/// <summary>
/// Setup constructor
/// </summary>
public WorkItem( BackgroundWorker worker, AtmosphereBuildModel model, AtmosphereBuildParameters parameters )
{
m_Worker = worker;
m_AtmosphereModel = model;
m_AtmosphereBuildParameters = parameters;
}
/// <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>
/// Builds a 2D lookup texture containing optical depth for a paramerization of the view frame
/// </summary>
/// <param name="buildParams">Atmosphere build parameters</param>
/// <param name="pixels">Atmosphere texture pixels</param>
/// <param name="progress">Progress indicator</param>
/// <returns>Returns true if the build completed (wasn't cancelled)</returns>
private unsafe bool BuildOpticalDepthTexture( AtmosphereBuildParameters buildParams, byte* pixels, AtmosphereBuildProgress progress )
{
// TODO: AP: Because we know that the view to pos angle range will never be > pi, can optimise this later
int viewSamples = buildParams.OpticalDepthResolution;
int heightSamples = buildParams.OpticalDepthResolution;
float viewAngleInc = Constants.Pi / ( viewSamples - 1 );
float heightRange = ( m_OuterRadius - m_InnerRadius );
float heightInc = ( heightRange * 0.9999f ) / ( heightSamples - 1 ); // Push height range in slightly to allow simplification of sphere intersections
float* rAccum = stackalloc float[ 3 ];
float* mAccum = stackalloc float[ 3 ];
float height = m_InnerRadius;
for ( int heightSample = 0; heightSample < heightSamples; ++heightSample, height += heightInc )
{
Point3 pos = new Point3( 0, height, 0 );
// Start the view angle at pi, and count down to 0. This is because it is quickest to address
// the 2D texture using the dot of the view vector and the view position, saving a (1-th) operation
float viewAngle = Constants.Pi;
Point3 lastAtmInt = pos;
for ( int viewSample = 0; viewSample < viewSamples; ++viewSample, viewAngle -= viewAngleInc )
{
Vector3 viewDir = new Vector3( Functions.Sin( viewAngle ), Functions.Cos( viewAngle ), 0 );
// NOTE: If ray intersection fails, the previous intersection position is used...
Point3 atmInt;
// if ( !GetRayPlanetAndAtmosphereIntersection( pos, viewDir, out atmInt ) )
if ( !GetRayAtmosphereIntersection( pos, viewDir, out atmInt ) )
{
atmInt = lastAtmInt;
}
else
{
lastAtmInt = atmInt;
}
Vector3 step = ( atmInt - pos ) / m_AttenuationSamples;
rAccum[ 0 ] = rAccum[ 1 ] = rAccum[ 2 ] = 0;
mAccum[ 0 ] = mAccum[ 1 ] = mAccum[ 2 ] = 0;
CalculateOutScatter( pos, step, rAccum, mAccum );
// float oR = CalculateCombinedOutScatter( pos, step, m_RayleighCoefficients[ 0 ], m_MieCoefficients[ 0 ] );
// float oG = CalculateCombinedOutScatter( pos, step, m_RayleighCoefficients[ 1 ], m_MieCoefficients[ 1 ] );
// float oB = CalculateCombinedOutScatter( pos, step, m_RayleighCoefficients[ 2 ], m_MieCoefficients[ 2 ] );
float attR = ExtinctionCoefficient( rAccum[ 0 ], mAccum[ 0 ] );
float attG = ExtinctionCoefficient( rAccum[ 1 ], mAccum[ 1 ] );
float attB = ExtinctionCoefficient( rAccum[ 2 ], mAccum[ 2 ] );
pixels[ 2 ] = ( byte )( Utils.Clamp( attR, 0, 1 ) * 255.0f );
pixels[ 1 ] = ( byte )( Utils.Clamp( attG, 0, 1 ) * 255.0f );
pixels[ 0 ] = ( byte )( Utils.Clamp( attB, 0, 1 ) * 255.0f );
pixels += 3;
}
if ( progress != null )
{
progress.OnSliceCompleted( heightSample / ( float )( heightSamples - 1 ) );
if ( progress.Cancel )
{
return false;
}
}
}
return true;
}
/// <summary>
/// Builds a 3D lookup texture containing the scattering values for a parameterization of
/// the view frame and sun position.
/// </summary>
/// <param name="parameters">Parameters for the build process</param>
/// <param name="progress">Progress object</param>
/// <param name="voxels">3d texture voxels</param>
/// <returns>true if build completed (wasn't cancelled)</returns>
private unsafe bool BuildScatteringTexture( AtmosphereBuildParameters parameters, byte* voxels, AtmosphereBuildProgress progress )
{
float viewAngle = Constants.Pi;
float viewAngleInc = Constants.Pi / ( parameters.ViewAngleSamples - 1 );
float heightRange = ( m_OuterRadius - m_InnerRadius );
float heightInc = ( heightRange * 0.9999f ) / ( parameters.HeightSamples - 1 ); // Push height range in slightly to allow simplification of sphere intersections
float sunAngleInc = ( Constants.Pi ) / ( parameters.SunAngleSamples - 1 );
for ( int viewAngleSample = 0; viewAngleSample < parameters.ViewAngleSamples; ++viewAngleSample, viewAngle -= viewAngleInc )
{
Vector3 viewDir = new Vector3( Functions.Sin( viewAngle ), Functions.Cos( viewAngle ), 0 );
float sunAngle = 0;
for ( int sunAngleSample = 0; sunAngleSample < parameters.SunAngleSamples; ++sunAngleSample, sunAngle += sunAngleInc )
{
Vector3 sunDir = new Vector3( Functions.Sin( sunAngle ), Functions.Cos( sunAngle ), 0 );
float height = m_InnerRadius;
for ( int heightSample = 0; heightSample < parameters.HeightSamples; ++heightSample, height += heightInc )
{
ComputeScattering( viewDir, sunDir, height, voxels );
voxels += 4;
}
}
if ( progress != null )
{
progress.OnSliceCompleted( viewAngleSample / ( float )( parameters.ViewAngleSamples - 1 ) );
if ( progress.Cancel )
{
return false;
}
}
}
return true;
}
//
// 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 );
}
