[Radiance-general] Illums and 3 Sided Polygons

[Greg Ward]

Hi Marcus,

The trouble with your floor is that you have specified a very small, 
but non-zero value for the surface roughness.  Radiance employs a 
hybrid technique for computing specularity, which does a closed-form 
computation of specular highlights related to light sources (including 
your illum's) plus a Monte Carlo sampling of other soft-specular 
reflections.  If you set the roughness to zero, you will see a perfect 
mirror reflection of the window, without artifacts.  Presumably, this 
is not what you are after.  Alternatively, you may increase your 
surface roughness value and your reflection will improve.  Finally, you 
can set the surface roughness to zero, then apply the included 
"gloss.cal" function to achieve a pure Monte Carlo sampling of 
reflected rays.  This will lead to greater noise in the result, but 
will eliminate the source sampling artifacts you are now seeing:

wood_pattern texfunc mc_roughness
4 gloss_dx gloss_dy gloss_dz gloss.cal
0
3 .02 .02 .02

mc_roughness plastic wood_flooring
0
0
5 .5 .2 .05 .02 0

Of course, I am only guessing on the parameter values, but this should 
give you an idea of what I mean.

-Greg

> From: "Marcus Jacobs" <marcdevon at hotmail.com>
> Date: September 6, 2004 3:52:37 PM PDT
>
> Greg,
>
> I did read through the previous threads about the illums about one 
> month ago. Though my question was similar in nature, I was looking for 
> a more theoretical explaination of what goes on because I had been 
> encountering some issues with some illums that I had placed outside of 
> a window.
>
> As I mentioned before, I was getting some very strange looking 
> reflections on the floor of my renders. Here is an example:
>
> http://pg.photos.yahoo.com/ph/marcdevon/detail?.dir=b3e9&.dnm=1736.jpg
>
> The window has a dog ear arch. The rectangular bottom part of the 
> window I probably could have made into an illum while placing a 
> separate illum outside for the upper portion of the window. Instead I 
> used illums placed outside the entire window. While reading through 
> RWR in the section explaining the adaptive source subdivision, it 
> mentions that large light sources can be a significant source of error 
> in a standard ray-tracing calculation. Because of this, the illum 
> plane I created consisted of 8 sub-planes (similar to the illum planes 
> created in the museum scene in RWR. The only difference is mkilum 
> create the distribution instead of winbright). For some reason this is 
> what is causing a significant problem with the reflection. Why is 
> this? It seems that the rendering would be better because the illum  
> sources are smaller. Anywho, I did take your advise and I made 1 large 
> plane instead of the multiple subplanes. The reflection did improve. 
> Here is what I came up with:
>
> http://pg.photos.yahoo.com/ph/marcdevon/detail?.dir=b3e9&.dnm=d860.jpg
>
>
> You can see that the reflection is improved. It still isn't completely 
> natural but it isn't as disurbing as the first picture. I set -ds to 
> 0.1 and -dj to 0.75 for this rendering. I attempted to do three things 
> in order to improve the reflection. None exactly worked. The first 
> method I tried was decreasing the -ds parameter. I used a rediculously 
> low number (0.001) with -dj set to 0.3 to see what would come out. 
> Unfortunately,there wasn't any improvement. Here is the result:
>
> http://pg.photos.yahoo.com/ph/marcdevon/detail?.dir=b3e9&.dnm=1dad.jpg
>
> I do not know if the lack of improvement was do any limit in Radiance 
> for the -ds parameter. The second method I used to attempt improve the 
> reflection was to increase the sampling density (i.e., increase the 
> image size). I rendered the reflection at 12 times the final image 
> size. There wasn't any improvement using this method either. Here is 
> the result:
>
> http://pg.photos.yahoo.com/ph/marcdevon/detail?.dir=b3e9&.dnm=dfa5.jpg
>
> Any improvement over the previous image was due to the higher direct 
> jittering setting (0.65 vs 0.3). The last method I tried was 
> increasing the direct jittering parameter to 0.85. Here is the result:
>
> http://pg.photos.yahoo.com/ph/marcdevon/detail?.dir=b3e9&.dnm=5814.jpg
>
> There is a noticable improvement although not completely natural. 
> There were no failures with the illums (there were created as perfect 
> squares) but all my other light sources generated "aiming failure" 
> error messages so this isn't a good solution.
>
> Does any know of any ways that I maight improve the reflection? Please 
> help.
>
> Marcus