Home

Adding atmosphere (literally and figuratively) to your tracks.

Introduction

The default OpenGL fog method has been useful, but these days things can get much nicer. Racer enables the use of atmospheric scattering to get beautiful sky gradients. Keywords here are rayleigh scattering, mie scattering and aerial perspective. Below is an example at 19:30 with some Mie scattering (haze).

NOTE: most of the parameters here are now (v0.8.23+) driven by TOD (time of day) curves. See here for more information on setting time of day parameters.

The theory

Background material that explains more about Rayleigh & Mie scattering due to atmospheric particles and air follows. You may skip this, since it isn't necessary to understand technical implementations to use the atmosphere system.

• ATI Light Scattering. This PDF explains an older method of doing a relatively simple sky color. I'm using this partly in the code, especially the Mie contribution (which I think looked better than the Hofman/Preetham attempts).
• Precomputed Atmospheric Scattering; although quite costly, I used the wavelengths from this paper to get a slightly redder sky.
• Modeling Skylight and Aerial Perspective; a great paper also about Rayleigh and Mie scattering by Preetham from ATI.
• A Practical Analytic Model for Daylight; this paper talks about an expensive but beautiful method to render skies. Although not targeted at realtime, the theory in the paper is interesting to understand the effects.

More links on atmosphere and sky rendering van be found at VTerrain.

In short, the atmosphere bounces light back & forth, which gives the sky a non-black appearance (blue in fact ;-) ). Roughly spoken, two types of scattering exist:

• Rayleigh scattering, which is due to air particles. As the amount of scattering is dependent on the wavelength, blue overcomes mostly, which explains the blue sky. 0 Rayleigh scattering means a black sky, more goes to blue, then orange.
• Mie scattering, which is due to bigger air particles (dust for example). This can most easily be explained with the term 'haze'. It is not really affected by wavelength, so things turn grayish (more a shade of brown) as more Mie scattering takes place. Take a lot of Mie scattering and you get a fog-like appearance.

Aerial perspective means that as you look further away, the sky and geometrical elements (track objects) blend further into each other. So distant objects become blueish on a clear day.

Racer supports all this since v0.7.2.

What is what?

To apply an atmosphere to a track, you have to be aware of the different components:

• special.ini; the track's special.ini defines things such as sun intensity, fog characteristics and time of day.
• Racer shaders; Racer materials in other words. These are Quake-style shader definitions, defining textures and vertex/fragment shaders for materials in your models (DOF files).
• Cg shaders; a subcomponent of Racer shaders, but the core power behind per-pixel lighting, bumpmapping etc.

The steps

In short, the steps for adding an atmosphere are:

• Add a skydome to your track. For example, if you have a skydome named sky.dof, add this to geometry.ini:
  sky
  {
    file=sky.dof
    ; Move with camera to avoid pop
    flags=64+8
  }

• Add shaders for the sky.dof object's materials to your track.shd file (which is in the track directory):

  shader_sky~vf_sky
  {
    sky=1
    layer0
    {
      map=sky_sun_alpha.tga
      texenv=replace
      depthwrite=0
    }
  }

• Add a vf_sky template to the track.shd file somewhere near the start of the file. As you can see above, the shader_sky references the vf_sky tree, which looks like this:

  vf_sky
  {
    vertex_shader
    {
      file=sky_v.cg
    }
    fragment_shader
    {
      file=sky_f.cg
    }
  }

• Add any textures used by the sky object to the track directory.

Modifying special.ini

In special.ini, take note of the following parameters dealing with atmosphere:

To play around, just start the track and use the script command 'fog rayleigh <v>', 'fog mie <v>', 'sun intensity <v>' and 'exposure <v>'. Also use the 'time' command to try various times of day to see how things hold. After finding suitable values, insert them into the special.ini file (in data/tracks/<mytrack>).

(last updated November 13, 2012 )