Find distance to boundary:d
Y
End (next photon)
N
Launch a photon
Move s
Set free path:s
Weight:w=w*m
Weight small?
Set free path:sleft
Set:sleft=0
Scatter & absorp
Transmit/Reflect
Move to boundary
Hit boundary:sd
PhotonDead?
Free path:s=0?
Survive Roulette?
