplate
4) enable exposure to 'affect background' in your renderer.
5) render.

The render looks as per your backplate?

From reading various posts the above seems to be the common workflow - correct me if I'm wrong.

To have a controlled test I did the following:

1) I rendered out a 32bit vray sky environment map from 3ds max; I disabled exposure to 'affect background' for this - this gave me a perfectly linear HDRI - and a HDRI that I know is scaled correctly.

2) I did a regular camera perspective render with exposure settings enabled - this is equivalent of my backplate.

So after a side by side inspection, there is absolutely no correlation between the values in the environment map and those in my perspective render. The environment map is WAY brighter with no values under 1.

And if I load the rendered environment map back into my environment slot, the result is identical.

So to me something is not right with this workflow. Or maybe I've been thinking about this way too much and my head's turned to putty!  Cheesy
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