Restore original camera orientation, scale when saving scene

[pierotofy]

During training the model is scaled and oriented.

Before outputting the output PLY, we should scale / rotate the scene back to its original CRS.

[pfxuan]

To reserve the metric scale and camera orientation, it might be easier to feed the original sparse point cloud to training code directly. Rotating the trained ply model file would introduce an extra complicity and quality degradation if it's not handled correctly.

[pierotofy]

Potentially useful:

• https://github.com/google/spherical-harmonics
• https://github.com/playcanvas/engine/blob/412d068cf0b1f6f7df400d75b61d0862c036fc7f/src/scene/gsplat/gsplat.js#L259

[pfxuan]

Yes, PlayCanvas's implementation works almost faultlessly. It would be even better if OpenSplat can use the compressed ply format as one of saving options along with the rotation and scaling.

• https://github.com/playcanvas/super-splat/blob/main/src/splat-convert.ts#L129
• .splat universal format discussion mkkellogg/GaussianSplats3D#47 (comment)