func (self *SurfacePoint) NextDirection(r *rand.Rand,
inDirection Vector3f,
outDirection_o *Vector3f,
colour *Vector3f) bool {
reflectivityMean := self.Tri.Reflectivity.Dot(VECTOR_ONE) / 3.0
/* russian-roulette for reflectance 'magnitude' */
isAlive := r.Float32() < reflectivityMean
if !isAlive {
return false
}
/* cosine-weighted importance sample hemisphere */
_2pr1 := math32.Pi * 2.0 * r.Float32()
sr2 := math32.Sqrt(r.Float32())
/* make coord frame coefficients (z in normal direction) */
x := math32.Cos(_2pr1) * sr2
y := math32.Sin(_2pr1) * sr2
z := math32.Sqrt(1.0 - (sr2 * sr2))
/* make coord frame */
t := self.Tri.Tangent()
n := self.Tri.Normal()
var c Vector3f
/* put normal on inward ray side of surface (preventing transmission) */
if n.Dot(inDirection) < 0.0 {
n.Neg()
}
c = n.Cross(t)
/* scale frame by coefficients */
tx := t.Mulf(x)
cy := c.Mulf(y)
nz := n.Mulf(z)
/* make direction from sum of scaled components */
sum := tx.Add(cy)
*outDirection_o = sum.Add(nz)
/* make color by dividing-out mean from reflectivity */
*colour = self.Tri.Reflectivity.Mulf(1.0 / reflectivityMean)
/* discluding degenerate result direction */
return isAlive && !outDirection_o.Is_Zero()
}
func (self *Triangle) SamplePoint(r *rand.Rand) Vector3f {
/* get two randoms */
sqr1 := math32.Sqrt(r.Float32())
r2 := r.Float32()
/* make barycentric coords */
c0 := 1.0 - sqr1
c1 := (1.0 - r2) * sqr1
/* make barycentric axes */
a0 := self.Vertexs[1].Sub(self.Vertexs[0])
a1 := self.Vertexs[2].Sub(self.Vertexs[0])
/* scale axes by coords */
ac0 := a0.Mulf(c0)
ac1 := a1.Mulf(c1)
/* sum scaled components, and offset from corner */
sum := ac0.Add(ac1)
return sum.Add(self.Vertexs[0])
}
func (self *Vector3f) Mag() float32 {
return math32.Sqrt(self.X*self.X + self.Y*self.Y + self.Z*self.Z)
}
func (self *Triangle) Area() float32 {
/* half area of parallelogram (area = magnitude of cross of two edges) */
normalV := self.NormalV()
return math32.Sqrt(normalV.Dot(normalV)) * 0.5
}