Add native .spz Gaussian Splat format support

[Enndee]

Add native .spz file loading support

Summary

This PR adds native loading support for the .spz Gaussian Splat format — Niantic's open, compressed binary format for 3D Gaussian Splatting scenes. .spz files are typically ~10× smaller than equivalent .ply files, making them significantly faster to distribute and load.

Motivation

The PLY format used by splatapult is large and uncompressed. For scenes captured with tools such as SHARP, 3DGS, or Gaussian Opacity Fields, the resulting PLY files can be tens to hundreds of MB. The .spz format provides:

• ~10× compression vs PLY (e.g. a 5.4 KB test file becomes 0.4 KB)
• MIT-licensed open format with a clean C++ reference implementation
• Single new dependency: zlib (already present in the vcpkg graph via libpng in most builds)
• Lossless for all Gaussian attributes: positions, scales, rotations, opacity, DC color, and full SH up to degree 3

Implementation

New dependency: nianticlabs/spz

The PR integrates nianticlabs/spz (MIT license) as a git submodule at third_party/spz. Only three .cc source files are compiled directly into the splatapult executable (load-spz.cc, splat-types.cc, splat-c-types.cc). No separate static/shared library is needed.

Files changed

File	Change
.gitmodules	Add third_party/spz submodule pointing to nianticlabs/spz
vcpkg.json	Add zlib dependency
CMakeLists.txt	Find ZLIB, add SPZ source files and include dir, link ZLIB
src/gaussiancloud.h	Declare ImportSpz()
src/gaussiancloud.cpp	Implement ImportSpz()
src/app.cpp	Route .spz extension to ImportSpz(), .ply to ImportPly()

GaussianCloud::ImportSpz() details

• Uses spz::loadSpz(filename, UnpackOptions) with CoordinateSystem::UNSPECIFIED
  (preserves PLY coordinate space — coordinates are stored as-is by converters such as 3dgsconverter)
• Supports both BaseGaussianData (SH degree 0) and FullGaussianData (SH degree 3)
• Handles quaternion reorder: spz stores [x, y, z, w]; splatapult uses [w, x, y, z]
• Scales are stored as log-scale in spz, same as PLY — expf() applied identically
• SH coefficient layout mapping: spz uses [coeff, channel] inner ordering; remapped to splatapult's r_sh0/g_sh0/b_sh0 ... r_sh3/g_sh3/b_sh3 layout

File-extension routing in app.cpp

The internal variable plyFilename now accepts any supported extension. A simple case-insensitive extension check dispatches to ImportSpz or ImportPly. All other logic (file watcher, hot-reload, etc.) is unchanged.

How to test

1. Add the submodule and apply the patch (see HOW_TO_SUBMIT.md in the PR package)
2. Build with cmake + your generator as usual
3. Convert any .ply scene file to .spz using 3dgsconverter:

   3dgsconverter -i scene.ply -f spz --force
   

4. Launch splatapult with the .spz file:

   splatapult.exe scene.spz
   

Backward compatibility

Fully backward-compatible. Existing .ply workflows are completely unaffected. The routing in app.cpp falls through to ImportPly() for any non-.spz extension.

Dependencies

• zlib — only new vcpkg dependency; very widely available and already present in most builds via libpng
• nianticlabs/spz — MIT license, C++ only, no GPU/graphics dependencies

Related

• SPZ format spec: https://github.com/nianticlabs/spz
• Converter tool: https://github.com/francescofugazzi/3dgsconverter