the ambient cache is one of Radiance’s most powerful features, and
offers many ways to get things wrong. The fundamental rule is that you
should not change any settings of the ambient calculation unless you
delete the cache file.
I have reviewed the rtrace document in detail and this part was very
attractive (-af option):
."…by creating an ambient file during a low-resolution rendering,
better results can be obtained in a second high-resolution pass. "
Here are my questions:
- I actually wonder for the low-resolution rendering, which
parameters do not affect the accuracy of the last high-resolution
pass, negatively. For instance setting -ab 1 for the first pass,
then for the final pass setting it -4 will work? Which parameters
dramatically reduce the computation time? The syntax for my case
is as follows:
You should definitley not modify the -ab parameter when re-using the
cache. Otherwise, you would interpolate between pre-calculated results
accounting for only 1 ambient bounce, and other added values accounting
for 4 bounces. This would be physically not meaningful and likely cause
The idea in the sentence you quote is that you first generate only the
ambient cache (the low-resolution rendering would in most cases get
discarded). Then, you use the pre-calculated values when running your
actual simulations. Note that the cache is continuously growing.
However, starting the simulations with the pre-filled cache avoids
artefacts since some basic info on the ambient illumination is known
before rendering the first pixel.
In some cases, one may reduce the sampling accuracy in the final
simulation, if the initial pre-calculation was accurate enough. This may
include e.g. slightly increasing -aa. However, I would advice against it
unless you do thorough tests ensuring that this does not introduce
significant error in your particular case. And probably, that would not
save you much time then.
Note that if you plan to later move toward CBDM, the ambient cache will
not be available.
rtrace -af output\ambientFile -e output\efile.txt -I -h -dp 2048 -ms
0.063 -ds .2 -dt .05 -dc .75 -dr 3 -st .01 -lr 12 -lw .0005 -ab 2 -ad
1000 -as 20 -ar 300 -aa 0.1 output\trial.oct <
sensorpoints\subdivided.pts > output\trial.dat
I would typically start with -as set to half of -ad. That allows
Radiance to refine the cache where necessary. -ab 2 is low for typical
interior scenes. If you really need to account for interior
inter-reflections, e.g. by bright, diffuse walls, you should increase
-ab to 5 or 6 at least. A sensitivity analysis for few exemplary sensor
points may reveal the required ambient bounces.
- The output file created by rtrace includes 3 columns, why is that?
Because, all the 3 columns are exactly the same for the regarding
rows. I think this may increase the computation time, am I right?
Some part of the output file is as follows:
The columns reflect that Radiance calculates with three channels, which
are typically assigned values for red, green and blue. Since you are
obviously working with grey values, the three channels will have