public static void GetSunlight(float altitude, float turbidity, Vector3D sunDirection,
out Vector3F directSunlight, out Vector3F scatteredSunlight)
{
_spectrum.SetSolarSpectrum();
sunDirection.TryNormalize();
double cosZenith = sunDirection.Y;
Vector3F direct, indirect;
if (cosZenith > 0)
{
// Daylight - Sun is above horizon.
double zenithAngle = Math.Acos(cosZenith);
_spectrum.ApplyAtmosphericTransmittance(zenithAngle, turbidity, altitude, _spectrumDirect, _spectrumIndirect);
direct = _spectrumDirect.ToXYZ();
indirect = _spectrumIndirect.ToXYZ();
}
else
{
// Twilight - Sun is below horizon.
// We lookup luminance based on experimental results on cloudless nights.
// Get sun angle in degrees for table lookup.
float solarAltitude = (float)MathHelper.ToDegrees(Math.Asin(sunDirection.Y));
// Get luminance from table (linearly interpolating the next two table entries).
int lower = (int)Math.Floor(solarAltitude);
int higher = (int)Math.Ceiling(solarAltitude);
float a, b;
TwilightLuminance.TryGetValue(lower, out a);
TwilightLuminance.TryGetValue(higher, out b);
float Y = InterpolationHelper.Lerp(a, b, solarAltitude - lower);
// We use fixed chromacity values.
float x = 0.2f;
float y = 0.2f;
// Convert xyY to XYZ.
float X = x * (Y / y);
float Z = (1.0f - x - y) * (Y / y);
// Get sunlight from slightly above the horizon.
const float epsilon = 0.001f;
const double zenithAngle = ConstantsD.PiOver2 - epsilon;
_spectrum.ApplyAtmosphericTransmittance(zenithAngle, turbidity, altitude, _spectrumDirect, _spectrumIndirect);
direct = _spectrumDirect.ToXYZ();
indirect = _spectrumIndirect.ToXYZ();
// Blend between table values and sunset light.
float blend = MathHelper.Clamp(-solarAltitude / 5.0f, 0, 1);
direct = InterpolationHelper.Lerp(direct, new Vector3F(0, 0, 0), blend);
indirect = InterpolationHelper.Lerp(indirect, new Vector3F(X, Y, Z), blend);
}
// Convert XYZ to RGB.
directSunlight = GraphicsHelper.XYZToRGB * direct;
scatteredSunlight = GraphicsHelper.XYZToRGB * indirect;
}
