Developer builds browser-based 3D marble run simulator with deterministic MP4 export

A developer has launched MarbleRun.online, a free browser-based 3D marble run sandbox where users can place ramps, pegs, spinners, and balls on a virtual wall and simulate realistic physics. The tool runs entirely client-side with no login, no uploads, and no AI involvement, and is intentionally limited to desktop browsers to preserve the 3D experience. Users can export their simulations as vertical 9:16 MP4 videos at 1080x1920 resolution with synchronized impact audio, a feature the developer says took three times longer to build than the editor itself. The export challenge centered on achieving determinism — ensuring the same level produces an identical video every time — which required offscreen rendering at a locked 30fps, a fixed-timestep physics clock, and frame-accurate generated audio instead of microphone capture. The developer noted that starting with the export pipeline rather than the editor would have saved significant rework, as determinism was not considered early enough in the build process.