[Radiance-general] ambient light

[bei.xiao at gmail.com]

We tried placing the scene and camera within the context of a larger box and
this seemed to solve the problem that we were having.  Thank you everyone
for all your help.
-Bei

On Tue, Nov 25, 2008 at 2:02 PM, <bei.xiao at gmail.com> wrote:

> Thank you very much! I will try this and get back to you about the result.
>
> Bei
>
> On Tue, Nov 25, 2008 at 12:55 PM, Thomas Bleicher <tbleicher at arcor.de>wrote:
>
>> Hello Bei.
>>
>> On 25 Nov 2008, at 16:49, bei.xiao at gmail.com wrote:
>>
>>  Hello,
>>>
>>> I am trying to render an image using only ambient light.  I noticed that
>>> the reflected light appears to be non-uniform across the objects in the
>>> scene.  The scene is a open box with the open part facing the viewer and
>>> only surfaces inside the box are lit (outside of the box is dark).  It
>>> appears that the light has some directionality to it.
>>>
>>
>> The ambient light setting in Radiance is different to other renderers
>> - or maybe it's not if I think about it. Anyway, it's not supposed to
>> replace
>> a proper light source in your scene (a 'light' or 'source' material for
>> example). It's just there to 'take over' when the proper ray tracing
>> does not produce a light source in an acceptable time. That means
>> it's only applied it the renderer has not found a light source after
>> tracing a ray for '-ab' bounces through the scene.
>>
>> In your case with a box open to the dark void every ray that leaves
>> the box adds nothing to the illuminance level of a point. The closer
>> you are to the (front) edge the more rays escape the box and the
>> darker this point will be in the image.
>>
>> The points at the back of the box are only 'bright' because the
>> renderer assigns an artificial brightness to the ray after the first
>> bounce (-ab 1). You can probably achieve a smoother gradient
>> in your image by increasing the -ab value.
>>
>> If you enclose your open box and the camera with another
>> closed box you will probably see that there is no directionality
>> to the ambient light and everything will be uniformly grey (if
>> there is only one material). Every ray not hits a surface and
>> the brightness is only modified by the reflectance of the
>> material.
>>
>> Regards,
>> Thomas
>>
>>
>>
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>
>
>
> --
> Quote of the Day
>
> Writing your PHD thesis is one chance that you can be bold at your
> arguments  because it will receive no reviews!
>
> -David Brainard (my advisor)
>
> Bei Xiao
> Room 330 C
> 3401 Walnut Street C Wing
> Department of Neuroscience
> University of Pennsylvania
> Philadelphia, PA 19104, USA
> Web: www.psych.upenn.edu/~beixiao
> flickr:www.flickr.com/photos/slowtempo
>



-- 
Quote of the Day

Writing your PHD thesis is one chance that you can be bold at your arguments
 because it will receive no reviews!

-David Brainard (my advisor)

Bei Xiao
Room 330 C
3401 Walnut Street C Wing
Department of Neuroscience
University of Pennsylvania
Philadelphia, PA 19104, USA
Web: www.psych.upenn.edu/~beixiao
flickr:www.flickr.com/photos/slowtempo
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