Based on: NVIDIA OptiX Ray Tracing Engine [C++/C/CUDA]
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
OptiX: Ray Trace optimized CUDA compiler
- ray tracing framework: flexible, low level
- state-of-the-art GPU accelerated intersection
- regular releases: improvements, new GPU tuning
- shared C++/CUDA context eases development
NVIDIA expertise on efficient GPU/multi-GPU 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
Mesh Fixing with OpenMesh surgery
Color indicates material assigned. Prior to fix, assignments incorrect for all photons heading towards Dayabay AD top lid.
For topologically correct meshes, without holes:
V - E + F = 2 (Eulers Characteristic)
Intersection boundaries determines photon material, bad meshes cause incorrect material assignments
G4Polyhedron Tesselation Bug
- Some union solids become cleaved meshes
- close parallel faces cause flickering render
- ~25/250 Dayabay meshes have issues
- two critical ones fixed: IAV, OAV.
OpenMeshRap finds/fixes cleaved meshes
- based on open source project: OpenMesh
- extracts real topological meshes
- finds close parallel faces between the meshes
- deletes the extra faces
- stitches the split mesh together with added triangles
Tesselated PMT geometry : convenient
Tesselated PMT geometry : unrealistic disco ball effect
Analytic PMT geometry : more realistic, faster, less memory
- parse geometry XML to give CSG tree : solids with boolean intersections, unions
- partition 5 solids into 12 single primitive parts
- splitting at geometrical intersections avoids implementing general CSG boolean handling
Analytic PMT in 12 parts instead of 2928 triangles
Geometry provided to OptiX in form of ray intersection code
PMT intersection by comparison with part intersections: cylinder and partial sphere intersection from quadratic roots
Volume to Surface translation
Volume heirarchy: Pyrex/Vacuum/Bialkali puts photocathode inside vacuum but as coincident boundaries it makes no difference for volume description.
Coincident boundaries do not work for surface description, must adopt correct heirarchy: Pyrex/Bialkali/Vacuum
Sphere intersection only 2 cases, Cylinder 10 cases (axial, walls, endcap, outside/inside)
OptiX Ray Traced Analytic PMT geometry
Near clipped, orthographic projection.
Analytic PMTs together with triangulated geometry
Dayabay Ray Trace: PMTs analytic, the rest triangulated
PMT Opticks/Geant4 step comparison TO BT [SD] : position(xyz), time(t)
PMT Opticks/Geant4 step comparison TO BT [SD] : polarization(abc), radius(r)
PmtInBox issues : velocity of photon propagation
Opticks propagating at phase velocity, Geant4 at group velocity
(vacuum wavelengths)
- time match obtained by kludging G4 GROUPVEL to be phase velocity
Working on fix:
- have extended GPU texture to hold GROUPVEL
- need to redo G4DAE export to populate the property and adjust propagation to use it
https://en.wikipedia.org/wiki/Dispersion_(optics)
G4MaterialPropertiesTable::SetGROUPVEL
G4Track::CalculateVelocityForOpticalPhoton
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.
Photon Propagation Times Geant4 cf Opticks
| Test | Geant4 10.2 | Opticks Interop | Opticks Compute |
|---|---|---|---|
| Rainbow 1M(S) | 56 s | 1.62 s | 0.28 s |
| Rainbow 1M(P) | 58 s | 1.71 s | 0.25 s |
| PmtInBox 0.5M | 41 s | 0.81 s | 0.15 s |
- Opticks > 200X Geant4 with only 384 core mobile GPU
- multi-GPU workstation up to 20x more cores
- photon propagation time will become effectively zero
- Interop uses OpenGL buffers allowing visualization, Compute uses OptiX buffers
- Interop/Compute : perfectly identical results, monitored by digest
- Compute (and G4) propagations visualized by loading into Interop viewer
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.
