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Rendering with Arnold in 3ds Max using the MaxtoA plug-in.
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How to get accurate light levels?

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Message 1 of 7
mmlodzienski
982 Views, 6 Replies

How to get accurate light levels?

Hey all! I work for a forensic engineering firm, specifically for automotive crash reconstruction, and one of the bigger struggles we've encountered is getting accurate lighting. In particular, we've had a string of cases involving visibility levels in low light conditions, where the ability to perceive, or not perceive, something like a pedestrian helps determine who is at fault.

Now, I know that Arnold is supposed to have accurate GI, inverse square falloff, etc. However, near as I can tell the light intensity doesn't correspond to real units. We are able to measure the intensity of light sources in terms of lumens, candles, etc. Is there a workflow that will allow us to say that, alright, for a light source of this intensity, it would be exactly this value in Arnold?

Secondly, is there a way to be able to match the exposure of the camera in Arnold as well? Say, if you know the camera, lens, have all the data like F-stop and ISO and shutter speed, and match that in Arnold? We already are taking multiple exposures and comparing them on site visually, and recording the exposures that most closely match the lighting we are seeing. The end goal would be to be able to say, with good deal of certainty, that the light levels in our renders matches what would be perceived by the human eye.

I know that this is likely a bit of a broad subject by this point in the post, I am more looking to be pointed in the right direction. I haven't managed to turn up anything helpful on this subject. In particular, if anyone knows of any academic papers on the subject those would be especially useful as they can be cited in court.

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Message 2 of 7

Yes, Arnold is physically based, so you have things like light falloff. But all units in Arnold are arbitrary.



// Stephen Blair
// Arnold Renderer Support
Message 3 of 7
mmlodzienski
in reply to: mmlodzienski

@Stephen Blair but, is there any workflow to use those arbitrary values? Say, assign some sort of conversion rate of x lumens = y intensity, when using a certain exposure?

Message 4 of 7
Ben_Cayce
in reply to: Stephen.Blair

Are you saying that light intensity of 1 is equal to some random amount of energy? The only way to figure out that number (in lumens) is to calibrate it with a real world source?

Message 5 of 7
emuranovic
in reply to: Ben_Cayce

Curious to know if anybody found a solution for this?

Message 6 of 7
CiroCardoso3v
in reply to: emuranovic

Elska, at the discord server, posted this explanation to a similar question.

 

An "RGB renderer" (in fact, any) is an oversimplification of reality and does not behave like a digital camera (or film) and does not reproduce light as in the reality (rather simulates it in a user-friendly way).
Most of those renderers feature a light system based on the radiometric domain ("radiant intensity", steradian).

Measuring via "watt" is highly misleading and often misinterpreted, even in photography/cinematography since it's an electrical unit and results in an inappropriate "light measurement", omitting light efficacy for instance.

Last but not least, the rendered data is (and should be) OpenEXR encoded (saved as), which is a floating-point high-dynamic range file format (for more: https://www.elsksa.me/scientia/cgi-offline-rendering/file-format-debunk). This also marks a difference with digital cameras which encode ("more or less") HDR data in an integer fashion (a magnitude more "limited" than computer-rendered OpenEXR encoded 3D scenes).

Lead Enviroment Artist @Axis Studios

Arnold Discord Server


Ciro Cardoso

EESignature

Message 7 of 7
imagestud
in reply to: mmlodzienski

I have the same concerns. Mental Ray allowed me to create lights in real world units. Were you ever able to find a paper or do any research that could be defended in court?

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