[Radiance-general] Shadow above highlight?

Thu Aug 13 16:31:25 PDT 2009

John,

I'm running your scene and am witnessing the same effect.  Interestingly the
dark spot still exists, it is just much less pronounced.

I am both able and unable to understand why this works.

Years ago I remember someone mentioning the impossibility of ambient
sampling normal to a surface and it's lingered in my mind ever since.  I've
always struggled with the ambient calculation's ability to sample the
horizon which technically contributes nothing to illuminance at a point
while it was unable to sample the zenith which contributes most to
illuminance.

I've thought about suggesting the range for ambient sampling switch from
[0,1) to (0,1]  but couldn't grasp the ramifications of the possibility of
sampling the zenith from all of the zenith adjacent ambient divisions.

It seems to me that statistically it isn't possible for this to make a
difference so maybe there is a bug in the ambient super sampling,  but it
also seems to me that statistically this must make a difference.

So I guess I'll just have to wait for Greg and others to weigh in...

Andy

On 8/13/09 4:05 PM, "schwaj3 at rpi.edu" <schwaj3 at rpi.edu> wrote:

> Wow, reducing the -as to 0 seems to have relieved the problem. I had no
> idea that they don't shoot directly up (I had guessed it might have been
> something along those lines, but couldn't track it down).
> 
> The RwR book seems to suggest that you use a -as equal to 1/4 your -ad,
> so I can't imagine that reducing the -as to 0 should be the correct
> solution... Perhaps the model I am using simply exacerbates this issue?
> 
> Could you explain when it's reasonable to use -as 0 and a large -ad, as
> opposed to the values recommended in the book? Does it depend on the size
> of the highlight on the floor (smaller = more/less -as  )?
> 
> Naturally like you suggested Andrew, this model is much faster to do with
> radiosity. It's really just a test model, and naturally more complex
> models would have to be used. However after I saw this, I naturally
> couldn't proceed with image comparisons until I figured out what I was
> doing wrong! As you've probably seen, I've sent out the scene, so if you
> have any further insights, especially into what rpict options I should be
> using, do tell.
> 
> --John Schwartz
> 
> ==============Original message text===============
> On Thu, 13 Aug 2009 18:01:09 EDT Andrew McNeil wrote:
> 
> 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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