[Radiance-general] gendaylit vs gendaymtx

[Jan Wienold]

Hi Eleonora,

I had a presentation at the radiance workshop in Boulder (uff I think 
this was in 2013) - there you can find how gendaylit treats 
"problematic" cases (close to sunrise and sunset).
I assume (and hope), that the differences are only for these hours. The 
core code of gendaylit was checked several times against the publication 
from R. Perez and was not changed in the last 20years.

best,
Jan




On 05/12/2017 05:25 PM, Greg Ward wrote:
> Hi Eleonora,
>
> The gendaymtx command and gendaylit are separate implementations based 
> on the Perez model with no code in common.  The gendaylit program was 
> developed first, long ago by Jean-Jacques Delaunay, then more recently 
> expanded, updated and improved by Jan Wienold and Wendelin Sprenger. 
>  In contrast, the gendaymtx program was derived a few years ago from C 
> code provided by Ian Ashdown.  The two programs have been spot-checked 
> against each other, but as far as I know, no one has done a thorough 
> comparison of either the methods or the outputs to ensure they behave 
> the same.  In fact, there is some room for interpretation in Perez's 
> model, and numerous places where exceptional values must be caught and 
> dealt with.  Finally, I don't think all the options of gendaylit are 
> included in gendaymtx.
>
> If you can be more specific by checking when, where and by how much 
> your output vectors differ, this would help.  Or at the least, attach 
> the relevant part(s) of your London weather file so we can check 
> ourselves.
>
> Cheers,
> -Greg
>
>> *From: *Eleonora Brembilla <E.Brembilla at lboro.ac.uk 
>> <mailto:E.Brembilla at lboro.ac.uk>>
>>
>> *Date: *May 12, 2017 8:16:18 AM PDT
>>
>> *
>> *
>>
>> Hi everyone!
>>
>> I have a doubt on how gendaymtx works. Or better, what’s the 
>> difference between the Perez model implementation in gendaymtx and in 
>> gendaylit?
>>
>> I was comparing the output of these two methods, for each hour of the 
>> year:
>> gendaymtx -m 1 GBR_London.wea > London.dmx
>> gendaylit 1 4 9.5 -a 51.15 -o 0.18 -m -0 -W 178 76 | genskyvec -m 1 > 
>>  london_0104h9_rad.skv
>>
>> And I was expecting to get an identical result, but instead the two 
>> data series are somehow different. The data I used for the gendaylit 
>> -W option are taken from the same .wea file I used for gendaymtx.
>>
>> Sorry if you discussed about this previously, I searched a bit in the 
>> mailing list but I didn’t find a precise answer.
>>
>> Thanks a lot!
>> Eleonora
>>
>> *
>>
>> *
>> *Eleonora Brembilla*, MSc
>> /PhD Student/
>>
>> *Building Energy Research Group*
>> *Daylight and CBDM
>> *School of Civil & Building Engineering
>> Loughborough University
>> LE11 3TU
>> UK
>>
>> @EleBrembilla <https://twitter.com/EleBrembilla>
>
>
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-- 
Dr.-Ing.  Jan Wienold
Ecole Polytechnique Fédérale de Lausanne (EPFL)
EPFL ENAC IA LIPID

http://people.epfl.ch/jan.wienold
LE 1 111 (Office)
Phone    +41 21 69 30849

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