That's the point I concluded to as well, after all this.
Formerly, I just thought that the two-pass was better
because one could save time on the first pass and
then reuse it for second pass with larger resolution,
as well as smoothening errors. At the beginning, I
didn't know that low_error_factor was doing just
that.
Then, I found out about low_error_factor, and I
guess that I screwed something up somewhere, because
my latest experiments never took so long again...

But since always_sample only reduces sample-taking
on the final trace to a bare minimum, one could even
leave it switched on for the first pass, and switch it off
for the second pass.

To wrap it up: the two-pass method with increasing
resolution from first to second pass, shoots several more
samples than with low_error_factor alone and thus results
in better quality of image. I can't consider times, as my
experiments showed to be faster for the two-pass method,
but this is highly scene-dependant.
Additionally, one can tamper with several other settings
than just error_bound on the second pass as well, and
thus, one has more control. But that's the main idea of
doing several passes most of the time, isn't it?

But I'm glad that I've understood more now than in
the beginning. Shows me that it was worth it. :-)

Oh, and thanks Mael and Christoph, for the help
and suggestions!

Regards,
Tim

-- 
Tim Nikias v2.0
Homepage: http://www.digitaltwilight.de/no_lights


> > Hm. So the actual difference lies in the samples on the
> > final trace. If always_sample would really kill samples on
> > the final trace, then there'd be no difference, and the whole
> > two-pass issue would be possible with one-pass.
>
> I'm not sure but I think that the two-pass method works well precisely
> because of the quite large number of samples disponibles (when averaged,
> with the bigger error_bound, it smooths out possible errors). If you don't
> have the samples from the final trace of the 1st pass you might loose the
> benefit of the two-pass.
>
> M
>
>


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