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Caustics with path engines
Path-only engines such as PATHOCL and BIASPATHOCL, as well as their non-OpenCL CPU versions are not suitable for any complex lighting scenes with a lot of indirect lighting, i.e. where light needs to bounce many times before it can be seen.
One example of relatively complex lighting are scenes with caustics and relatively small light sources, such as an area light. The larger the light source, the easier it is for camera to find the light. This article includes some example test cases on rendering caustics using Path engines. Note that they are not at all effective for this, which the examples will also show, and you should consider engines that currently work only on the CPU such as Bidir and BidirVM instead.
If you're interested in how to create beautiful caustics with Bidir, you should read this article instead.
Scene used for testing
The following scene was used to test the efficiency of PATHOCL and BIASPATHOCL engines for rendering interesting caustics on GPU + CPU hardware:
The scene includes a diamond object with smooth shading, Glass1 Material with 1.52 IOR, #B7F5FF Transmission color, one bottom plane object with Matte material with Sigma 30, and one area light sized 0.1 to produce the caustics.
The format of image descriptions is Engine/Sampler, followed by any additional settings and the approximate render time in seconds, minutes or hours. All images except Bidir (used for reference only, being the most accurate representation) were rendered on GPU+CPU. Not all of the settings and parameter changes are included in the image description, but they should not result in a very significant difference from default parameters. Other factors such as availability of system resources may slightly vary between the images. With all slight differences considered, this should be taken as an approximate and not exact comparison.