Use precise grid-space ray-tracing

[fangq]

MCX has been using a 1-grid advance step size since the beginning, derived from the tMCimg era. My previous argument regarding this inaccurate in-grid ray-tracing scheme is that after sufficient sampling of the paths, the solution will converge to the correct result. See this mailing list comment:

https://groups.google.com/forum/?hl=en#!searchin/mcx-users/precise$20/mcx-users/I7YCRf4aNnY/8qunTY6UPo4J

However, for this to converge, it may take a lot more photons than I originally expected. I recently compared MCX and MMC (which does precise ray-tracing) and found that the fluence distributions from the two solutions match very well for homogeneous solutions, but not for heterogeneous cases with scattering contrasts (inclusions with absorption contrast also matches very well). This suggested the problem of photon-path related inaccuracy exist in MCX.

Implementing precise and fast grid-space ray-tracing in MCX is expected to solve this problem.

[fangq]

Implementation is progress. Initial result is very positive. MCX and MMC now match perfectly for both homogeneous and heterogeneous cases with scattering contrasts.

The boundary reflection code was also significantly simplified due to this new change.

Computational speed did get a hit - about 50% slow-down for Fermi (GTX 590) and 30% for Maxwell (GTX980Ti). NVVP profiler shows hot spots in the new ray-grid ray-tracing code. Need to further optimize to improve the speed.

Stay tuned.

[fangq]

Changes are completed

c7f9625

the precise ray-tracing algorithm does make a difference when an inclusion present scattering contrast (either in mus or in g). New algorithm returns smooth fluence profiles while the old algorithm generate a small discontinuity.

The computational speed was improved guided by NVVP profiling results. A bunch of register variables, which were actually stored in the local/global memory, were optimized by using shared memory. see my forum post at

https://devtalk.nvidia.com/default/topic/868118/?comment=4646254

These changes improved the performance on Maxwell by 50%; however, there is no noticeable speed improvement on Fermi or Kepler.

The old way of photon fluence accumulation, i.e. adding the photon weight to enclosing voxel at a fixed length (1mm) can still be simulated by setting "-F 1" or "--faststep 1". By default, faststep is disabled.

[fangq]

issue now closed