[Radiance-general] genskyvec with HDR sky view?

[Achim Geissler]

Hi Greg,

thanks for the pointers. I am afraid, though, that I was unable to really
make heads or tails of them.
a) HDR file (Photosphere)
	i, the (test) HDR gives min/max values in falsecolor view of 39 to
2900, whatever the unit, there
	ii, it gives histogram min/max values  for Luminance of 34.8 to
4.43E+06
	iii, it does not seem to be clipped
b) There is a sun (albeit behind a thin cloud cover). Simply adding a sun
definition to my sky description did not really change anything, though.
Also, at this point I do not know which r,g,b radiance values to use for the
solar "light" material and do these influence the result? And, actually, how
to exactly position the HDR so that the sun positions tally.
c) I took a look at the genskyvec.pl script, however, I am not perl-literate
so that is a bit difficult. I seem to be missing out on where this script
>could< use values from the HDR file to generate the sky vectors. Part of
the code seems to be a copy of the setting up of patches from reinhart.cal.
Also, the "skyfunc glow skyglow" and "skyglow source sky" are written to the
(an) octree in the script. Can I ignore that? Or does that let me leave
these definitions out of my sky description? Which definition has precedent?
I can't really see where the "skybright.cal" values from a standard gensky
sky would come into play - this should be what is replaced by values from
the HDR?
d) using the sky description as below but with "skyfunc" gives 
rpict: warning - compute error for colorpict "skyfunc"
rpict: Division by zero
 when trying to generate a view of a scene.

I must admit the flow of data (input / output) in all the definitions is a
bit of a mystery, for me. I did find a kind of description for colorpict in
p_data.c, but that didn't really help, sadly. Looking at your code, I get
the feeling I am not literate in any programming language ... :o(

If you (or, of course, anyone else out there that has done this before) can
give any further help or tips, it would be much appreciated.

Best
Achim


-----Ursprüngliche Nachricht-----
Von: Greg Ward [mailto:gregoryjward at gmail.com] 
Gesendet: Samstag, 28. Mai 2016 01:27
An: Radiance general discussion
Betreff: Re: [Radiance-general] genskyvec with HDR sky view?

Hi Achim,

I wouldn't call what you are trying to do "easy," even if others have done
it.  Your HDR file has to be in the correct units or scaled by the right
amount to get calibrated radiance values, and even then, the sun is usually
out of range(clipped) and won't be properly sampled if it is a clear sky.

As you noted, such a sky cannot be given as input to genskyvec, which
expects a very specific set of primitives output by gensky or gendaylit.
Specifically, you need to name your colorpict pattern "skyfunc" and add a
solar source if the sky is sunny.  You could try to modify your description
to match, but you'd have to look at the Perl script src/util/genskyvec.pl to
figure out how to satisfy the input requirements.

Once you have a sky description, you can use rtrace with the -I option to
compute hemispherical irradiance, from which you can create an adjustment
factor to apply to your glow based on horizontal irradiance.

It's all possible, but the details are not very straightforward.

Best,
-Greg

> From: Achim Geissler <achim.geissler at intergga.ch>
> Date: May 27, 2016 12:29:35 PM PDT
> 
> Hi all,
> 
> after searching high and low without clear results apart from a lot of
pointers that “everybody seems to be doing it” (so it should actually be
easy and I am probably missing the forest due to seeing trees, as they say)
a short question: The goal is to use HDR (fisheye) views of the sky for
radiance calculations. All I could manage to date are pretty pictures with
HDR skies. Ideally, the workflow would be “the same” as when using a sky
from gensky or gendaylit. E.g., it would be nice to be able to generate the
sky vectors (genskyvec) from the HDR sky view. Is that possible? How? Trying
to simply pipe a sky definition along the lines of
> 
> void colorpict skyfunc_hdr
> 7 red green blue sky_image.hdr map_sky.cal u v
> 0
> 0
> 
> skyfunc_hdr glow sky_glow_hdr
> 0
> 0
> 4 1 1 1 0
> 
> sky_glow_hdr source sky_hdr
> 0
> 0
> 4 0 0 1 180
> 
> into genskyvec didn’t work - well, would have been too simple (the
map_sky.cal is from the PhD by Coralie Cauwerts, 2013). The following
approaches resulted in the “pretty pictures” (replacing “source” above)
> 
> sky_glow_hdr bubble sky_hdr
> 0
> 0
> 4 0 0 0 2000
> 
> or
> 
> !genbox sky_glow_hdr boxenv 2000 2000 1000 -i | xform -t -1000 -1000 -500
> 
> A further topic will be a “calibration” of the HDR sky views - we have
global horizontal and diffuse horizontal radiation values available. The
current idea of a (possibly slightly rough) method of calibration would be
to use the HDR sky to calculate the corresponding sensor value with radiance
and “somehow” introduce a scaling factor into the HDR sky such that the
measured and calculated global horizontal radiation values tally.
> 
> Thanks for any hints & ideas!
> 
> Best
> Achim
> 
> achim.geissler at intergga.ch

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