[Radiance-general] DGP

[Jan Wienold]

Hi Alejandro,

the question is not that easy... Usually you compare two cases and then 
look for the differences (as you want to do)
But the first question is, what is the "reference" you want to compare 
to. A comparison with a black surface is definitely not "fair" - it 
overestimates the influence of the facade. I would rather use a case 
without an opposite building or using one with a diffuse surface (but 
then again, what color do you assume as "reference"... if you are on 
greece islands, this would be white around 70% reflectance, in northern 
Germany red brick around 20% reflectance...). So not to end in 
discussions about a meaningful reference, I would go for the "no 
building" reference. So then you have two cases and you end up with two 
different values, so you can state "the facade causes an increase of the 
DGP by XX compared to an unobstructed view to the surrounding.

If you are dealing with specular reflections of the sun on your facade, 
then you have to be careful to use a larger image size.  (at least 
1200x1200, better 1500x1500 in order to have enough pixels to see the 
reflexion of the sun)- even evalglare is getting very slow. and be 
careful about pixel sampling, jittering settings and the -lw limit.

And just to be sure - don't use any of the existing simplified methods 
to calculate DGP (they are not based on a full rendering), because they 
are based on illuminance calculations using rtrace -I - in that case you 
will miss the specular reflection of the sun (it doesn't matter if it is 
glass, plastic or metal) if you don't use a special material mix on your 
surface containing also the mirror material.

And finally: Be aware, that DGP is just a measure for discomfort glare 
INSIDE a building. It does not include disability glare and is also no 
measure for discomfort glare OUTSIDE a building, where you have much 
higher adaptation levels.

Best,

Jan


Am 4/22/15 um 4:21 PM schrieb J. Alstan Jakubiec:
> Hi Alejandro,
>
> One option is to run the evalglare program with the -d and -c parameters.
> > evalglare -d -c check.hdr image.hdr
>
> This will do two things: (*1*) the -d option will give you a list of 
> all contrast-based glare sources, including reflections from your 
> facade; (*2*) the -c option will colour the separate contrast-based 
> glare sources randomly in the check.hdr file. You can even plug the 
> values from 1 into the contrast half of the DGP equation.
>
> Of course running the calculations either with the building as a black 
> hole or (more realistically) as not present at all produces some 
> comparative value as well.
>
> Best,
> Alstan
>
> On 4/22/2015 9:58 PM, Alejandro Pacheco Diéguez wrote:
>>
>> Hi everyone,
>>
>> I have a question about DGP.
>>
>> I want to assess the DGP at a certain point in my building caused by 
>> the façade across the street.
>>
>> What I have done is running the simulation DGP simulation, then 
>> setting the facade as a black body and run it again (this way I 
>> account for everything but the façade). Then I subtracted both 
>> results to get the DGP produced by the façade.
>>
>> However, I am kind of suspecting that DGP is not that simple (since 
>> it also considers factors like contrast) and one cannot operate with 
>> it in that way.
>>
>> If so, can someone think of an alternative way to measure glare 
>> produced by a certain elemhent?
>>
>> Thanks in advance,
>>
>> Alejandro
>>
>>
>>
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>
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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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