[Radiance-general] pmap, mkillum and photon ports

[Lars O. Grobe]

Hi all,

I just want to share my experiences with the pmap for radiance 3.7p2, 
the use of photonports and mkillum. Maybe it is interesting to someone 
here, and maybe I even get some comment on what I am doing ;-)

First, to introduce, the problem with the pmap is that, for large angle 
sources, it is only performing well when used with photon ports. The 
drawback of photonports is, that no interreflection calculation happens 
outside the port. That means that, when modeling a simple room with a 
window, only unobstructed parts of the visible environment contribute - 
buildings, ground planes etc. do not contribute any reflected illuminance.

To solve this, I played a bit with the combination of pmap and illum 
surfaces. Carsten did that before, but with a different scene, different 
set-up and different goals. To get the indirect component of light flux 
into my photon ports, I have to place my illums outside the ports, so 
that they become (unobstructed) light sources accounting for the light 
reflected from outside geometry.

I have set up a simple scene as follow: a room 10mx10mx5m, with its 
floor level at +5m. In the floor of that room, a hole of 5mx5m (pointing 
DOWN!). At -1m, a ground geometry (disk, diameter 50m). A photon port in 
the 5mx5m hole. And five mkillum-surfaces, again 10mx10mx5m, so the 
geometry of the room mirrored over the rooms floor. This means that the 
bottom mkillum surface is located at +-0m, just 1m above the the ground 
disk. This bottom surface is generated by gensurf, and is actually an 
array of 20x20=400 squares of .5mx.5m each.

I place a view at 0,0,5.1 (10cm above the photon-port), pointing 
upwards, using vth-fisheye view type with 180 degree vertical and 
horizontal. And now I render four images and sum up all pixel values for 
each image:

scene classic (no illum, no pmap, ab 10): 1180668
scene with pmap (no illums, but also no photonport): 1013533
scene with illums (no pmap): 1522379
scene with illums and pmap: 1695772

mkillum was started with default values and -ab 10. Rendering with 
photonport and without mkillum does not really work, as the port does 
not see a light source.

So for now, the result is that, as long as mkillum is involved, the 
results are pretty stable. I would not call the two results close, but 
at least they have similar scales ;-) Having the two pixel value sums 
for the runs without mkillum being that much lower, I got worried 
however. Would I have to set much higher ambient parameters, or is there 
something wrong in my set-up? Is the use of exterior mkillum-koffers 
allowed?

Cheers, Lars.