CUDA Path Tracer
University of Pennsylvania, CIS 565: GPU Programming and Architecture, Project 3
- Zach Corse
- Tested on: Windows 10, i7-6700HQ @ 2.60GHz 32GB, NVIDIA GeForce GTX 970M (personal computer)
I'm constructing a pathtracer on the GPU using CUDA. Pathtracing models physically-based light interactions in a scene by casting rays into the scene, testing for intersections, then shading using a variety of physically-based scattering models. My particular implementation handles box, sphere, and triangle intersections, and can simulate diffuse, reflective, and refractive surfaces. What differentiates these shading models is their BxDFs, which are characterized by unique probability distribution functions that encode the probability that a ray is re-directed in a given direction as a function of angle of incidence and surface normal. Rays in the scene are launched with a "bounce depth", which is simply the number of bounces they have to reach an emissive source (light) before they are terminated with the color black. If they do reach a light, the pixels they were launched from get the color they've accumulated by interacting with various surfaces in the scene.
Various optimizations speed the processing of parallel rays, and I've included several here. The first handles the early termination of rays. Within a given iteration (a max of 8 bounces by default), a ray might terminate early. This happens for one of two possible reasons: the first is the ray hits an emissive source. This means its path to the camera is complete and can therefore be terminated early. The second is the ray does not intersect any geometry in the scene. This means the photon ray will have no further interactions in the scene and will not subsequently reach a light (it is lost to the aether). It would be wasteful to launch dead rays in the various kernels called within a single ray bounce (which runs up to ray bounce depth times within a given iteration). Stream compaction reorders rays in a buffer according to a simple check -- is the ray dead or not? If it isn't, it is moved to the front of the buffer. We maintain a pointer to the transition index between alive and dead rays then only launch rays in subsequent iterations up to this index.
A second optimization handles variable shading computation times, which vary according to the material the ray intersects. Within a warp, if one ray intersects a diffuse surface and another intersects a refractive one, the time it takes to compute the ray's new direction and color varies considerably (diffuse is simpler). It might therefore be prudent to first sort rays by the materials they've intersected, such that warps handling simpler interactions (eg diffuse) can be freed earlier and directed to other tasks. Consequently, my implementation includes a toggleable radix sort by key (material id). As shown below, in general this does not provide the speedup one might expect.
A final feature / optimization I include checks ray intersections with arbitrary OBJ mesh bounding boxes before testing more computationally intensive triangle intersections. If a ray does intersect the mesh's bounding box, it is then naively checked against all triangles in the mesh. A more computationally efficient approach would be store the mesh in a KD-tree, but I haven't managed to implement this just yet. Finally, I use tiny_obj_loader to handle the broad span of OBJ mesh qualities found in open source libraries.
Present features included in my pathtracer. See below for sample renders and performance analysis!
- Basic Pathtracing
- Diffuse BxDF scattering
- Scene construction
- Sphere and Box intersections
- Terminated Ray Stream Compaction
- Per iteration
- Only launches ray threads that have not yet terminated
- Material sorting
- Sorts ray intersections pre-shading
- Reflection BxDF
- Perfect specular reflection
- Refraction BxDF
- Includes Fresnel effects using Schlick's approximation
- Camera rays are jittered, enhancing scene convergence
- Improves edge representation at the cost of first bounce caching
- OBJ mesh loading with tiny_obj_loader
- Supports all mesh types
- Includes mesh bounding box intersection test optimization
- Uses ray-triangle intersection methods
- Choose between barycentric normal interpolation and facet normals
- Supports all BxDFs mentioned above
- Does not include textures ... yet
- Does not include KD-tree acceleration ... yet
- Without acceleration, practical limitations on mesh size
A Standard scene. Cornell box with diffuse boxes and specular spheres.
Ideal, specularly reflective spheres.
Refractive spheres using Schlick's approximation.
Arbitrary mesh loading. Only relatively low-poly meshes supported.
Stream Compaction Analysis
As described above, stream compaction terminates rays that intersect emissive sources and rays that don't intersect any scene geometry. Within a single iteraction we can observe this as follows:
We see that the number of rays we need to launch decreases with ray depth per iteration. The cost of stream compaction is executing the algorithm described in the introduction. In practice, we see real speedup effects for ray bounce depths greater than or equal to 29 for an 800 x 800 resolution camera:
Until this transition point, the cost of stream compaction outweighs its benefits.
As one might expect, stream compaction is particularly useful in non-closed scenes, such as the Cornell box, because rays that are directed away from scene geometry can be terminated early. The following table demonstrates the cost of closing the Cornell box in milliseconds per iteration (for a trace depth of 8). The relative cost is greater for stream compaction than it is for path tracing with no stream compaction.
|No Stream Compaction||37.448||40.231|
Both iterations (with stream compaction and no stream compaction) take longer to execute because there is more scene geometry, but the relative cost of stream compaction is greater because fewer rays terminate.
Material Sorting Analysis
Material sorting efficiency boils down to whether or not it speeds up material shading sufficiently to offset the cost of sorting. For simpler scenes that involve fewer materials, and for scenes that primarily involve diffuse shading interactions, the cost of material sorting does not offset longer shading computation times. This is shown in the graph below, which measures milliseconds per iteration in a simple scene that includes three spheres (one diffuse, one refractive, and one reflective):
Three sample materials in a scene.
While we do in fact see a marginal decrease in shading computation time using material sorting, as with stream compaction, the cost of sorting outweighs its benefits.
Anti-aliasing improves image convergence. Rays are launched within individual pixels with random offsets per iteration, such that initial ray bounces are not identical. Caching these initial bounces would otherwise be useful, but anti-aliasing renders this practice useless while yielding superior edge representation. The two images below, taken after an identical number of iterations, demonstrate the utility of anti-aliasing.
With AA. Jagged edges are smoothed. 150 iterations.