GenUtahSky

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

What is this?

Users of the Radiance Synthetic Imaging System 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.

Here are slides from a presentation about Radiance skies and a video of 24 hours of skies over the Grand Canyon.

How to use it

To build the genutahsky executable, you will need a C compiler. This is already installed on most Linux systems, but in case you need it, run either:

sudo dnf install gcc
sudo apt-get install gcc

Then to build the software just run:

git clone --depth 1 https://github.com/markstock/GenUtahSky.git
cd GenUtahSky
make

If you want to use the libnova library instead, first install it and then build with the following:

sudo dnf install libnova-devel		(for Fedora)
sudo apt-get install libnova-dev	(for Ubuntu)
LIBNOVA=ON make

Run a simple test with

genutahsky 6 23 19

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.

Better star map

It makes no sense to put a 500MB file in this repository, so we provide a script to download and convert a high-resolution starfield for use with GenUtahSky. On a Fedora machine with >2GB of drive space available, you should be able to do this with:

sudo dnf install curl ImageMagick
./makestarmap.sh

Then just edit stardome.rad and replace TychoSkymapII.t5_08192x04096.hdr with starmap_2020_16k_scaled.hdr.

To Do

Use CMake to build and install the package (putting necessary files into radiance library location and binaries into /usr/local/bin
Create a cal file to render the correct moon phase

Copyrights and licensing

The genutahsky.c program uses code from Radiance, (c) Greg Ward Larson.

All sky color numbers are from the original Preetham, Shirley, Smits paper.

The program optionally links with the LibNova library for astronomical calculations, maintained by Liam Girdwood and Petr Kubanek. It also uses Astronomy if libnova is unavailable.

The starfield included in this repo 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. Original data. The makestarmap.sh grabs a newer dataset with a much nicer-looking Milky Way and a lot more stars. Original data is here.

Citing GenUtahSky

I don't get paid for writing or maintaining this, so if you find this tool useful or mention it in your writing, please please cite it by using the following BibTeX entry.

@Misc{GenUtahSky2009,
  author =       {Mark J.~Stock},
  title =        {GenUtahSky: Radiance implementation of Preetham, Shirley, Smits model for sky color, plus more},
  howpublished = {\url{https://github.com/markstock/GenUtahSky}},
  year =         {2009}
}