[Radiance-general] Shadow above highlight?

[Andrew McNeil]

John,

I'd also like to see your scene, but while I wait I'll go out on a limb:
AFAIK in radiance, an ambient sample ray is never emitted normal to a
surface.  I suspect your shadow results from the (slightly) decreased
probability of sampling the highlight from the area directly above the
highlight.  This effect can be compounded by the -as setting (which never
samples the upper edge of a division, but can sample the lower edge of a
division).

My suggestion: try a high ad value coupled with no ambient super samples.
-ad 20000 -as 0  


As far as your comparison between raytracing and radiosity, You pick a scene
that plays to all the strengths of radiosity.  Radiosity with a dense mesh
produces a highly accurate result in relatively little time for scenes that
have very simple geometry and are limited to completely diffuse reflection
and completely specular transmission.

If you had semi-specular reflection, semi-diffuse transmission or complex
geometry raytracing is more likely to result in higher accuracy in less time
than radiosity.

Andy



On 8/13/09 1:58 PM, "Greg Ward" <gregoryjward at gmail.com> wrote:

> Can you send me your scene description, so I can have a look at it?
> This is the worst possible timing for me, but you're going to get
> nowhere unless you share your scene files.  It's impossible to deduce
> what's going wrong from an image.
> 
> -Greg
> 
>> From: schwaj3 at rpi.edu
>> Date: August 13, 2009 1:42:40 PM PDT
>> 
>> Hmm, well no suggestions thus far has helped solve the problem I am
>> having. I think it would help if I explain what I am trying to do. I
>> am
>> attempting to validate radiosity calculations against a raytracer (in
>> this case Radiance). I am doing this with a simple example of a box
>> lit
>> only by direct sun and ambient bounces, so there is no sky
>> distribution.
>> 
>> I have attempted many different rendering options, such as -aa 0 and
>> many
>> other "high quality" level options as recommended in the Rendering
>> with
>> Radiance book. These do not get rid of the problem. As you can see
>> in my
>> gensky below, the sun shouldn't be approaching the zenith, as it has
>> been
>> revealed can cause problems.
>> 
>> I was reccommended to try using mkillum to turn the window into an
>> illum
>> surface. I have attempted this, but it doesn't appear to do
>> anything, so
>> I could be using it incorrectly? I am only using a single ambient
>> bounce
>> in my calculations because the shadow is still there even on higher
>> amounts, such as -ab 20. I decided -ab 1 would make it easier to track
>> down the problem as well as take less computation time.
>> 
>> Any more thoughts on what could be causing this? Could it be a problem
>> with gensky similar to the zenith problem maybe?
>> 
>> --John Schwartz
>> 
>>> From: schwaj3 at rpi.edu
>>> Date: July 27, 2009 3:41:40 PM PDT
>>> 
>>> I am fairly stumped as to what is going on here. I have a box with a
>>> square hole, 1/3 x 1/3 the width of the box, to allow sunlight into
>>> my room.
>>> 
>>> I am using the following gensky to attempt to only light the room
>>> using
>>> sunlight.
>>> !gensky 6 21 12:00EST -a 40.77 -o 73.967 -g 0.0 +s
>>> skyfunc glow sky_glow
>>> 0
>>> 0
>>> 4 0 0 0 0
>>> 
>>> sky_glow source sky
>>> 0
>>> 0
>>> 4 0 0 1 180
>>> 
>>> This picture was rendered using -ab 1
>>> http://i639.photobucket.com/albums/uu111/radiancebox/radiance.jpg>
>>> You'll note on this image, that there is a strange shadow on the  >
>>> ceiling.
>>> I am unable to figure out why.
>>> 
>>> If I instead place a light in the room, also 1/3x 1/3 the size of the
>>> box, I do not have this problem. This picture is taken from the
>>> "back" of
>>> the box, with the light on the ceiling instead of the floor. There
>>> is no
>>> gensky used on this picture.
>>> 
>>> http://i639.photobucket.com/albums/uu111/radiancebox/Light.jpg
>>> This  > picture was rendered using -ab 0>
>>> Changing the camera angle doesn't effect the shadow seen in the first
>>> picture. I have determined that the shape of the window is not
>>> causing
>>> the shadow, instead the highlight on the floor causes the shadow on
>>> the
>>> ceiling.
>>> 
>>> If anyone can help clear this up for me, I would appreciate it.
>>> 
>>> --John
>> 
> 
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