Sky / Atmospheric Rendering

subsections: Clouds, Fog, Rain, and Lighting

Sky Color

• Sky dome
  • A common approach is to model a dome, and either apply a texture map or use vertex colors to provide the impression of a sky.
  • a texture map dome can be extremely realistic looking, but has drawbacks:
    • difficult to make it change with time of day
    • if it contains pictures of clouds, they must be very distant clouds, otherwise as the viewer moves around a landscape, they would notice the same clouds staying overhead
  • vertex color dome
    • in general, daytime sky color is a gradual fade from "horizon color" to "zenith color" overhead
    • for example, a clear blue sky usually goes from a light cyan at the horizon to a rich medium-blue
    • at dusk and dawn, there is an additional fade of warm color from a broad stretch of the east/west horizon to the zenith, as shown in the photograph to the right
    • by choosing an attractive set of colors, you can just interpolate between them for all times of day and night
    • the colors of a sunset are typically: yellow at the edge, then orange-red, then purple-slate blue
• Sky Domes Tutorial at the game developer site Flipcode
  • includes source code
  • uses blending of luminosity textures, draws a flare around the sun
• The Brightness of the Night Sky answers questions such as "How do man-made light pollution, natural air glow and light from interplanetary dust affect the darkness of the night sky? What about elevation, transparency and atmospheric particles?" 

Some academic work

• Precomputed Atmospheric Scattering (pdf)
  • Eric Bruneton and Fabrice Neyret, EGSR 2008
  • Abstract: "Method to render the atmosphere in real time from any viewpoint from ground level to outer space, while taking Rayleigh and Mie multiple scattering into account [..] reproduces many effects of the scattering of light, such as the daylight and twilight sky color and aerial perspective for all view and light directions, or the Earth and mountain shadows (light shafts) inside the atmosphere"
  • there is a demo with full source code (including GLSL)
  • 
  • I was able to build and run the demo, and although it is not the full algorithm from the paper (no elevation, so no light shafts) the effect is fast and very nice, especially sunset/sunrise.
  • Implemented in the open-source terrain library Proland.
• Real-Time Rendering of Planets with Atmospheres (pdf)
  • T. Schafhitzel, M. Falk, T. Ertl, In Journal of WSCG 07, 2007
  • From the abstract: "A real time technique for planetary rendering and atmospheric scattering effects. Based on Nishita's atmospheric model, taking into account air molecules and aerosols, and on a CLOD planetary renderer. Interactive frame rates by combining the CPU bound spherical terrain with the GPU computation of the atmospheric scattering."
• Real Time Rendering of Atmospheric Scattering Effects for Flight Simulators (pdf)
  • Ralf Stokholm Nielsen, Sept. 2003
  • an excellent overview of the whole field of atmospheric rendering, and well-documented implementation of realistic effects using vertex-shader programming
• A Practical Analytic Model for Daylight
  • A. J. Preetham, Peter Shirley, Brian Smits, Siggraph 99
  • goes into rigorous depth: "Sunlight and skylight are rarely rendered correctly in computer graphics..."
    • Stephan Heigl's notes on implementing the Preetham Day Sky model includes formulas and pointers to source code for the sky color part of the Preetham paper
• A fast, simple method to render sky color using gradient maps, 2006
  • method and whitepaper by Jesús Alonso Abad, aka Kencho
  • uses a single texture to hold the color gradient for a whole day, with the texture U coordinate for time
  • there is a related open-source addon for the Ogre3D engine, called caelum
• Display Method of the Sky Color Taking into Account Multiple Scattering
  • Tomoyuki Nishita, Yoshinori Dobashi, Kazufumi Kaneda, Hideo Yamashita, Pacific Graphics '96
  • a fast method to calculate the spectral distribution of sky radiation for single scattering, and an efficient method to calculate the sky color taking into account multiple scattering.
  • Nishita seems to be the most-referenced author in this field.  Many recent papers involve implementation of his atmospheric model, with optimizations like using the GPU.
• Erweiterung und Verbesserung eines strahlenphysikalischen Ansatzes zur Simulierung der Globalstrahlung und ihre Anwendung bei der Visualisierung windbewegter Wasseroberflächen (German only, no longer online?)
  • Bertram Walter, between 1996 and 2000
  • Google translates the title as "Extension and improvement of a radiate-physical beginning for simulating the global radiation and their application with the visualization of wind-moved water surfaces" and does a reasonable job on the paper itself
  • reportedly an excellent treatment of the subject of atmospheric optical modelling
• GPU-based multi-layer perceptron as efficient method for approximation complex light models in per-vertex lighting
  • Konrad Pietras, March 2005
  • "display method of the sky color on GeForce FX hardware", with plenty of math, source code, and resulting images
  • On Athlon 1700+, GeForce FX 5500, in 1280x1024x32 resolution, the program runs with speed of 9 - 11 frames per second.

Sky Rendering Libraries

• SilverLining is a cloud rendering library which also includes some very extensive sky rendering: sky color, sun, moon, stars, lightning, etc.  More about SilverLining on the Clouds page.
• The VTP software has some support for sky dome color, sun, moon and stars.
  • The sun is positioned correctly based on earth location and date/time using SPA.
  • The sky color is implemented with vertex colors, allowing for a sunrise/sunset glow in the east and west.
  • The sun and moon are implemented as texture quads, the stars as OpenGL points.
  • Alternately, the sky dome can use a static texture, upon which the sun glow is rendered additively.  This is the most photogenic approach because a sky photograph with clouds, etc. can be used.
• osgEphemeris is a nodekit for OSG which implements sky dome color, sun, moon, and stars
  • Rendered very efficiently with OpenGL and GLSL, positioned correctly based on earth location and date/time.
  • The sky color is implemented with a 1D texture, so color is the same in all directions.
  • The sun is a texture quad, the moon is a full 3D spherical model, the stars are a GLSL shader.

Sky dome textures

• SkyPaint ($85) is a small program for painting panoramic sky images
• Philo's Sky Collection has 16 nice, free skydomes
• Continuum has over 25 nice, free skydomes including some 'fantasy' skies, even an animated skydome
• Roel Reijerse's skyboxes contains 4 very nice free skies, although they are in the alternative "box" layout which loses 1/2 of the area to blank space and another 1/4 to the blank area below the horizon.
• Blender Textures: Sky has some free sky textures (CC-by-nc-sa license, but the owner says "the only restriction is that you can't sell them")
  • Here are the realistic skies from that page, resized to 4096x1024 for use in the VTP software, just drop them into your "Data/Sky" folder:
    Sky_madcow_4096.zip (1.3 MB)
  • Or resized to 2048x512 for older ATI cards:
    Sky_madcow_2048.zip (477 KB)

• commercial skydome texture images are actually fairly rare
• Johannes Schlörb's Sky Backgrounds
• MARLIN STUDIOS - Panoramica - Land & Sky ($250)
  •  includes 20 full hemispherical textures and 24 360° sky panoramas
  • has a free (but not redistributable) sample at 1500x525 resolution
• 1000 Skies sells a large number of sky textures for $100 each at 14,000 pixels wide, $50 at 7000 wide

• Hyperfocal sells collections of high-res sky images at $100-220 per CD, or $60 each for HDRI sky images.  They also have a few free low-res samples (1635x512) which are just fine for realtime 3D.

Sun & Moon

• computer displays don't have the dynamic range to actually draw a bright object like the sun
• one thing you can do is put a "glow" in the sky
  • using a intensity map, an additive texture map, or a solid white texture with a gradient alpha map, as in the example to the right (128x128 TGA):
  • lens flare effects might help
  • using an "additive" texture map allows it to blend smoothly into the sky dome
• for the moon, brightness isn't as much of an issue, since it isn't much brighter than a CRT is capable of
  • again, a well-designed additive texture map would probably give the expected result, even when the moon appears in the day against a blue sky
  • however, in order to show phase of the moon, might want to try a physically modelled sphere and let the rendering library light the illuminated region
• the size of the sun and moon in the sky is almost exactly the same
  • diameter, distance of sun: 1400130 km, 149597870 km
  • diameter, distance of moon: 3474.8 km, 384401 km
  • diameter of earth: 12756 km
  • angular size of objects as seen from earth = arctan( diameter/distance )
  • sun diameter/distance: 0.00936 = 0.53623 degrees as seen from earth
  • moon diameter/distance: 0.00904 = 0.51791 degrees as seen from earth
• A Physically-Based Nightsky Model
  • Henrik Jensen, Frédo Durand, et. al., Siggraph 2001
  • describes the physical basis for combining light from the stars, moon, atmosphere, and 'zodiacal light'
  • includes a BRDF for the Moon!
• from the flipcode tutorial:
  • "The moon has to be rendered in 2 passes. First, from the moon texture, which also has an alpha component, a mask is generated that has alpha values of 1.0 for texels inside the moon and alpha values of 0.0 for texels outside the moon. This mask is rendered onto the dome without blending using the current sky color. This is done to remove any stars that might appear behind the moon. Next, the actual moon texture is blended onto the sky dome. The reason it is blended is because during the day, the moon will show a bit of blue or red hue of the sky."
• Placing the Sun, Moon and stars correctly in the sky requires understanding Celestial Coordinates
  • Sky Coordinates by John E. Ross
  • Positional Astronomy by Fiona Vincent
• How to compute the positions for the Sun, Moon and the major planets
  • Computing planetary positions, by Paul Schlyter
  • Astronomical Calculations, by Keith Burnett
  • NREL (National Renewable Energy Laboratory) hosts some algorithms for the Sun, with C source:
    • Solar Position Algorithm (SPA)
    • Solar Codes & Algorithms (SOLPOS)
  • libnova is a general-purpose, open-source celestial mechanics and astronomical calculation library
• Sunlit Designs InfoSearch is a very nice glossary of sun/solar terminology
  • they also produce the Sun API, a Solar and Sundial Calculation freeware program (free)

Stars

• Star location data is available from the public-domain bright star catalog (BSC) in celestial coordinates, which are a kind of polar coordinates
• One formula for conversion of declination and right ascension (abbreviated as (dec, ra)) to (x, y, z) are:
  • x = -cos (dec) * sin (ra);
  • y = cos (dec) * cos (ra);
  • z = sin (dec);
• Setting a maximum magnitude (minimum brightness) of:
  • 5.5 gives 2887 stars
  • 5.8 gives 4103 stars
• As these magnitudes are barely visible under the best of conditions, you can see that a relatively small number of stars is sufficient to draw an accurate sky
• Excellent article High-quality rendering of 2.0 stars (via google translate, or see the original in German: Hochwertiges Rendern von Sternen) by Chris Maiwald, 2009

Rainbows

• first rainbow: red on outside, violet inside
• the second rainbow of a double rainbow: red inside, violet outside, fainter and wider
• at what angle, relatives to the sun's direction, does the rainbow appear?
• wikipedia Rainbows gives the answer as 40-42° for the primary and 52-54.5° for the secondary rainbow
• the sky inside the rainbow is slightly lighter
• Light and Optics weather site gives detail on other atmospheric effects such as halos and sundogs
• Prisms and Rainbows: A Dispersion Model for Computer Graphics (ps, Musgrave 1989) gets deep into the math and physics of the rainbow
• Clint Brewer's technique (GDC 2004): NVidia technical report: How to render a real Rainbow.
  • That's no longer online, but there is NVidia SDK Sample: Rainbow Fogbow (user guide / whitepaper / sample code), which is the same material
  • Covers the basic physics of Rainbows, Fogbows and Coronas, how to implement them using a pre-calculated lookup texture in a programmable shader, and blend them realistically with a 3D scene.

General Atmospheric Rendering

• Night Rendering, University of Utah, covers the moon and stars and the impact of nighttime lighting conditions to the terrain scenery
• Jaroslav Sloup's Modeling and Rendering of Atmospheric Effects project at CCG Prague (2000-2003),  covered the color of the sky, plus "halos, mirages, green and red flashes, rainbows, etc.", seems to be gone as of 2007

Atmosphere from Space

• the earth's atmosphere looks different from space
• "Display of The Earth Taking into Account Atmospheric Scattering"
  • Nishita, Sirai, Tadamura, Nakamae, Siggraph 93
  • algorithm for efficient calculation of optical length and sky light
  • Real-Time Atmospheric Scattering, GameDev article by Sean O'Neil
    • describes how to do a fast, practical implementation of the Nishita approach
• ORBITER Space Simulator (free, but closed source) demonstrates the nice effects of using a land/water mask for surface reflectance of the earth, as well as atmospheric haze and cloud shadows on the surface:
   
• Sean O'Neil's project site 'A Real-Time Procedural Universe' contains demo apps (with full source) using OpenGL with GLSL to render Atmospheric Scattering both from space and from within the atmosphere:
  
  I didn't have much luck running his Planet_Quad demo a few year back (with a ATI Radeon 9800) but it might work fine for your system

Open-source Apps

• Stellarium and Celestia are open-source applications that do full sky and star rendering, so perhaps parts of them could be adapted for use in terrain rendering application