[Radiance-general] rtrace result varies for each calculation

[Ji Zhang]

Thank you very much, Chris!

Ji


On Tue, Jul 13, 2010 at 9:54 PM, Christopher Rush <Christopher.Rush at arup.com
> wrote:

> -ar - (ambient resolution) This may also improve accuracy by setting the
> -ar parameter higher than the default. This would help mainly if you have a
> large scene with a small area of interest (eg a model of an entire large
> office tower but seeking high level of accuracy for points within one small
> private office).
>
> -Chris
>
> -----Original Message-----
> From: radiance-general-bounces at radiance-online.org [mailto:
> radiance-general-bounces at radiance-online.org] On Behalf Of Thomas Bleicher
> Sent: Tuesday, July 13, 2010 4:40 AM
> To: Radiance general discussion
> Subject: Re: [Radiance-general] rtrace result varies for each calculation
>
> 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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