[Radiance-general] Re: physically-based landscapes

[Greg Ward]

> From: Rob Guglielmetti <rpg@rumblestrip.org>
>
> I knew about the -ae trick.  I just thought that since the exterior 
> would still have values, that they could get used inside.  Now I 
> realize that the direct calculation is one thing, *computed* ambient 
> values are another, and the *approximated* ambient values (by way of 
> the -av parameter) are another, and they all are separate.  Only 
> computed ambient values live in the ambient cache, and only computed 
> ambient values can be re-used elsewhere.  Yet another concept that 
> seems obvious now, but didn't an hour ago.  OK, so the easy cheat is 
> to exclude all the exterior objects from the ambient calculation, and 
> live with dark shadows on the mountain.  But Carsten says that the -ar 
> is based on the scene bounding cube, so even if I exclude the exterior 
> values I need to crank it up, yes?

Yes, though you could set -ar 0 and you might get around this problem.  
The disadvantage is that the calculation can go a bit nuts in the 
little corners, but it's only a problem on high-resolution renderings.  
Rtrace shouldn't be much affected.

> I just wanna make sure I understand this.  This is the correct way to 
> achieve what I asked at the end of my email, yes?  A method for using 
> HDR lightmaps to illuminate the scene and have a pleasant (and 
> photometrically accurate) view out the window?  This would be, in a 
> word, cool.
>
> As I have never used colorpict and have limited experience with illum, 
> I just wanna make sure I get this:
> 1. I take a hemispherical HDR image of the site.
> 2. Colorpict and the fisheye.cal file takes the highres picture, and 
> applies it to a plane, and is rotated into the proper orientation.  
> Can you explain the fish_u fish_v parameters?

The fish_u and fish_v parameters are computed lookups into the image 
that convert angles looking out the window into pixel positions.  See 
Chapter 4 in RwR for details.

> 3. An illum source is created from the low res version of the pic, 
> mapped to the same polygon?  Why do you use a low res image for the 
> illum?

You could use mkillum to compute the distribution, but the effect would 
be to reduce the resolution of the original image, which can be done 
much faster with pfilt.

> The illum's luminous distribution function is the result of applying 
> the lightmap to the window pane, just the same as if I were to use 
> gensky?  The colorpict is purely for the view out, it does not 
> contribute to the illuminance of the interior space?

No, it's the same as if you used mkillum.  The only difference is that 
rpict -vta -vh 180 -vv 180 computes the window's light distribution 
from a single viewpoint, where mkillum would average it over the entire 
window.  If your window is small relative to the closest geometry, the 
difference is vanishingly small.

> Seems like a lot of work, 'specially for this brain, but it could be 
> worth it.  In the short term, I think I need to try one of the other 
> tacks.

It's actually not that difficult -- try it.

-Greg