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In pinhole cameras, straight lines look straight. That's what a regular projection matrix gives you with rasterization. With non-pinhole cameras, straight lines look curved. You can't rasterize this directly. 3D Gaussian splats have an issue with this, addressed by methods like ray tracing. It's very useful to train on non-pinhole cameras, because in real world they can capture a wider field of view.

In Gaussian Splatting, a first order approximation takes ellipsoids in camera space to ellipses in image space. This works ok for pigeonhole cameras where straight lines remain straight and more generally conic sections are taken to other conic sections. For high distortion models like fisheye, this approximation probably breaks. However this method presumably does not rely on approximation since it is ray traced.

looks like it’s some sort of fisheye camera with a super wide fov. it might be simulating rays bending due to lens effects. a pinhole camera could just look “normal” ie straight lines stay straight (except for horizon convergence perspective effects)