smallpt

smallpt is a global illumination renderer written in just 99 lines of C++ by Kevin Beason in 2007. It implements a fully functional, physically correct unbiased Monte Carlo path tracer with Russian roulette path termination, tent filter anti-aliasing, and support for diffuse, perfectly specular, and ideal refractive (glass) surfaces. The entire scene — a modified Cornell Box with two spheres — is hardcoded in the source. Despite its extreme brevity, the algorithm is not an approximation: it produces a mathematically correct solution to the rendering equation.

The renderer uses only spheres as geometric primitives, performs brute-force ray-sphere intersection testing without any acceleration structure, and outputs a single PPM image file. It has no external dependencies beyond the C++ standard library and OpenMP pragmas for optional parallelism. The 99-line version is deliberately compressed and nearly unreadable; the educational value comes primarily from annotated and expanded versions created by the community, which typically span 200-300 lines. Configuration is limited to resolution and sample count via command-line arguments or source code modification.

smallpt is arguably the most famous minimal renderer in existence and occupies a unique niche in the rendering ecosystem. It has spawned an entire family of educational extensions — smallptGPU, smallpaint, smallvol, and numerous annotated versions — that collectively form one of the best introductory resources for understanding path tracing. It is universally cited in university computer graphics courses, conference talks, blog posts, and textbooks as the canonical example of how simple a physically correct global illumination renderer can be. It is the "hello world" of path tracing.