[Radiance-general] Re: physically-based landscapes

[Greg Ward]

Hi Rob,

You can certainly exclude any geometry you like from the ambient 
calculation using the -ae option.   Each -ae value adds a material to 
exclude from ambient calculations.  Named materials will get the -av 
value rather than incurring any new interreflection calculations.  This 
means that the mountains will have a rather flat appearance, and the 
shadows will be too dark if you don't choose a reasonable outdoor value 
for -av, which in turn could be too bright for your interior.  (It's a 
problem.)

Another option is to capture (using HDR photography) or render the 
scenery using a fisheye lens in a separate step, then apply the results 
to the window as a luminance distribution using a fisheye perspective 
mapping.  The Radiance ray/lib/fisheye.cal does a lookup on a 180 
degree angular fisheye image (-vta -vh 180 -vv 180).  I recommend using 
two pictures -- a high-resolution for the view out the window and a 
low-resolution for the light distribution.  You can compute the low 
from the high using pfilt with the -1 option:

pfilt -1 -x 32 -y 32 -r 1 window_hires.pic > window_lores.pic

The scene specification for a west-facing window might look like so:

void colorpict window_pict_hires
9 red green blue window_hires.pic fisheye.cal fish_u fish_v -rz 90
0
0

window_pict_hires glow window_glow
0
0
4 1 1 1 0

void colorpict window_pict_lores
9 red green blue window_lores.pic fisheye.cal fish_u fish_v -rz 90
0
0

window_pict_lores illum window_illum
1 window_glow
0
3 1 1 1

window_illum polygon window
0
0
12
	window_with_inward_normal

------
I haven't tried this, so I can't say for sure if it will work.  I 
started this e-mail before I saw Carsten's, but didn't get a chance to 
finish it until today.

-Greg

> From: Rob Guglielmetti <rpg@rumblestrip.org>
>
> I have to create some renderings of a space, which offers panoramic 
> views of a distant mountain range.  We're doing some glare studies, so 
> the luminance of the mountain is important to me.
>
> I could model the mountains, but because they are so far from my model 
> I'll get ambient leaks, right?  Is there some way to exclude the 
> distant geometry from adding values to the ambient cache?  I'm 
> interested in doing some daylight animations, so if I could do it with 
> representative 3D geometry (vs. using a texture map) I'd be ever so 
> happy.
>
> Failing that, is there a way to apply a picture of the mountains to a 
> local object?  My concern there is that if I have a large object close 
> to the window it will reflect more light into the space than what 
> would actually happen, with the mountain a mile away.
>
> Any pointers appreciated.
>
> P.S.  If I had a lightmap of this site, could I use it to do an 
> image-based rendering?  All the image-based renderings I've seen are 
> of objects being directly illuminated by the lightmap.   But if I put 
> a local ground plane and my building into the center of a lightmap, 
> would the interreflection be computed, and the interior of the space 
> be rendered properly?  Would it be "accurate"?