[Radiance-general] Sunlight hours calculation in radiance

[Thomas Bleicher]

On Tue, Feb 15, 2011 at 7:28 AM, Giovanni Betti <
gbetti at fosterandpartners.com> wrote:

I would be interested as well in seeing how other people approached the
> same problem though.
>

In the UK you have to calculate sunlight hours on the ground for planning
purposes.

We used a grid (as opposed to an image) and traced rays from each grid point
to the sun (direction was calculated from the gensky sun angle information).
We modified the rtrace command to print the material of the surface it hits
instead of the radiance. If the ray hits the sky (or nothing) it returns
"*", if it hits an obstruction it returns the material name. Then you just
need to count the "*"s in the output file to know your number of sunlight
hours.

For planning this is done on one day only, so we ran it with a 5 minute
resolution. It only took minutes for a reasonably sized grid and that
included all the preparation and evaluation in Lua (Ecotect). This method
generates a data array (x,y,n) which you have to visualise in a separate
step.

[On second thought you can probably combine this method with an vwrays based
preprocess to find all the surface points in the scene. For these scene
points you then evaluate the number of sunlight hours as above and return
the number as value for the image. Sounds like a longish simulation,
though.]

If you want pictures your idea with rtcontrib and optimised suns seems very
promising. A Tregenza subdivision is not optimal because the sun only
occupies about a quarter to a third of the sky. But you can run rtcontrib on
defined light sources so you just need to give all your suns a unique
identifier and you're set (the command line length might be somewhat
excessive, though). You can also interpolate between the sun positions
without loosing too much accuracy so you really only need to evaluate a few
hundred positions. It might be worth to split the summer from the winter.

If you want to read the sun light hours directly from the final image you
have to use a unified sun source and combine all the images with a cosine
corrected intensity for this particular surface which results in a 1 for
each bright pixel on that surface. The final image gives you a direct
representation of the sunlight hours. If you have more than one surface
(more than one surface orientation, that is) you should go with Lars'
recommendation and apply a threshold value to convert all bright pixels to a
1 value, regardless of the intensity.

Regards,
Thomas
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