[Radiance-general] genskyvec with HDR sky view?

[Greg Ward]

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