[Radiance-general] Questions about spotlight and specularity

Sat Jul 29 07:42:14 PDT 2017

Hi Joe,

The only time specularity == reflectance is when you have a plastic material whose first three (RGB) arguments are all zero.  Otherwise, there is a diffuse component that gets added to the specular in the following manner:

	R_total_refl = (1 - A4)*A1 + A4
	G_total_refl = (1 - A4)*A2 + A4
	B_total_refl = (1 - A4)*A3 + A4

This is explained (more or less) in the following document:

	http://radsite.lbl.gov/radiance/refer/materials.pdf

Good -- you found the presentation from Germain.  That's the one I was looking for.

Your understanding of the "mirror" material is correct.  For every light source in the scene, a virtual spotlight is created that encompasses the surface modified by "mirror", "prism", or "prism2".  The number of reflections searched is set by the "-dr" option, which should be set to 1 in this case because you neither care about nor wish to burden the calculation searching for secondary reflections.

You should be able to create images using the virtual light source calculation Germain used.  The photon-mapping approach ought to work as well.  You don't need to "cheat," although getting a reasonable tone-mapped exposure might be challenging.  You can play around with pcond, especially the veiling glare simulation, to give you the kind of flare effect you are looking for in a post-process.

Cheers,
-Greg

> From: Joe <solarjoe at posteo.org>
> Date: July 28, 2017 11:10:42 PM PDT
> 
> Hello Greg,
> 
> so, as defined in Radiance, is specularity == reflectivity == reflectance?
> 
> I guess you mean this presentation:
> 
> https://www.radiance-online.org/community/workshops/2008-fribourg/Content/Augsburger/GermainAugsburgerPresentation.pdf
> 
> He was using Radiance like SolTrace, to calculate the flux distribution on the receiver.
> 
> He calculated the surface normals at the receiver panels sent them
> into the scene using rtrace.
> 
> I am not sure if I understood that correctly:
> The material "mirror" uses sort of mimics the spotlight, so only
> shooting rays within a fixed cone angle towards the light sources
> or other illumination surfaces?
> 
> I will also look into the photon mapping, have never used that before.
> 
> Well, I would like to create realistic renderings of these power plands at different values of solar irradiance, e.g. 500 and 1000 W / m^2.
> 
> Images like
> 
> http://www.dlr.de/dlr/en/Portaldata/1/Resources/bilder/portal/portal_2011_7/turmkraftwerk.jpg
> 
> http://www.bine.info/fileadmin/content/Publikationen/Themen-Infos/II_2013/themen_0213_29.jpg
> 
> https://s-media-cache-ak0.pinimg.com/originals/2b/b8/fb/2bb8fb6e9d4194bb87d32a5b0855182b.jpg
> 
> But instead of "cheating" myself into a glowing receiver, I would
> rather have it to be illuminated by the heliostats.
> 
> Maybe also with some mist:
> 
> https://www.e-education.psu.edu/eme812/sites/www.e-education.psu.edu.eme812/files/Lesson07/7.2solucarps10.png
> 
> Do you think it is possible to create images like that at all?
> 
> Kind regards and thanks for your patience :)
> 
> Joe
> 
> 
> 
> Am 28.07.2017 um 19:24 schrieb Greg Ward:
>> I should add that Roland Schregle's photon mapping routines might help you with this calculation.  Saves creating as many virtual light sources as you have mirror elements.  (This number approaches infinity when you try to model curved mirrors.)  Unfortunately, I am not knowledgeable enough about it to even tell you how to set it up, but I guess it will involve a portal for the sun, somewhere.
>> Cheers,
>> -Greg
>>> From: Greg Ward <gregoryjward at gmail.com>
>>> Date: July 28, 2017 10:13:07 AM PDT
>>> 
>>> Hi Joe,
>>> 
>>> Regarding reflectivity, I suppose you could say that for any opaque material, reflectivity==reflectance.  Since Radiance materials (with the possible exceptions of "dielectric," "interface" and "mist") don't really consider volumes, this equality generally applies.
>>> 
>>> I don't really know anything about SolTrace, but from your brief description, it sounds like it might be the right tool for your purpose.
>>> 
>>> The basic problem with Radiance is that as a light-backwards ray-tracer, it needs to know where to look for light sources, particularly tiny ones like the sun.  There is a trick you can use, which is the "mirror" material light, which enables a virtual light source search.  That brings other issue, especially once you go to a full array.  By the way, it employs the "spotlight" mechanism internally to avoid unnecessary source tests, so you had a good idea with that.
>>> 
>>> I remember clearly that someone covered this in a Radiance workshop presentation.  Can someone post the link for me?  I haven't managed to locate it with any of my searches.
>>> 
>>> Cheers,
>>> -Greg
>>> 
>>>> From: Joe <solarjoe at posteo.org>
>>>> Date: July 28, 2017 2:18:13 AM PDT
>>>> 
>>>> Hello Greg,
>>>> 
>>>> thank you very much for your quick response and the good answers.
>>>> Abbout reflectivity / reflectance, I just found this
>>>> https://en.wikipedia.org/wiki/Reflectance#Reflectivity
>>>> 
>>>> I am also using SolTrace, also from NREL, maybe you know it. It's a
>>>> more technical raytracer (path tracing, starting at the light source),
>>>> not considering colors, only energy.
>>>> 
>>>> Now I would like to make a rendering of a concentrating solar tower
>>>> plant. But since I have only very little experience with rendering so
>>>> far I can't get the materials right, or I am missing something else.
>>>> 
>>>> A solar tower plant consists of thousands of tracking mirrors reflection solar radiation
>>>> onto a receiver at the top of a tower.
>>>> 
>>>> Tower and receiver:
>>>> http://cspworld.org/sites/default/files/map/images/eSolarBabcockWilkoxReceiver.jpg
>>>> 
>>>> Heliostats scattered around the tower
>>>> http://www.solartowersystems.com/mediapool/99/994662/resources/big_21459290_0_350-279.jpg
>>>> 
>>>> This is what I have so far
>>>> 
>>>> http://imgur.com/Vgf6Zuz
>>>> 
>>>> But I was not able to see a light spot of the heliostat on the receiver (large black cylinder with lid).
>>>> I would have expected it at the spot marked in blue.
>>>> 
>>>> The scene is illuminated by a sun disk in zenith (0, 0, 1, from gendaylit) and a downward
>>>> pointing spot (red) directly above the heliostat (green). Heliostat diameter is 10 m.
>>>> 
>>>> This is the sun I am using at the moment
>>>> 
>>>> void light sun
>>>> 0
>>>> 0
>>>> 3 7.131e+004, 7.131e+004, 7.131e+004
>>>> 
>>>> gendaylit originally returned
>>>> 
>>>> void light sun
>>>> 0
>>>> 0
>>>> 3 7.131e+006, 7.131e+006, 7.131e+006
>>>> 
>>>> but this oversaturates my scene completely and it looks like a white-out.
>>>> 
>>>> I tried several materials for the heliostat (glossy colors, glass with no transmittance, mirror).
>>>> 
>>>> So far I tried a red plastic material for the receiver and played a bit with the specularity
>>>> and roughness but could not get it to reflect anything towards the camera.
>>>> 
>>>> Do you have some ideas how I could get this to work?
>>>> 
>>>> Kind regards,
>>>> Joe