NVIDIA OptiX GPU Ray Tracing Framework
Pixels calculated by tracing rays from image plane into geometry, recursively doing reflection, refraction, shadow calculations.
Ray Tracing more realistic (closer to physics) than rasterized projection
Apply Image Synthesis Tool to Photon Simulation
Same rate determining step : geometry intersection
OptiX Provides:
CUDA compiler optimized for Ray Tracing
- state-of-the-art accelerated intersection
- regular improvements, new GPU tuning
NVIDIA expertise on efficient GPU(s) usage
- persistent warps sized to fill machine
- load balancing between warps, GPUs
IHEP Performance Check
Used IHEP 4 GPU workstation to verify near linear performance scaling across multiple GPUs
https://developer.nvidia.com/optix
https://research.nvidia.com/publication/optix-general-purpose-ray-tracing-engine
Opticks Absolute Reflection compared to Fresnel expectation
Comparison of simulated absolute reflection of S and P polarized single events against expectation from Fresnel formula. Using uniform planar incident cyclindrically directed light.
Opticks Prism Deviation vs Incident angles for 10 wavelengths
Prism geometry and Snell's law at two refractions allows deviation angle vs incident angle to be predicted. Comparison of simulation results with expectations for 10 wavelengths using refractive index of Schott F2 Flint Glass.
Multiple Rainbows from a Single Drop of Water
Caustic bunching at least deviation causes rainbow
- Primary bow, single reflection : ray f at bow angle
- Secondary bow, double reflection : ray g at bow angle
- Deviation angles 0:180 degrees hemisphere opposite to incident rays
Jearl D. Walker, 1975, Multiple rainbows from single drops of water and other liquids http://patarnott.com/atms749/pdf/MultipleRainbowsSingleDrops.pdf
Disc beam 1M Photons incident on Water Sphere (S-Pol)
Photons shown by lines with color representing polarization direction. S-Polarized (perpendicular to plane of incidence) intersection by disc radially directed polarization. Geodesic icosahedron tesselation just for OpenGL visualization, actual OptiX geometry is perfect sphere.
2ns later, Several Bows Apparent
Rainbow deviation angles
Deviation angle distribution of all 3M photons. Photon wavelengths from Plankian 6500K blackbody spectrum (implemented with inverse CDF GPU texture lookup). Simulated "images" obtained from wavelength spectrum of each bin using CIEXYZ weighting functions converted into sRGB/D65 colorspace. The two images exposed for luminance (CIE-Y) of max bin of 1st and 2nd bows.
Rainbow Spectrum for 1st six bows
Spectra obtained by selecting photons by internal reflection counts. Colors obtained from spectra of each bin using CIEXYZ weighting functions converted into sRGB/D65 colorspace. Exposures by normalizing to bin with maximum luminance (CIE-Y) of each bow. White lines indicate geometric optics prediction of deviation angle ranges of the visible range 380-780nm. 180-360 degrees signifies exit on same side of droplet as incidence.
1M Rainbow S-Polarized, Comparison Opticks/Geant4
Deviation angle(degrees) of 1M parallel monochromatic photons in disc shaped beam incident on water sphere. Numbered bands are visible range expectations of first 11 rainbows. S-Polarized intersection (E field perpendicular to plane of incidence) arranged by directing polarization radially.
Performance Comparison Opticks/Geant4
Average Propagate Time for 1M photons
MacBook Pro 2013, NVIDIA GeForce GT 750M 2GB (384 cores)
| Rainbow Test | Geant4 10.2 | Opticks Interop | Opticks Compute |
|---|---|---|---|
| 1M (S-Pol) | 56 s | 1.62 s | 0.28 s |
| 1M (P-Pol) | 58 s | 1.71 s | 0.25 s |
- Opticks ~200X Geant4 with 384 core GPU
- expect > 1000X with multi-GPU workstation
- photon propagation time --> zero (effectively)
Opticks Interop/Compute Modes
- Interop: uses visualizable OpenGL buffers
- Compute: uses OptiX buffers
- perfectly identical results, monitored by digest
- Compute (and G4) propagated events visualizable by loading into interop mode viewer
