Curvature is an experimental hybrid raytracing / raymarching 3D engine written from scratch in C++ with one goal in mind: allowing light to be curved efficiently.
Curvature implements common ray-tracing optomization such as BVH acceleration and multithreading, while also implementing a novel technique for curved light-path integration, which uses distance functions to define safe, spherical zones where some calculations can be skipped, resulting in a 1.5x-4x speedup in render times.
The engine makes very few assumptions about the distortion fields; they are represented as arbitrary functions of not only position, but direction, and they are not required to have a closed-form integral. The only requirement is that they be non-zero within a restricted, bounded region. Multiple fields are allowed to overlap, in which case their effect is combined additively. This gives immense freedom in implementing fields that generate strange space curvature.
You can look into Curvature's source code and more detailed documentation here on my GitHub.
GIF above: orbiting around a 7x7 grid of spheres with two light-repelling fields and a light-bending field at the center, which twists light in a fashion similar to the way magnetic fields bend the trajectory of electrons. The original 180 frame, 1920x1080, 2x anti-aliased render took about 8 hours to complete on a 8-core i9 2019 15" MacBook Pro running an earlier version of the software.