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====== Bidirecitonal path tracer ======

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. 

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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.