/// <summary>
/// Sets up stored and derived parameters from an atmosphere model
/// </summary>
/// <remarks>
/// The inner radius is diminished by a constant amount, because otherwise, the quantised
/// view directions hit the inner sphere and cause a blue band to appear when the actual
/// view direction approaches it.
/// </remarks>
private void SetupModelAndParameters( AtmosphereBuildParameters parameters, AtmosphereBuildModel model )
{
m_RayleighCoefficients = ( float[] )model.RayleighCoefficients.Clone( );
m_MieCoefficients = ( float[] )model.MieCoefficients.Clone( );
m_InnerRadius = model.InnerRadiusMetres;
m_OuterRadius = model.InnerRadiusMetres + model.AtmosphereThicknessMetres;
m_InnerSphere = new Sphere3( Point3.Origin, m_InnerRadius * model.GroundRadiusMultiplier ); // NOTE: AP: See remarks
m_OuterSphere = new Sphere3( Point3.Origin, m_OuterRadius );
m_AttenuationSamples = parameters.AttenuationSamples;
float heightRange = ( m_OuterRadius - m_InnerRadius );
m_InvRH0 = -1.0f / ( heightRange * model.RayleighDensityScaleHeightFraction );
m_InvMH0 = -1.0f / ( heightRange * model.MieDensityScaleHeightFraction );
m_InscatterDistanceFudgeFactor = model.InscatterDistanceFudgeFactor;
m_OutscatterDistanceFudgeFactor = model.OutscatterDistanceFudgeFactor;
m_MieFudge = model.MieFudgeFactor;
m_RayleighFudge = model.RayleighFudgeFactor;
m_OutscatterFudge = model.OutscatterFudgeFactor;
}
/// <summary>
/// Setup constructor
/// </summary>
public WorkItem( BackgroundWorker worker, AtmosphereBuildModel model, AtmosphereBuildParameters parameters )
{
m_Worker = worker;
m_AtmosphereModel = model;
m_AtmosphereBuildParameters = parameters;
}
//
// Discrepancies in atmospheric modelling papers:
// - HG:
// - Neilsen: HG(t,g) = (1-g2)/(4pi(1 + g2 - 2cos(t))^1.5)
// - Wolfram: HG(t,g) = (1-g2)/(1 + g2 - 2.g.cos(t))^1.5
// - O'Neil (adapted HG): HG(t,g) = 3(1-g2)/2(2+g2) . (1+t2)/(1+g2 - 2gt)^1.5
// - O'Neil and Wolfram work well. Neilsen doesn't (denominator can evalute to < 0 before raising to 3/2 power)
// - Scattering coefficients: (# is wavelength)
// - Neilsen Rayleigh coefficient: 8pi^2(n2-1)^2/3N#^4
// - Neilsen Mie coefficient: 0.434c.pi.4pi^2/#^2K
// - K=non-wavelength dependent fudge factor for Bm: ~0.67
// - c=concentration factor(0.6544T-0.6510).10e-16 (T=turbidity. ~555nm)
//
//
//
//
// Realtime rendering of atmospheric scattering for flight simulators:
// http://www2.imm.dtu.dk/pubdb/views/edoc_download.php/2554/pdf/imm2554.pdf
//
// Display of The Earth Taking into Account Atmospheric Scattering: (Nishita)
// http://nis-lab.is.s.u-tokyo.ac.jp/~nis/abs_sig.html#sig93
//
// Implementation of Nishita's paper:
// http://www.gamedev.net/reference/articles/article2093.asp
//
// A practical analytical model of daylight:
// http://www.cs.utah.edu/vissim/papers/sunsky/sunsky.pdf
//
// Discussion about atmospheric shaders:
// http://www.gamedev.net/community/forums/topic.asp?topic_id=335023
//
// Paper that this model is based on:
// http://www.vis.uni-stuttgart.de/~schafhts/HomePage/pubs/wscg07-schafhitzel.pdf
//
// Heyney-Greenstein on Mathworld:
// http://scienceworld.wolfram.com/physics/Heyney-GreensteinPhaseFunction.html
//
/// <summary>
/// Builds a 3D lookup texture
/// </summary>
/// <param name="model">The atmosphere model</param>
/// <param name="parameters">Parameters for the build process</param>
/// <param name="progress">Optional progress object</param>
/// <returns>Returns a 3d texture data. Returns null if cancel was flagged in the progress object</returns>
public unsafe AtmosphereBuildOutputs Build( AtmosphereBuildModel model, AtmosphereBuildParameters parameters, AtmosphereBuildProgress progress )
{
Texture2dData opticalDepthTexture = new Texture2dData( );
Texture3dData scatteringTexture = new Texture3dData( );
scatteringTexture.Create( parameters.ViewAngleSamples, parameters.SunAngleSamples, parameters.HeightSamples, TextureFormat.R8G8B8A8 );
SetupModelAndParameters( parameters, model );
progress = progress ?? new AtmosphereBuildProgress( );
fixed ( byte* voxels = scatteringTexture.Bytes )
{
if ( !BuildScatteringTexture( parameters, voxels, progress ) )
{
return null;
}
}
if ( progress.Cancel )
{
return null;
}
opticalDepthTexture.Create( parameters.OpticalDepthResolution, parameters.OpticalDepthResolution, TextureFormat.R8G8B8 );
fixed ( byte* pixels = opticalDepthTexture.Bytes )
{
if ( !BuildOpticalDepthTexture( parameters, pixels, progress ) )
{
return null;
}
}
return new AtmosphereBuildOutputs( scatteringTexture, opticalDepthTexture );
}