it’s the babiest of baby steps but I finally have a real-time path tracer working. it’s about one level of complexity above the simplest possible one because it’s doing next event estimation (NEE), i.e. instead of bouncing rays completely at random it’s also checking at each bounce whether a light is visible from that position. path tracing is noisy—it’s probabilities stacked on smaller probabilities all the way down—so any sampling strategy that can reduce that, like NEE, is super helpful for the final image quality.
this feels like a milestone because I’ve spent years reading articles and papers and code about this stuff and going “ok that makes sense” but this is my first real implementation of it. debugging the code helped a ton with some concepts I’d been unwittingly (or wittingly) shaky on. hooray for learning.
next steps:
- more lights: each ray bounce will have to choose a subset of them to sample from, which will add noise
- specular transmission, i.e. glass—requires more rays because the path is likely to pass through multiple surfaces before getting somewhere that it can see the light from
- denoising: even the fanciest sampling in the world (i.e. way fancier than mine) will not make a real-time path-traced image not noisy; the image needs some post-process smoothing
- spatial data structure of some kind, probably a BVH or octree—with this few objects it’s not important, but as that number rises it’ll help to be able to skip unneeded intersection tests
lots to do—I’m excited.
got it set up to handle multiple lights and while that part is working great you can also see the noise is only getting noisier. I experimented briefly with a basic spatial denoise—3×3 bilateral filter—which honestly made things worse since it just looked muddy. I think what might save me is doing something temporal, i.e. blending from frame to frame, but generating motion vectors (which you need to reproject the last frame’s pixels, so you know which bits to blend the current frame’s pixels with) is a little trickier with a purely raytraced thing like this. but I was gonna need that anyway so it’s just shifting the order of things around a bit. onward.
