POV-Ray : Newsgroups : povray.binaries.images : hdr : Re: hdr Server Time
16 Oct 2021 17:06:18 EDT (-0400)
  Re: hdr  
From: Trevor G Quayle
Date: 12 Apr 2007 10:45:02
Message: <web.461e451cc3e2923bc150d4c10@news.povray.org>
"Tek" <tek### [at] evilsuperbraincom> wrote:
> My light dome's not adaptive like that. I sample each light from an average
> of 20 points on the dome to get less aliasing, but for the light positions I
> just messed with some formulae until I got a nice even distribution of
> lights over the dome (I couldn't be bothered coding a geodisic dome light
> placement). My formula for spreading lights over a sphere is:
>
>
> To be honest I don't know why it gives such a good distribution, I found it
> by trial and error, I have a knack for messing with pow() functions until I
> get something that looks right!
>
> Your weighted sampling technique sounds good, I assume you use bigger area
> lights if they're representing a region with less lights in (if you see what
> I mean). My area lights are setup so I can specify an angular size for them,
> which matches the region I'm sampling on the dome, the angular size was
> chosen using a simple sun-dial test scene adjusted until the distinct
> shadows overlapped enough that they looked like 1 smooth shadow.
>

My development process progressed as follows:

1) I started out with a full geodesic distribution and sampling.  Geodesic
distribution has a slight downfall in that the spacing at the mid latitudes
(+- 45 deg)  is greater than at the poles and equator.
2) So then I switced to an equal spacing distribution. (basically divide the
circumference at the given height by the desired spacing and round to the
nearest integer).
Even distribution gave good results with a high number of lightsources (2000
or so) but was essentially oversampling in the low-light areas, needlessly
adding more lights than required.
3) At this point I switched to a random point selection with 2 levels.
Above a given brightness threshold, I chose a given number of light
sources, and below the threshold another number of light sources.
This allowed me to get more weighting to the bright lights, but I found I
was now undersampling the low light too much (ie, not getting that blue
ambient tinge in outdoor scenes).
4) Now I added centroidal voronoi tesselation techniques to gravitate the
chosen point lights to a natural even brightness distribution.
This was a little better, but was very computationally intensive.
5) At this point I came across median cut method and tried implementing
this.  In order to do this efficiently, a few precalcs needed to be done.
This involved prebuilding summed area tables for the HDR.  Because I had to
use a finite size to store the SATs, I had to choose a subdivision level for
the HDR (ie. pixelate it).
This worked out quite well and was much more effective and quick at given a
nice equal brightness distribution with much fewer light sources (I use 256
typically).  One downfall is that at low subdivision (large pixel size)
small point lights in the HDR could be lost.  This is remedied by
increasing the subdivision (which adds to the parse time, but not render
time)
Along with the median cut method technique, I have the central voronoi
tesselation technique still available to tweak the calculated distribution
(though I never use it because of computation time).

A few other features have been added along the way:
1) previewing of light distribution: renders a preview of the HDR itself
showing the calculated light distribution.  Makes tweaking the numbers and
seeing the results easy and quick to do.
2) Clipped area rebuilding algorithm:  artificially rebuilds clipped areas
of HDRs (Some HDRs I have still have a cutoff threshold becuase of
limitations in their creation).  This can also be used to try to adapt LDR
images to the system, though the result may not be great and may need a lot
of tweaking of the variables.
3) Area lights: allows area lights to be used instead of point lights.  The
size of the area light is determined by the size of the representative area
of the specific light source.
4) Automatic/manual brightness adjustment:  Allows the system to either
automatically determine a good overall brightness of the lighting based on
the brightness of the HDR or allow the user to target a given effective
light brightness.

And much more including a few experimental techniques/features like gamma
adjustment and brightness weighting.

It has turned out to be quite complex, but I am quite satisfied with the
results I have been getting and feel that I am very near to being satisfied
enough to release it.

The process also taught me a great deal about a number of items:
-HDR images and concepts
-programming methods
-voronoi tesselations
-median cut method
-summed area tables

I hope this helps you along the way with your progress. Whether what you end
up doing turns out to meet your expectations or not, you can still learn a
lot about POV and other topics along the way.

Feel free to ask more questions and I can try to answer or help if/when I
can.


-tgq


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