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BDPT works by tracing paths from both emitters and camera. Generated nodes on paths are joined in every possible way then. Meaning all points of geometry intersection are connected and their contribution to pixel energy is evaluated. In efficiency it can give both speed up or slow down comapared to Path Tracing (PT) which traces rays only from camera. Firstly cpu cost is much higher on BDPT since all connections have to be checked for shadow by other geometry. On the other hand in some situations generation of path can be a lot easier by doing it from light side. Finally number of samples per pixel is drastically decreased but each sample gives a lot better convergence to correct value compared to PT.
BDPT can give better results than PT in situations where it is hard to reach emitter from camera, like bulb hidden under lampshade. It is also hard to reach emitter on path that is a caustic. Connecting all the nodes on emitter and light paths can give better result in interior light evaluation. It has to be pointed out that BDPT is not a cure for all rendering problems. Situations where PT has no problems can't be evaluated faster on BDPT.
Here is list of situations where PT (old engine) will generally perform better:
Situations where BDPT will perform better:
BDPT is set as default engine. It can be changed both in NOX or Max/Blender. This selection is kept in scene file.
It has to be told that paths from light are camera-independent and this leads to following two problems:
- in bad scenario you can sample paths from emitters that won't reach camera at all (cpu time waste)
- two buffers are needed to create image. This is because pure light paths (totally camera independent) samples can't be averaged like rest of paths. They are averaged per whole image in contrast to camera-dependent samples which are averaged per pixel.
If one insists on using BDPT on exterior scene it is suggested to keep it small. This is because for whole scene there is created virtual portal for sun to make it as starting points for emission. Unnecessary big portal gives lower samples per area which leads to higher noise. What I mean sun will illuminate parts of scene that gives small or none contribution to image - cpu time waste.
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