Jim nous apporta ses lumieres en ce 2007/09/22 22:21:
> I have been experimenting with POV's photon mapping, and I'm pretty familiar
> with the basic idea. I've read some of Jensen's papers, and the Nathan Kopp
> paper. After reading and experimentation I still have some pretty basic
> questions about settings. Obviously all of the settings interact. But right
> now I have questions about spacing and count. Quoting the docs:
> http://povray.org/documentation/view/3.6.1/425/
> 
> #  If spacing is used, it specifies approximately the average distance
> between photons on surfaces.
> 
> # If count is used, POV-Ray will shoot the approximately number of photons
> specified.
> 
> That quickly covers the theory, but I could use more practical knowledge. My
> (mis)understanding, at a basic level, is as follows - please correct,
> augment, etc.
> 
> 1) The quality of the final result is influenced by the precision of the
> generated map. If you don't have sufficient photon density, subtle details
> will not be visible or rendered poorly. More photons are better, at least
> to a point. More photons take more time.
> 
> 2) Using "count" sets a rough upper bound on the time to generate a map for
> an arbitrary scene. But there is no guaranty that there will be sufficient
> precision in the map to show desired features. But surprisingly small
> numbers of photons can yield very pleasing results.
> 
> 3) Using "spacing" sets the approximate level of precision (photon density)
> in the final map, but that can result in arbitrarily long map generation
> times. Using extremely small spacing won't guaranty more visible detail,
> but it will almost definitely result in longer map generation times.
> 
> 4) Specific details about scenes makes all of these numbers "fuzzy" because
> of complexity of the light transport problem. If there was any
> easier/faster way to do it, there wouldn't be a need for photon maps in the
> first place.
> 
> So, assuming I'm not totally wrong about how it all should work, this leads
> me to several questions:
> 
> 1) In what situations is it "better" to use count vs. spacing, and why?
If you have several light_source that emit photons and several objects set as 
target, spacing is probably a better solution.
If you have only one light and few target objects, they are equivalent.
> 
> 2) Are there good rules of thumb for setting these values? (POV seems to
> choose pretty good defaults - was it just trial and error, or was there a
> method?) What are the major factors in the decision?
You need smaller spacing if, on average, one POV-unit ocupy a large image area.
> 
> 3) What do the photon statistics indicate about the scene, and the choices
> for the spacing/count values selected? Just for instance: If there are a
> large number of "Gather radius expanded" in relation to "Gather function
> called" then perhaps it would be good to reduce spacing or increase count.
> Yes? No? Maybe?
> 
> 
> 
> Also, this is a little "off topic" - but just occurred to me... if
> dispersion is used, I know the effects will show up for photons - does it
> affect the generated photon map at all, or is dispersion a "post gather"
> type of operation?
> 
> 
> 
> 
> 
If you use dispersion, the generated photon map will be affected as it is 
generated during the photon shooting step. This will greatly increase the 
photon's count. If you use 7 dispersion sample, each refracted photon will get 
splitted in 7 coloured ones. You will probably need more dispersion sample than 
that to get smooth results.

Also, if you use blured reflection for a target surface, it will multiply every 
photon hitting it. Same for blured refraction, only worst: there are always two 
surfaces contributing, and you also can have dispersion contributing...

You should note that if you also use radiosity, the photons effect will affect 
the radiosity results.

-- 
Alain
-------------------------------------------------
HAMMER:  Originally employed as a weapon of war, the hammer nowadays is used as 
a kind of divining rod to locate expensive bike parts not far from the object we 
are trying to hit.

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