Gathering for Free in Random Walk Radiosity

We present a simple technique that improves the efficiency of random walk algorithms for radiosity. Each generated random walk is used to simultaneously sample two distinct radiosity estimators. The first estimator is the commonly used shooting estimator, in which the radiosity due to self-emitted light at the origin of the random walk is recorded at each subsequently visited patch. With the second estimator, the radiosity due to self-emitted light at subsequent destinations is recorded at each visited patch. Closed formulae for the variance of the involved estimators allow to derive a cheap heuristic for combining the resulting radiosity estimates. Empirical results agree well with the heuristic prediction. A fair error reduction is obtained at a negligible additional cost.