[Radiance-general] ambient light

[Thomas Bleicher]

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