public Shader <Vector2, Vector3> Configure(FMatrix4x4 projViewInv, AtmosphereConfiguration atmosphereConfiguration)
{
return(ProxyIn <Vector2>(uv => new PixelInput {
UV = uv,
CameraProjViewInv = projViewInv,
AtmosphereConfiguration = atmosphereConfiguration
}));
}
public static Vector3 Compute(Vector3 queryDirectionUnit, AtmosphereConfiguration c)
{
// Calculate the step size of the primary ray.
var(tlow, thigh) = Geometry.TryFindRayAndCenteredSphereIntersection(c.CameraPosition, queryDirectionUnit, c.AtmosphereRadius);
if (tlow > thigh)
{
return(Vector3.Zero);
}
thigh = MathF.Min(thigh, Geometry.TryFindRayAndCenteredSphereIntersection(c.CameraPosition, queryDirectionUnit, c.PlanetRadius).tlow);
const int iSteps = 200;
var iStepSize = (thigh - tlow) / (float)(iSteps);
// Initialize the primary ray time.
var iTime = 0.0f;
// Initialize accumulators for Rayleigh and Mie scattering.
var totalRlh = Vector3.Zero;
var totalMie = Vector3.Zero;
// Initialize optical depth accumulators for the primary ray.
var iOdRlh = 0.0f;
var iOdMie = 0.0f;
// Calculate the Rayleigh and Mie phases.
var mu = Dot(queryDirectionUnit, c.SunDirectionUnit);
var mumu = mu * mu;
var gg = c.MiePreferredScatteringDirection * c.MiePreferredScatteringDirection;
var pRlh = 3.0f / (16.0f * M_PI) * (1.0f + mumu);
var pMie = 3.0f / (8.0f * M_PI) * ((1.0f - gg) * (mumu + 1.0f)) / (MathF.Pow(1.0f + gg - 2.0f * mu * c.MiePreferredScatteringDirection, 1.5f) * (2.0f + gg));
// Sample the primary ray.
for (int i = 0; i < iSteps; i++)
{
// Calculate the primary ray sample position.
var iPos = c.CameraPosition + queryDirectionUnit * (iTime + iStepSize * 0.5f);
// Calculate the height of the sample.
var iHeight = iPos.Length() - c.PlanetRadius;
// Calculate the optical depth of the Rayleigh and Mie scattering for this step.
var odStepRlh = Exp(-iHeight / c.RayleighScaleHeight) * iStepSize;
var odStepMie = Exp(-iHeight / c.MieScaleHeight) * iStepSize;
// Accumulate optical depth.
iOdRlh += odStepRlh;
iOdMie += odStepMie;
// Calculate the step size of the secondary ray.
const int jSteps = 20;
var jStepSize = Geometry.TryFindRayAndCenteredSphereIntersection(iPos, c.SunDirectionUnit, c.AtmosphereRadius).thigh / jSteps;
// Initialize the secondary ray time.
var jTime = 0.0f;
// Initialize optical depth accumulators for the secondary ray.
var jOdRlh = 0.0f;
var jOdMie = 0.0f;
// Sample the secondary ray.
for (int j = 0; j < jSteps; j++)
{
// Calculate the secondary ray sample position.
var jPos = iPos + c.SunDirectionUnit * (jTime + jStepSize * 0.5f);
// Calculate the height of the sample.
float jHeight = jPos.Length() - c.PlanetRadius;
// Accumulate the optical depth.
jOdRlh += Exp(-jHeight / c.RayleighScaleHeight) * jStepSize;
jOdMie += Exp(-jHeight / c.MieScaleHeight) * jStepSize;
// Increment the secondary ray time.
jTime += jStepSize;
}
// Calculate attenuation.
var attn = Exp(-(Vec3(c.MieScatteringCoefficient) * (iOdMie + jOdMie) + c.RayleighScatteringCoefficient * (iOdRlh + jOdRlh)));
// Accumulate scattering.
totalRlh += odStepRlh * attn;
totalMie += odStepMie * attn;
// Increment the primary ray time.
iTime += iStepSize;
}
// Calculate and return the final color.
return(c.SunIntensity * (pRlh * c.RayleighScatteringCoefficient * totalRlh + pMie * c.MieScatteringCoefficient * totalMie));
}