func DiffusePhoton(scene []*geometry.Shape, emitter *geometry.Shape, ray geometry.Ray, colour geometry.Vec3, result chan<- PhotonHit, alpha geometry.Float, depth int, rand *rand.Rand) {
if geometry.Float(rand.Float32()) > alpha {
return
}
if shape, distance := ClosestIntersection(scene, ray); shape != nil {
impact := ray.Origin.Add(ray.Direction.Mult(distance))
if depth == 0 && emitter == shape {
// Leave the emitter first
nextRay := geometry.Ray{impact, ray.Direction}
DiffusePhoton(scene, emitter, nextRay, colour, result, alpha, depth, rand)
} else {
normal := shape.NormalDir(impact).Normalize()
reverse := ray.Direction.Mult(-1)
outgoing := normal
if normal.Dot(reverse) < 0 {
outgoing = normal.Mult(-1)
}
strength := colour.Mult(alpha / (1 + distance))
result <- PhotonHit{impact, strength, ray.Direction, uint8(depth)}
if shape.Material == geometry.DIFFUSE {
// Random bounce for color bleeding
u := normal.Cross(reverse).Normalize().Mult(geometry.Float(rand.NormFloat64() * 0.5))
v := u.Cross(normal).Normalize().Mult(geometry.Float(rand.NormFloat64() * 0.5))
bounce := geometry.Vec3{
u.X + outgoing.X + v.X,
u.Y + outgoing.Y + v.Y,
u.Z + outgoing.Z + v.Z,
}
bounceRay := geometry.Ray{impact, bounce.Normalize()}
bleedColour := colour.MultVec(shape.Colour).Mult(alpha / (1 + distance))
DiffusePhoton(scene, shape, bounceRay, bleedColour, result, alpha*0.66, depth+1, rand)
}
// Store Shadow Photons
shadowRay := geometry.Ray{impact, ray.Direction}
DiffusePhoton(scene, shape, shadowRay, geometry.Vec3{0, 0, 0}, result, alpha*0.66, depth+1, rand)
}
}
}
