[Radiance-general] What's causing these artifacts???

[Marcus Jacobs]

Dear Group

Thanks for all of the suggestions regarding my rendering problems. I know 
its taken me a while to reply back about this issue to the group. I have 
spent some time investigating these issues that I am having. This way, I at 
least try to solve my problems myself before bringing to this group.

First, the dark splotches:

http://i35.photobucket.com/albums/d178/marcdevon/AFrame2_Camera6_final.jpg
http://i35.photobucket.com/albums/d178/marcdevon/AFrame2_Camera03_final.jpg

I think I have some idea as to what is causing this issue. Generally when I 
render, I perform a small overture calculation at a 100 x 100 pixel 
resolution. The ambient settings are generally at -aa 0.1 and -ar 0. The 
next step is to take a medium resolution overture usually at 500 x 500 
pixels. The settings are generally set at -aa (.15 - .2) -ar = maximum scene 
size as determined by getinfo -d in inches (basically a 1 inch grid).  The 
final render is performed at -aa (.25 - .3) and -ar = maximum scene size as 
determined by getinfo -d in inches/(2 to 10) (basically a 2 - 10 inch grid) 
depending on the illumination contrast. The -ad and -as settings are 
generally set based on the illumination contrast in the scene. For these 
renderings, I set -ar very low since I had already rendered it at the same 
resolution with higher -ar settings. Essentially, I wanted to force it to 
interpolate using the existing ambient data. The splotches usually come in 
as the -ar parameter degrades. The number of splotches depends on how low 
this parameter has been set. The reason why I ask is because this seems to 
me to have started occuring recently even though I have used the 
aforementioned technique for some time now.

For the bright splotches here:

http://photobucket.com/albums/d178/marcdevon/th_room2a_view2_render1.jpg

it looks like mkillum saves the day again. I tried Greg's suggestion of 
setting an illum at skylight openings at the surface level of the ceiling. 
This seems to have done the trick. It's funny but I ran dozens upon dozens 
of renderings and for some reason I thought these artifact were due to the 
bright spots on the floor rather than the skylight openings. Here are the 
results:

http://i35.photobucket.com/albums/d178/marcdevon/room2_view3.jpg
http://i35.photobucket.com/albums/d178/marcdevon/room2_view2.jpg

May I ask, why is the brightness of the skylight openings more prone to 
causing these artifacts rather than a bright spot on the floor or wall?

When initially attempting to find a solution to this problem, I tried to go 
back to issues that Rob Guglielmetti  was experiencing a couple of years 
back with a museum that he was modeling. At one time he documented his 
progression on his blog but the page has since been deleted. I did remember 
a couple of threads from July/2004 which discussed whether to place illums 
inside or outside the scene:

http://www.radiance-online.org/pipermail/radiance-general/2004-July/001871.html
http://www.radiance-online.org/pipermail/radiance-general/2004-July/001953.html

The reason why I have been hesitant to using illums inside is because I 
thought that mkillum could only simulate illumination that's a smooth 
function with no discontinuities. I thought that this may be an issue when 
windows have lites. I did a little experiment with another old scene of 
mine. It is a window that I knew would receive direct illumination from the 
sun. I wanted to place the illums on the inside to see if mkillum would pick 
up the lites. Here is the result:

http://i35.photobucket.com/albums/d178/marcdevon/room2_camera1_tm.jpg

I am impressed that mkillum has able to detect the presence of lites in the 
windows as you can see their occluding effect by the direct sunlight that is 
on the floor.  By placing the illums on the inside of this room, I was also 
able to eliminate some bright splotches (although less conspicuous than the 
previous scene) from this room.

In my investigation of course there are more questions/issues that has 
arisen.  From another test room, I wanted to see how much the smoothness the 
indirect calculation would improve by placing an illum inside its skylight. 
This is what's happened:


http://i35.photobucket.com/albums/d178/marcdevon/room1_12_amb_view4.jpg
http://i35.photobucket.com/albums/d178/marcdevon/room1_12_amb_view5.jpg
http://i35.photobucket.com/albums/d178/marcdevon/room1_12_amb_view6.jpg
http://i35.photobucket.com/albums/d178/marcdevon/room1_12_amb_view8.jpg

The indirect illumination in the corner I do not believe is correct. I know 
that this problem isn't due the presence of the illum in the skylight at the 
surface level of the vaulted ceiling. This seems to occur sort of 
haphazardly. Here is the same room with the same day/time settings, albeit 
with different materials, and this problem didn't occur:

http://i35.photobucket.com/albums/d178/marcdevon/room1_b_view4_render2.jpg
http://i35.photobucket.com/albums/d178/marcdevon/room1_b_view5_2.jpg
http://i35.photobucket.com/albums/d178/marcdevon/room1_b_view6.jpg

This problem that I am receiving looks similar to issues that I am having 
with this rendering (right side near the corner):

http://i35.photobucket.com/albums/d178/marcdevon/KountryKitchen_view2_render3_3.jpg

Also, for the previous rendering and the next, I do not know what's causing 
the gray artifacts on the floor nearby the dishwasher and lower cabinets:

http://i35.photobucket.com/albums/d178/marcdevon/KountryKitchen_view1_render3_99.jpg


Generally, what I wish to be able to acquire is achieving a higher level of 
predictability so that I may take a scientific approach to rendering rather 
than constantly tinkering. Sometimes when I think I got it, I later realize 
that I don't. That's pretty much it in a nutshell.


Thanks everyone for all of your help.


Marcus