[Radiance-general] rtrace result varies for each calculation

[Ji Zhang]

Dear Thomas,

Thank you very much for your detailed explanation! We'll try out the
parameters according to your suggestions later.

May I also ask:
1) how are these parameters relates to model size?
2) The formula (6) in Axel's tutorial (p.33) is about the radius of the
"splotches". If the model is fairly large, e.g. 100m, then what would you
suggest for "aa" and "ar"?

Thank you again!

Ji


On Tue, Jul 13, 2010 at 4:40 PM, Thomas Bleicher
<tbleicher at googlemail.com>wrote:

> Ji.
>
> You can always expect some variations in Radiance calculations. The
> trick is to modify your ambient parameters (in general options
> starting with "-a") until you're satisfied that the error has no
> relevant impact on the results. The "right" ambient parameters depend
> on your scene complexity and and the rendering time you can afford to
> spend.
>
> A few further comments on your parameters are below:
>
> On Tue, Jul 13, 2010 at 5:41 AM, Ji Zhang <hope.zh at gmail.com> wrote:
> > The rtrace command I'm using is:
> >
> > cat SENSORS.pts | rtrace -I -aa .2 -ab 8 -ad 1024 -as 512 -h -w -oov
> > scene.oct | rcalc -e '$1=$1;$2=$2;$3=$3;$4=179*(.265*$4+.670*$5+.065*$6)'
> >
> > RESUTLS_lx.dat
>
> -aa - (ambient accuracy) seems a bit high (although I don't know
> anything about your scene). A value of 0.2 allows for 20% error in the
> ambient calculation. If the sky in your scene is hardly visible this
> will lead to significant errors. In general I prefer values of 0.05
> and less. Reducing this value will increase your rendering time. If
> you set it to 0 you will turn ambient interpolation (and the
> associated errors) off but it will take much longer to calculate.
>
> -ab - (ambient bounces) is generous. If you have a simple scene where
> the sky is visible from most locations you can safely reduce this to 6
> or even 5 which will reduce your rendering time to a quarter of the
> previous run. Note that your average results will increase a bit with
> each additional bounce. You should do a series of calculations with
> increasing -ab values to estimate the effect of one bounce more or
> less.
>
> -ad - (ambient divisions) could be a bit higher. I use 4096 these
> days. It doesn't have a big impact on rendering time.
> In a simple room with one or two large windows and no external shading
> 1024 will do. If you have lots of small windows or external shading
> devises use a larger number.
>
> -oov - You will find that the origin (second "o") is reported
> differently from your input file. This is an effect of the -I setting
> of rtrace. Don't worry about it. If you want to convert it you have to
> reduce the z-value by 1.
>
>
> > However, I found that the output values in the RESULTS_lx.dat file varies
> > each time I execute this rtrace command in the Terminal in Ubuntu 10.04.
>
> As stated above, some variation is normal. For you can add "-u-" to
> your rtrace command line which changes the random sampling algorithm
> used to a "low-discrepancy sequence". That and setting "-aa" to 0
> should give you the most reproducible results. However, that alone
> does not guarantee that the results are correct.
>
> Homework:
>
> Reduce your set of sampling points to a line from the front (window)
> of the room to the back. Then start with your command line above and
> modify the parameters as discussed. Do more than one calculation with
> the same set of parameters and plot and compare the results. Also keep
> a record of the rendering times.
>
> A good set of options generates a smooth DF curve from the front of
> the window to the back of the room that does not change a lot between
> separate calculation runs. If the curve is not smooth your error is
> too high and you have to increase/decrease some values.
>
>
> Regards,
> Thomas
>
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-- 
ZHANG Ji 张冀 (PhD) :: Research Fellow :: Centre for Sustainable Asian Cities
:: School of Design and Environment :: National University of Singapore :: 4
Architecture Drive,  Singapore, 117566 :: Contact: 65-6516 5046 :: Email:
sdezj at nus.edu.sg
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