Overview

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 a trusty standby, and has been shipped in several games.

The Hosek code is new, 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. The CPU-only and table variants are solid, as is the night transition. The overcast functionality and BRDF convolution might need a bit more tweaking.

I would like to provide some sample shaders, but for now look at FillTexture*, and the Table::SkyRGB() routines.

Results

Preetham

Clear Sky:

Overcast 50%/Day

BRDF Day/Sunset:

Hosek

Clear Sky:

Overcast 50%/Day:

BRDF Day/Sunset:

SunSky Tool

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.

Building

To build this tool, use

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

Or add those files to your favourite IDE.

Options

    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)

Examples

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