/// <summary>
/// Scatter a ray using the BRDF and Impulses
/// </summary>
public ScatteredRay Scatter(PositionNormalCoordinate surfel, float3 w)
{
float selection = random();
float impulseProb = 0;
foreach (var impulse in GetBRDFImpulses(surfel, w))
{
float pdf = (impulse.Ratio.x + impulse.Ratio.y + impulse.Ratio.z) / 3;
if (selection < pdf) // this impulse is choosen
{
return new ScatteredRay
{
Ratio = impulse.Ratio / abs(dot(surfel.Normal, impulse.Direction)),
Direction = impulse.Direction,
PDF = pdf
}
}
;
selection -= pdf;
impulseProb += pdf;
}
float3 wout = randomHSDirection(surfel.Normal);
/// BRDF uniform sampling
return(new ScatteredRay
{
Direction = wout,
Ratio = EvalBRDF(surfel, wout, w),
PDF = (1 - impulseProb) / (2 * pi)
});
}
}
public float3 EvalBRDF(PositionNormalCoordinate surfel, float3 wout, float3 win)
{
float3 diffuse = Diffuse * (DiffuseMap == null ? float3(1, 1, 1) : DiffuseMap.Sample(TextureSampler, surfel.Coordinates).xyz) / pi;
float3 H = normalize(win + wout);
float3 specular = Specular * pow(max(0, dot(H, surfel.Normal)), SpecularPower) * (SpecularPower + 2) / two_pi;
return(diffuse * WeightDiffuse / WeightNormalization + specular * WeightGlossy / WeightNormalization);
}
public float3 EvalBRDF(PositionNormalCoordinate surfel, float3 wout, float3 win)
{
float3 diffuse = Diffuse.Sample(TextureSampler, surfel.Coordinates).xyz / pi;
float3 H = normalize(win + wout);
float3 specular = Specular * pow(max(0, dot(H, surfel.Normal)), SpecularPower) * (SpecularPower + 2) / two_pi;
return(diffuse * (1 - Glossyness) + specular * Glossyness);
}
public IEnumerable <Impulse> GetBRDFImpulses(PositionNormalCoordinate surfel, float3 wout)
{
if (!any(Specular))
{
yield break; // No specular => Ratio == 0
}
float NdotL = dot(surfel.Normal, wout);
// Check if ray is entering the medium or leaving
bool entering = NdotL > 0;
// Invert all data if leaving
NdotL = entering ? NdotL : -NdotL;
surfel.Normal = entering ? surfel.Normal : -surfel.Normal;
float ratio = entering ? 1.0f / this.RefractionIndex : this.RefractionIndex / 1.0f; // 1.0f air refraction index approx
// Reflection vector
float3 R = reflect(wout, surfel.Normal);
// Refraction vector
float3 T = refract(wout, surfel.Normal, ratio);
// Reflection quantity, (1 - F) will be the refracted quantity.
float F = ComputeFresnel(NdotL, ratio);
if (!any(T))
{
F = 1; // total internal reflection (produced with critical angles)
}
if (WeightMirror + WeightFresnel * F > 0) // something is reflected
{
yield return new Impulse
{
Direction = R,
Ratio = Specular * (WeightMirror + WeightFresnel * F) / WeightNormalization
}
}
;
if (WeightFresnel * (1 - F) > 0) // something to refract
{
yield return new Impulse
{
Direction = T,
Ratio = Specular * WeightFresnel * (1 - F) / WeightNormalization
}
}
;
}