[Radiance-general] accurate rendering of concavities

[Chris Kallie]

Dear fellow Radiance users,

Recently, I did an experiment that involved the study of subtly-shaded, 
locally-concave surface patches, including contours that were indirectly 
illuminated in some areas. The problem was, in these deeper regions of 
the object, the ambient lighting term dominated the rendered patch.

In the physical world, there is probabilistically only one darkest 
region (i.e., a single darkest pixel in HDRI) in any particular image of 
a scene. However, with Radiance, I find large patches that share the 
same darkest luminance value, and the subtle shading information is 
lost. In my last experiment, I was able to overcome this problem with a 
hack: I added a large, dim area light for fill. This, in effect, caused 
the cosine reflectance function to dominate the shading values, which is 
not the same as indirect lighting effects caused by inter-reflections 
and contour changes. Unfortunately, although that hack worked for my 
purposes at the time, it would not be the right approach for my current 
project. How does one eliminate this problem in his/her renderings using 
Radiance?

In general, is there a way to recreate an environment using Radiance 
that ensures photometric accuracy in subtly-shaded concave regions of 
convoluted objects (e.g., caused by gradual orientation changes, umbras, 
self-occlusions, and inter-reflections)?

Thank you for your thoughtful considerations.

Sincerely,
Chris Kallie