[Radiance-general] bean counting the photon array

[Chris Kallie]

Thanks for your reply, Roland!

Your suggestions are extremely helpful, and much appreciated. And thanks 
to you and the whole gang for all the work put into the new official 
release...it was perfect timing for our current research.

Sincerely,
Chris

On 10/25/15 11:04 AM, Schregle Roland HSLU T&A wrote:
> 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
>
>