On Thu, Mar 5, 2015 at 6:24 AM, Christopher Sean Morrison <brl...@mac.com>
wrote:
>
> Astute observations. The OpenCL code was intentionally structured that
> way, not at all for performance obviously, but so we could perform a 1-1
> comparison to validate that the implementation behavior was correct. Our
> librt ray tracing logic is integrated in a number of scientific analysis
> applications, so we go to great lengths to ensure we consistently report
> hits/misses, that the ray intersection implementation is correct especially
> given inaccurate floating point math.
>
> Indeed, the first OpenCL translation of the ellipsoid logic — which seemed
> 100% identical to the C code — was wrong. It resulted in different
> calculations. The issues were sorted out a few days later, but this level
> of rigor and validation is necessary. Of course, this is not the desired
> end-state, but was done to easily test the implementation. The inefficient
> buffer allocation overhead was also intentional (and also temporary) just
> to get quick numbers on OpenCL’s overhead costs.
>
I see. But double precision floating point math is a lot slower than single
precision on certain architectures like the lower end NVIDIA cards. e.g.
the GeForce GTX 780 Ti has 5048 SP GFLOPS but only 210 DP GFLOPS. Even on
the higher end NVIDIA cards the DP FLOPS are like a third of the SP FLOPS.
The rendering system should be tunable for either speed or accuracy. That
kind of complicates things. As a first approach we can work only on getting
the double precision to work but the performance will suffer a great deal.
--
Vasco Alexandre da Silva Costa
PhD Student at Department of Information Systems and Computer Science
Instituto Superior Técnico/University of Lisbon, Portugal
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