[Radiance-general] visual transmittance

[Carsten Bauer]

Hi Christoph !

For some reasons this is an intersting topic for me, too, although I
don't know much about rtrace (never used it so far, I'm working only
with rpict/rview).

Under what circumstances did you perform the calculation ? Direct light
only, or ambient light also? Which were your parameter settings (direct
and ambient ?) Did you perform several consecutive runs with rtrace to
get a mean value ?  For this, parameters have to be changed slightly to
"trick" the internal random generators, otherwise you simply would get
the same result again. From my answer so far you can imagine where my
thoughts are heading at: Within the algorithms, a lot of random
fluctuation, ray direction jittering etc. is performed. This leads to
noticeable deviations of results. (In a comparison experiment, I once
observed relative brightness differences between 0 and 8% throughout the
main part of an image, near one shadow boundary up to 18% and higher,
for two runs of the same scene).


Carsten