Rain, Fog, Lighting, and Snow

Rain

• Markus Hein [email] writes:
  "I began to implement some rain-tests. Rain seems to be a tricky thing, consisting of a lot of components to make the scene real and not looking synthetic. The part I had good results with was the visible rain-effects "close to the lens". Falling particles, from an emitter that was moved with the camera, and 'ripples' and that was made with FIXED-aligned particles over the surface (interpolating the particle's size&alpha)  But, this is just a result like one of thousands of "GL-Rain-Demos" I have found on the net.  If we want to have a real rainy scene so we have to go further.  I think that the rain-component "far from the lens" is stuff for using Shader-Programs on modern 3D-hardware."
• rain demo [2002] at Kevin Hawkins' gamedev site [2003] shows rain falling as short grey lines simulating motion blur of the falling drops
• many of the "rain demos" out there aren't rain, but rather ripples, as if from raindrops
• Simulation of Rain in Videos, Starik and Werman, 2003 [pdf]
  • analysis of how rain appears in actual video footage, and description of how to simulate the same results, without any knowledge of the 3D scene
  • although this paper deals with video, it should certainly apply to realtime 3D rendering as well
  • interestingly, they don't use any kind of particle system, they don't track raindrops from frame to frame - apparently, humans can't see the individual drops nor even whether they are going up or down!
  • an important part of their approach is darkening the image (decreasing brightness and saturation)
  • corresponding videos are on the page Rain Simulation Results

Fog & Haze

• It's easy to turn on fog in OpenGL, which is a type of uniform haze.
• However, not only is it restricted to uniform density in all direction, you cannot vary it's color by direction either, so it does not co-exist well with a sky dome.
• For example, when the sunset makes the sky orange in the west, the fog color will clash with the orange in that part of the sky.
• Johan Hammes wrote:
  • Since your field of view is fairly narrow always (smaller than 60 degrees), it's possible to calculate the average color of the skydome at the horizon, and change the fog color to reflect this. I have done this in the past with stunning results.
  • Remember to also increase the amount of fog as you are looking into the sun. This simulates the strong forward scattering found in fog and clouds. When looking towards the sun through fog, because of forward scattering the fog appears brighter, and as a result you are able to distinguish less detail through it. This is most easily simulated by increasing the fog density as a function of the angle towards the sun.
• Another option is to render the skydome as fog, using the alpha channel as described by Clint Brewer
• DigiBen's OpenGL tutorials include a section on Volumetric Fog which shows how to use glFogCoordfEXT to accomplish it with one rendering pass

Lightning

• will need some lightning when we have clouds and rain
• The Digital Ceraunoscope: Synthetic Thunder and Lightning
  • Andrew Glassner, Microsoft Research, 1999
  • actually models the relationship between the shape of the lighting and the sound of the thunder it produces (!)
• Visual Simulation of Lightning (pdf requires ACM library membership)
  • Brian Wyvill and Todd Reed, University of Calgary, 1993
  • focus is on producing a ray-tracing model of randomized lightning
• Efficient Rendering of Lightning Taking into Account Scattering Effects due to Cloud and Atmospheric Particles
  • Yoshinori Dobashi, Tsuyoshi Yamamoto, Tomoyuki Nishita, Proc. of the 9th Pacific Conference, 2001
• Visual Simulation of Lightning Taking into Account Cloud Growth
  • B.Sosorbaram, T.Fujimoto, K.Muraoka and N.Chiba, Proceedings CGI2001
• Probabilistic Technique for the Synthetic Imagery of Lightning
  • P. Kruszewski, Computers & Graphics, Vol.2, No.2, 1999

Snow

• snow which is already on the ground (rather than falling) is generally treated as a problem of distributing the snow texture on the ground in a realistic fashion
  • see Geotypical Imagery and Detail Textures for a description of these approach
• "The Synthesis of Snowcovered Terrains", MIT, 1996 [no longer online]
  • academic paper which uses a raytraced heightfield rather than a texture
  • "creates the snow mesh according to the geometry of the terrain and other user inputs, e.g. temperature, snow quantity, density of virtual snow guages, etc."
• "A Modeling and Rendering Method for Snow by Using Metaballs", Tomoyuki Nishita. 1997
  • models the shape and shading model of snow, based on the physical phenomenon
• Computer Modelling Of Fallen Snow (pdf), Paul Fearing, 2000
  • simulates snow distribution with "an adaptive particle/surface hybrid"