Add serialization and deserialization methods to Octree

[lumurillo]

Add serialization and deserialization methods to the Octree.

Type

• Bug fix (non-breaking change which fixes an issue): Fixes #
• New feature (non-breaking change which adds functionality). Resolves #
• Breaking change (fix or feature that would cause existing functionality to not work as expected) Resolves #

Motivation and Context

Checklist:

• I have run python util/check_style.py --apply to apply Open3D code style
  to my code.
• This PR changes Open3D behavior or adds new functionality.
  • Both C++ (Doxygen) and Python (Sphinx / Google style) documentation is
    updated accordingly.
  • I have added or updated C++ and / or Python unit tests OR included test
    results (e.g. screenshots or numbers) here.
• I will follow up and update the code if CI fails.
• For fork PRs, I have selected Allow edits from maintainers.

[update-docs]

Thanks for submitting this pull request! The maintainers of this repository would appreciate if you could update the CHANGELOG.md based on your changes.

[Copilot]

Pull Request Overview

This PR adds binary serialization and deserialization support to the Octree class, complementing the existing JSON-based I/O functionality. The implementation allows Octree objects to be saved and loaded in a binary format (.bin) for more efficient storage and transmission.

Reviewed Changes

Copilot reviewed 8 out of 8 changed files in this pull request and generated 6 comments.

Show a summary per file

File	Description
cpp/open3d/geometry/Octree.h	Adds binary serialization method declarations to all Octree node classes
cpp/open3d/geometry/Octree.cpp	Implements binary serialization/deserialization for all Octree node types
cpp/open3d/io/OctreeIO.h	Adds function declarations for binary I/O operations
cpp/open3d/io/OctreeIO.cpp	Implements high-level binary I/O functions and registers .bin format support
cpp/open3d/io/file_format/FileBIN.cpp	Adds low-level binary file read/write utilities
cpp/tests/geometry/Octree.cpp	Adds unit test for binary stream serialization/deserialization
cpp/tests/io/OctreeIO.cpp	Updates existing tests to verify both JSON and binary formats
cpp/pybind/t/geometry/pointcloud.cpp	Minor formatting fix for function definition

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[ssheorey]

Hi @lumurillo thanks for this nice addition. Some thoughts:

• Can we use msgpack, instead of raw binary format for serialization? This is already used elsewhere in Open3D, is simple to use, should provide good speed and smaller size and will also be cross-platform. Raw binary format sometimes has issues if you write on Windows and read in Linux or vice versa. It will also enable reading / writing from plain Python through the Python msgpack library.
• msgpack in Open3D:

  Open3D/cpp/open3d/io/rpc/Messages.h

  Line 250 in fb7088c

  // macro for creating the serialization/deserialization code

  (and other places - search for msgpack). We use *.msgpack files to store tensor geometries in the Tensorboard plugin, for example.
• Tutorial: https://github.com/msgpack/msgpack-c/wiki/v2_0_cpp_tutorial

[ssheorey]

From Github Copilot, about the macOS errors:

The build is failing due to missing override specifiers for functions that override virtual methods in the Octree classes. The compiler treats this as an error because of the -Werror,-Winconsistent-missing-override flag. For example:

error: 'SerializeToBinaryStream' overrides a member function but is not marked 'override' [-Werror,-Winconsistent-missing-override]

To fix this, add the override keyword to all overriding methods in the derived classes. Specifically, in cpp/open3d/geometry/Octree.h (ref: 1fada55), update the relevant function declarations as follows:

For OctreeInternalNode:

bool SerializeToBinaryStream(std::string& out) const override;
bool DeserializeFromBinaryStream(const std::string& in, size_t& offset) override;

For OctreeInternalPointNode:

bool SerializeToBinaryStream(std::string& out) const override;
bool DeserializeFromBinaryStream(const std::string& in, size_t& offset) override;

For OctreeColorLeafNode:

bool SerializeToBinaryStream(std::string& out) const override;
bool DeserializeFromBinaryStream(const std::string& in, size_t& offset) override;

For OctreePointColorLeafNode:

bool SerializeToBinaryStream(std::string& out) const override;
bool DeserializeFromBinaryStream(const std::string& in, size_t& offset) override;

Add override to all other similar functions that are overriding a base class method but are missing it. This will resolve the compilation errors and allow the build to succeed.

You can see the problematic lines in Octree.h here:
https://github.com/isl-org/Open3D/blob/1fada55e1c46344d4b0bd63d3668240a0ff2f9dc/cpp/open3d/geometry/Octree.h