[Radiance-general] bean counting the photon array

[Schregle Roland HSLU T&A]

On Sun, 25 Oct 2015 00:26:38 +0200, Chris Kallie <kallie at umn.edu> wrote:

> Fellow Renderers,
>
> I wish to account for all scattering events (i.e., specular reflections  
> and refractions) that eventually make it to the viewpoint, and I wish to  
> render (or at least account for) scattering events incrementally, in  
> order to visualize the unfolding (and hopefully the order) of  
> contributing events. I have been successful (to some degree) when the  
> environment has a non-zero reflectance, but mkpmap fails when I set the  
> surrounding environment reflectivity to zero.

Hi Chris,

glad to hear you're trying this out! Photons only account for indirect  
irradiance on surfaces with non-zero diffuse component, which explains  
your failure case with the black surround.

> I have also been able to render 1+ scattering event(s) with mkpmap, plus  
> 1+ scattering event(s) with rpict, when including a non-zero reflectance  
> Lambertian environment, but I don't think those images are useful  
> because I don't think they describe any realistic physics

Well, basically you'll get a biased (underestimated) solution. With mkpmap  
you can limit the number of photon bounces with the -apm option. Normally  
this parameter is only relevant for pathological cases, i.e. when using  
surfaces with absurdly high reflectance.

> In summary, I would like to use a combination of mkpmap and rpict to  
> incrementally build the photon array, and perhaps use pcomb or some  
> other statistical or combinatorial technique to analyze the  
> contributions of scattering events as they relate to the viewpoint.

Without modifying the code, I believe you can only separate out the  
ambient component via commandline arguments (correct me if I'm wrong,  
Greg). To only get the ambient component, you can subtract a run with -ab  
0. This isn't specific to photon map tho.

The alternative is rcontrib, but the photon map only accounts for light  
source contributions, so that's no help in your case.

Best regards,

--Roland


-- 
Dr. Roland Schregle
Senior Research Associate

Lucerne University of Applied Sciences and Arts
School of Engineering and Architecture
CC Envelopes and Solar Energy (EASE)
www.hslu.ch/ccease

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