BTP is a path tracing process in which ray paths are generated by the camera as well as by all light sources in the scene and are linked together.
BPT in GroIMP is controlled via 3 parameters. The <key>Max. eye path depth</key> and <key>Max. light path depth</key> parameters can be used to set the maximum number of vertexes on the eye path and light path respectively. A light path depth of 1 can be used to simulate the operation of a normal PT. Conversely, with an eye path depth of 1, only caustic events are displayed. Values smaller than 1 are not permitted. The parameter <key>Heuristic exponent</key> controls the calculation of the weighting factor w_st, reference the Steidelmüller 2008 report for further details.