I’m new to Radiance and currently calculating illuminance values in an indoor scene with multiple luminaires. I’m curious about the differences in results between simulating each luminaire separately and then combining the results, versus simulating all luminaires together at once. Any insights or advice would be greatly appreciated. Thank you! ：）
Hi Kiki,
Welcome to the Radiance forum!
Numerically, there should be no difference between simulating the light sources individually and adding them together, versus simulating them all at once. Within Radiance, there are some caveats to this:
A) There are optimizations in the light source calculation that will allow some approximation errors in scenes with many light sources. To turn this optimization off, set -dt 0 on the rpict or rtrace command line. There will be a performance penalty of course, but this is the easiest way to detect if this is a source of error.
B) Multiple runs need to either be done with a different ambient file or no ambient file to be accurate. Otherwise, you will transfer contributions from one light source’s run to another, and over-count in this way. (Technically, ambient files should be unique to each scene configuration and are even sensitive to calculation parameters, so should be shared with care.)
C) As a Monte Carlo rendering technique, Radiance will typically show some variance from one calculation to another. This deviation may or may not be biased high or low, depending on the calculation parameter settings.
D) Any post-processing done with Radiance pictures (HDR images) is subject to a few percent error, usually +/-2% or so at each pixel, due to the limited accuracy of an 8-bit mantissa. This error can add to any variance from the above contributors.
I’m guessing I missed something, but you haven’t said exacty what you are seeing that led you to post this question. Further details would be useful.
Cheers,
-Greg
Hi Greg,
Thank you very much for your response. I posed the question out of curiosity, as I am contemplating whether the lighting environment is a linear system or a nonlinear system (considering each light’s contribution as an independent variable and illuminance as the dependent variable). Initially, my -dt setting was 0.1. Following your advice, I changed the -dt to 0 and found that the summation of individual simulations closely matches the total from the overall simulation (the difference is negligible). Thank you very much for your guidance!.
Cheers,
Kiki
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