[Radiance-general] rtcontrib and rtrace-parameters

[Thomas Bleicher]

Hi Martin.

On Sat, Mar 6, 2010 at 5:37 PM, Martin Klingler
<martin at lichttechnik.co.at>wrote:

> The first sky (k4) gives direct sun for the first two measurement points,
> with the second sky (k5) there is no sun entering the room.
> For the rtcontrib run I use reinhart.cal and compared two MF-settings (2
> and 4). The rtrace-options are –ab 6 –ad 4096 –as 2048 –aa 0 –dj 1 –lw
> 2e-10.
> With this settings I get the distributions in Graph1
>

[graph removed]


> To get the distribution I use genskyvec.
> Additional I did some rtrace runs with the same options und compared the
> results for sky k4 in the second graph
>

I think you wanted to write 'rtcontrib runs' - at least that's what your
labels say.

When you use genskyvec you also remove the sun 'source' from the sky
description and distribute its brightness to the surrounding sky patches.
rtace treats a sky with sun differently than a sky without sun. Therefore
you will not achieve identical results from rtrace and rtcontrib.

[graph removed]


> To this results the are some questions:
> 1.) Are this useful rtrace-options, am I on the right track?
>

That obviously depends on your scene complexity. If you have blinds in front
of your window I would say that the results you get are sufficiently
accurate for climate data studies.

If you have a simple geometry like in Axel's tutorial I think that the -as
2048 is overkill. You already have a high -ad setting which should account
for window openings in a typical room.

Effectively you have turned off ambient caching by setting -aa to 0.
rtcontrib does not use ambient caching anyway so that's probably not going
to make a difference. Ambient caching might have smoothed out your
individual point results a bit.

Your -dj value seems high, but given that there are no sources it should not
matter.


> 2.) What can I do, to get closer to the rtrace-results?
>

As I said above, you can not reproduce the direct calculation with
genskyvec. You could split out the direct component and do a separate
calculation just for the sun. It will just make things far more complicated,
though.

You could try and switch off Monte Carlo sampling with '-u-'. That should
give you a deterministic sampling pattern which should result in 'identical'
values.


> 3.) MF=2 seems to get smoother results, than MF=4 - dose that make
> sense?
>
4.) Can I reduce the differences between different rtrace-runs with the
> identical options?
>

rtcontrib-runs?

Perhaps you should investigate where the error originates in your
calculation chain. When you use rtcontrib to calculate illuminance values at
a point the only value affected by the rtrace options is the contribution of
each sky patch. This is only a fraction of the 'brightness' the patch is
ultimately assigned so small variations in the contribution can result in
noticeable changes of the illuminance values. I would start by comparing the
patch contributions between rtcontrib runs. If they are reasonably constant
your error is introduced somewhere else.

IIRC the multiplier for each patch contribution is calculated via genskyvec
at specified directions (saved in tregsamp.dat). That should result in
constant values for the multipliers but you can still check.


Overall I think that your current values are sufficiently accurate. In the
end you will apply these values to a large number of (inaccurate) sky
descriptions to then calculate an average from all these results. The error
you introduce by the use of insufficient sky models should largely exceed
the error resulting from the rtcontrib/genskyvec process.

Anyway, good that there is more interest in validating climate data
calculations.

Regards,
Thomas
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