[Radiance-general] Understanding Limit Reflection

[Carsten Bauer]

Hi Marcus,

-lr limits the reflections form any 'normal' specular surface (e.g. a plastic 
with fourth parameter greater than zero). If light from sources hits such a 
surface, its reflection doesn't get considered in the direct calculation 
anymore (you need the mirror material which produces virtual sources 
instead). However, the reflected light gets considered within the ambient 
calculation, so the -lr setting has an indirect influence on the accuracy of 
the ambient calculation. 
Otherwise, -lr is completely independent from the ambient settings and the 
virtual source settings, i.e. it doesn't interfere with -ab or -dr (number of 
relays from virtual sources)

Often the additional specularity of normal room material surfaces doesn't 
contribute that much to the illuminance, with exception perhaps of direct 
sunlight shining on a polished tabletop with glossy finish or the like. I 
remember this because of my experiments with the direct cache. In such a 
special case like above, for example, I observed relative errors of around 
10-12 % occuring when ommiting the specular part completely (as if one would 
set -lr = zero).So setting -lr = 2 shouldn't do much harm normally (I think 
the default is 6 ). A different thing however is the generation of those 
typical weird images with opposing specular surfaces where you see the 
reflection of the reflection oft the ... etc, in this case -lr needs to be 
(almost) infinite, of course. ( I once saw something on a website of one guy 
playing around with this feature, I forgot the url, you may ask pillo, I 
think he gave me the link..

Another hint: if you set a very high -lr, beware of -lw (weight), too, which 
is lurking in the background, often unnoticed...:), and cuts off every ray 
which doesn't contribute more radiance than the -lw setting to the 
calculation.

-Carsten