DiligentEngine

Diligent Engine is a cross-platform graphics abstraction layer. Write code once and run it against Direct3D 12, Vulkan, Metal, and WebGPU on Windows, macOS, Linux, iOS, Android, and the browser. Shaders stay in HLSL and the engine translates them to MSL, GLSL, or SPIR-V per backend. This is infrastructure for graphics programmers, not a game engine.

If you are writing a CAD tool, simulation, scientific visualization, or any real-time 3D application that has to run everywhere, you either use this, bgfx, or sokol, or you write platform-specific code for each backend. Diligent is more featured than sokol and more modern than bgfx.

Solo and small teams: use it freely under Apache 2.0. Metal support on macOS, iOS, and tvOS requires a commercial license from DiligentGraphics. Vulkan via MoltenVK stays free. Contact them for pricing on the Apple backends since it is not published.

The catch: low-level graphics is hard. Diligent abstracts the API surface but you still need to understand descriptor sets, command buffers, and pipeline state. This does not replace Unity or Unreal. It is what you reach for when those are too opinionated for what you are building.