[Radiance-general] ambient resolution and memory

[Carsten Bauer]

Gregory J. Ward wrote:

> 
>> Is there a patch for current radiance that allows an alternative
>> modifier (like void) for the indirect calculation? That is available
>> in Radzilla and might be very helpful, as I have colorpict maps on
>> most surfaces.
> 
> There is no such hack.  It's not really physical, is the problem.
> 

Hi Greg,

Since a long time already I have come to the conclusion that its better
to stay out of the discussion. I simply do some work based on and
inspired by the enourmous power and flexibility offered by Radiance,
including of course the fact that it is Open Source, and  quite
evidently this also means moving away form the original path, having a
different focus in mind, probably adressing a different group of users,
whatever.
So, to repeat myself, I normally avoid bothering the Radiance community
with my thoughts.

But being misunderstood is not a very nice feeling, so for an
exceptional case I dare to make a comment on this small example.
Please everyone don't feel offended :-) :-)

In the current example you have
a) a surface made out of a material with a complicated pattern, produced
either by a function or by mapping a photo. Its an architecture scene,
so the pattern is a common building material, marble, stone, wood,
whatever, showing a fine grain color modulation in the rage of some
millimeters or centimeters

b) you have a scene with bounding cube of some 10 meters to probably 100
meters, and an -ad setting of e.g. 1000. So ambient rays send out from
one point e.g. at the floor hit walls and other objects in the big room
at points maybe several 10 centimeters apart. How much of that fine
grain resolution of the applied pattern can really be sampled by this
'few' (1000 ambient rays *are* still few in a general sense) rays?

If you now specify an average color for that material, with correctly
determined values, I dare to state that this will be equally physical
correct in the limitations of the applied algorithm and general range of
accuracy the whole stochastic ray tracing method can provide.

I of course do not mean to set a blue paint as alternative ambient
material for a red painted wall. :-)

regards
-Carsten

PS: a lot of words for just one example, but this shows quite good how
difficult it is to avoid getting misunderstood.

PPS: unfortunately, experience has shown that the performace increase by
circumventing complicated pattern evaluations for the ambient rays is
only noticable for high ambient settings and scenes with lots of
pattern-modified surfaces (especially patterns which make heavy use of
the time consuming noise functions), then it can reach 10-15%, for low
ambient settings it is sometimes not noticable at all, so one really
cannot expect too much from this hack in this respect.

PPS: but there are other cases in which the alternative ambient material
can be useful, e.g. the topic of color bleeding (see the work of Marcus
Jacobs). Here it is a crude means to simulate the white balance adaption
occuring in the human visual system within the machine by simply
reducing the amount of color bleeding 'forwarded' by the ambient
calculation.
But this is a pure image generation issue, not a scientific one, so it
doesn't belong here in this list. Also, it is only an 'intuitive'
approach (probably there is even a way of performing this based on some
physio-psycho-physical model, but evidently I have by no means at all
the resources to pursue such an approach)