[Radiance-general] Sunlight hours calculation in radiance

[Dan Glaser]

Hi Giovanni and Lars!
   I think you guys have some good approaches.
   If # of images it produces is an issue for the original "brute force" 
method you proposed for checking direct sun exposure on clear days, you 
can overwrite the same image every-time after doing an image check and 
incrementing a variable.  Below is pseudo-code:

[loop over timebounds (can have a sun above horizon check here too)]
     [create/overwrite image SunCheckImage]
     [if (pixel value of representative point in SunCheckImage > threshold)
         num_timesteps_in_sun++]
[print "I've been in the sun for " + num_timesteps_in_sun + " time steps"]
[delete SunCheckImage]

   Sorry, I do not know off-hand an easy command for reading pixel 
values of .hdr images with Radiance (pcomb ?...), but this can be done 
elsewhere (e.g. ImageMagick...).
   Thresholds can be tricky, so take into account reflected, ambient, 
low angle, etc light (which is what you are doing with -ab...).
   You can also try intersecting the line between the surface and sun 
with the model geometry for each time-step...
   Good luck!

- Dan


On 2/15/2011 5:28 AM, Giovanni Betti wrote:
>
> Lars,
> the number of about 300 comes from subdividing each tregenza patch in 4
> to increase angular resolution and then looking only at the occupied
> patches, sorry for the confusion.
>
> I would be interested as well in seeing how other people approached the
> same problem though.
>
> Thanks,
>
> Giovanni
>
> -----Original Message-----
> From: Lars O. Grobe [mailto:grobe at gmx.net]
> Sent: 15 February 2011 12:16
> To: Radiance general discussion
> Subject: Re: [Radiance-general] Sunlight hours calculation in radiance
>
> Hi Giovanni!
>
>> I see how that's a possibility but it would mean running separate
> images
>> and to filter each of them.
> I guess it would not be impossible, as renderings should complete very
> fast when considering only direct.
>
>> simulation with multiple sun positions. Eventually as an optimization
>> the sun position can be clustereed according to tregenza patches and
>> each patch weighted based on the number of sun hours it represents (so
>> you don't even need 4000 sun but maybe only 300...)
>> Any hint on that?
> The tregenza sky introduces limitations of the angular resolution. You
> may try a two-step approach:
>
> First, for each patch, find out at how many time intervals the sun is
> covered by the patch. So a patch containing the sun solid angle 20 times
>
> a year, you would set the patch value to 20.
>
> Second, run rtcontrib to find out which patch contributes to which
> pixel. Here you can apply your threshold just as described in my former
> email, so any pixel that is lid by a patch is set to the value (in
> hours) for the patch from step 1.
>
> With a Tregenza grid, you would end up with 145 images, each pixel value
>
> corresponding to the number of hours of direct sun exposure from the
> related patch. Adding up all the 145 images in the third step should
> result in pixel values of direct sun exposure (hours) for all patches.
>
> BTW if you follow an approach as typically used when working with
> Tregenza skies, and split the sun over n neighbouring patches, you would
>
> of course set the pixel value to 1/n instead.
>
> I am not sure about better ways to achieve what you want to do, this is
> just one idea... any other proposals? I am sure others have done that
> before...
>
> Cheers, Lars.
>
>
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>


-- 
LF logo 	Daniel C. Glaser, PhD, LEED AP, IES
Principal
Light Foundry, LLC
T: 510.387.8890 | F: 315.410.2617
www.lightfoundryllc.com <http://www.lightfoundryllc.com>

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