The code in SunSky.* is an implementation of several sky models:

• The Preetham Clear Sky model. (From "A Practical Analytic Model for Daylight", Preetham, Shirley & Smits.)

• The Hosek Clear Sky model. (From "An Analytic Model for Full Spectral Sky-Dome Radiance", Hosek & Wilkie.)

• Various luminance-only CIE models:

  • The Clear Sky model
  • The Overcast Sky model
  • The Partly(!) Cloudy Sky model
  • The more recent fifteen CIE sky models from, e.g., "CIE general sky standard defining luminance distributions", Darula & Kittler. (There are many variants of this paper!)

• The sun model from Preetham (also used by Hosek), and helpers for converting from time and place to local sun direction.

In addition, for the Preetham and Hosek models, I have added:

• Table-driven versions for fast evaluation on the GPU (or indeed CPU) via 64 x 2 lookup table.

• Extended table-driven versions that use zonal harmonics to produce approximate mirror-to-diffuse BRDF power convolutions. These use 64 x 8 or 64 x 16 tables.

• Proper handling of night transitions. The original models assume the sun is above the horizon. The supplied code transitions to a dark blue sky as the sun fully sets, and then to black towards the end of twilight.

• An extension to allow mixing the clear sky models with an overcast sky via a simple linear 'overcast' factor. Generally it's best to use this to simulate high cloud, while low-lying clouds are represented explicitly in the shader. However, when used for shading rather than skybox display, it can also represent low-lying cloud cover.

The Preetham code is an old standby, and has been shipped in several games.

The Hosek code is newer, but has also been shipped several times, and is what I recommend now, particularly as it includes an integrated ground albedo factor that feeds into the sky model. It is a re-implementation of the Hosek paper using floats, with some minor optimisations, and an attempt to make the structure a bit more obvious.

See sky.sh for shader routines to evaluate the Hosek sky model, optionally with a roughness value, and some notes on how to set up the corresponding uniforms. The file skybox_fs.sc is an example of how to use these in a skybox context -- it also adds a sun disc.

Clear Sky:

Overcast 50%/Day

BRDF Day/Sunset:

Clear Sky:

Overcast 50%/Day:

BRDF Day/Sunset:

Included in SunSkyTool.cpp is a tool exercising most of the sky model functionality. Current options are below. It can be used to generate top-down 'hemisphere' views with or without fisheye projection, panoramic views, and cube maps, with various forms of tonemapping. Both LDR (png) and HDR (pfm) versions are output.

To build this tool, use 'make', or

c++ --std=c++11 -O3 SunSky.cpp SunSkyTool.cpp -o sunsky

Or add those files to your favourite IDE.

sunsky <options>

Options:
  -h : this help
  -t <time>          : 0 - 24
  -d <day of year>   : 0 - 365
  -b <tubidity>      : 2 - 30
  -x <ground_bounce> : 0 - 1
  -l <latitude> <longitude>
  -w <normalisation weight>
  -g <gamma>
  -e <tonemapType> : use given tonemap operator (default: linear)
  -a : autoscale intensity
  -i : invert hemisphere
  -f : fisheye rather than cos projection
  -c : output cubemap instead
  -p : output panorama instead
  -m : output movie, record day as sky.mp4, requires ffmpeg
  -v : verbose
  -s <skyType> : use given sky type
  -r <roughness:float> : specify roughness for PreethamBRDF

skyType:
  Preetham         (pt)
  PreethamTable    (ptt)
  PreethamBRDF     (ptb)
  Hosek            (hk)
  HosekTable       (hkt)
  HosekBRDF        (hkb)
  cieClear         (cc)
  cieOvercast      (co)
  ciePartlyCloudy  (cp)

toneMapType:
  linear           (l)
  exponential      (ex)
  reinhard         (rh)

Show noon sky for the current time of year using Preetham:

sunsky -t 12

Glossy version of the same sky:

sunsky -t 12 -s preethamBRDF -r 0.3

Hosek sky at 4pm with greenish albedo, high turbidity, and exponential tone mapping, saved to a cube map:

sunsky -t 16 -s hosek -x 0.2 0.5 0.2 -b 6 -e ex -c