GenUtahSky

Radiance implementation of Preetham, Shirley, Smits model for sky color, plus more

What is this?

Users of the Radiance Synthetic Imaging System (http://radsite.lbl.gov/radiance/) can use the software in this repository to generate and render realistic skies for anywhere on Earth at any requested time (no clouds, sorry). The colors of the sky come from the Preetham, Shirley, Smits (1999) paper. Extras added by this author include: realistic sun disk size and color, moon with proper brightness and position (but not phase), Jupiter and Venus, plus a glowing starfield at night.

Slides from a presentation about this software are at http://markjstock.org/radiance/radiance_harvard_09_stock.pdf and a video of 24 hours of skies over the Grand Canyon is at https://www.youtube.com/watch?v=BJWviVcu_Qo

How to use it

To build the genutahsky executable, you will need a C compiler and the libnova libraries. On Fedora or Ubuntu, you can get this by running, respectively:

sudo yum install libnova-devel
sudo apt-get install libnova-dev

Once you have those, grab this repository with:

git clone https://github.com/markstock/GenUtahSky.git
cd GenUtahSky
make

To use the system in a Radiance scene, simply copy all of the files in this directory into the directory containing your scene, and add a command like the following to one of your .rad files:

!genutahsky 6 23 19 -t 5.0 -a 45 -o 105

This will generate the proper materials for a sky at 45N 105W, on June 23 at 7pm local time, with a turbidity of 5 (2 represents very clear air, 10 is hazy).

If you don't want to clutter your working directory up, drop these files into your Radiance lib directory, often at /usr/local/lib/ray.

If you're going to use pcond on night scenes, use -v -s instead of -h. The latter will blur the stars unrealistically.

Copyrights and licensing

The genutahsky.c program uses code from Radiance, (c) Greg Ward Larson http://radsite.lbl.gov/radiance/

All numbers are from the original Preetham, Shirley, Smits paper, which can be downloaded from http://www.cs.utah.edu/~shirley/papers/sunsky/sunsky.pdf.

The program links with the excellent LibNova library for astronomical calculations, maintained by Liam Girdwood and Petr Kubanek. http://libnova.sourceforge.net/

The starfield is courtesy the NASA/Goddard Space Flight Center Scientific Visualization Studio, and was generated from the Tycho II Star Survey. I processed their original high-resolution 8bpp image to a smaller 12bpp hdr image. http://svs.gsfc.nasa.gov/cgi-bin/details.cgi?aid=3572

All other material is (c) 2009 Mark J. Stock