## On a surface, Russian Roulette determines whether a photon is absorbed or reflected.

The photon power is not scaled.

**Reflection or Absorption:**

*a: [0, 1] is a random variable*

*p: probability of reflection*

*phi: the power of incoming photon*

if( a < p ) reflect photon at power phi

else photon is absorbed

**Specular or Diffuse Reflection:**

*a: [0, 1] is a random variable*

*p_d: probability of diffuse reflection*

*p_s: probability of specular reflection*

*where ( p_d + p_s < 1 )*

Diffuse Reflection: a: [0, p_d]

Specular Reflection: a: [p_d, p_d + p_s]

Absorption: a: [p_d + p_s, 1]

## In participating media, Russian Roulette determines whether a photon is absorbed or scattered.

The probability of a photon being scattered is given by the scattering albedo ( rho / (rho + alpha) ) [Jensen 1998], where rho and alpha are the scatter and absorption coefficients.

**Scattering or Absorption:**

*a: [0, 1] is a random variable*

*p: probability of scattering*

*phi: the power of incoming photon*

if( a < p ) scatter photon at power phi

else photon is absorbed** **

## New direction of a photon being scattered from diffuse reflection or participating media:

Sampling techniques. Importance sampling is usually applied.