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Equation for film exposure - need help!

Posted: Wed Mar 30, 2011 11:03 pm
by limo1977
Hello,

I'd like to know how to calculate pixel-values of maxwell-rendering (32bit floating point value in EXR or TIF) into absolute units of cd/m2. So one idea is using exposure value form the camera. But you have to know a conversion factor K like in this equation given by G. Wrad for Radiance:

EXPOSURE = K * T * S / f^2

where:
T = exposure time (in seconds)
S = film speed (ISO)
f = f-stop
K = 2.81 (conversion factor 179*PI/200)

But the formula for film exposure in maxwell is nowhere documented. Can anyone help me figure it out? NL-Team?

thank you and best regards!

Re: Equation for film exposure - need help!

Posted: Wed Apr 06, 2011 6:17 pm
by dariolanza
Hello limo1977,

The distribution of energy prior to the Tone Mapping perceptual conversion is a proprietary development and we didn't publish it yet for general public.

But we are considering publish it in the future.

Sorry for not being able to provide more help.

Greetings

Dario Lanza

Re: Equation for film exposure - need help!

Posted: Wed Apr 13, 2011 10:20 pm
by limo1977
Hi Dario,

thank you very much for your answer!

we are using maxwell to simulate night appearance of automotive lighting since a few years. and I belive the physical correctness of maxwell-rendering (maxwell-core-engine is an excellent implementation of MTL algorithm). but a very big problem for me is to prove that. because the world of Illuminating Engineering works only with absolut luminance/radiance but the conversion form the pixel-brightness of maxwell-rendering to cd/m2 is unkown. in addition I can't get any a pure linear output-images without tonemapping (even as HDR, EXR or Tif in 32bit) which is a pity...

I think maxwell could miss the chance to open the door to the Society of Illuminating Engineering just because of this conversion-problem. to my knowledge: 99% of lighting engineer still simulate their Lights with Photon-Tracing which is weird (and old as well) for making the light-behaviour visible.

best regards

Mo Li

Re: Equation for film exposure - need help!

Posted: Thu Apr 14, 2011 11:28 am
by tom
limo1977 wrote:in addition I can't get any a pure linear output-images without tonemapping (even as HDR, EXR or Tif in 32bit) which is a pity...
Maxwell saves 32 -bit outputs with linear gamma. Could you provide an example failing?