Summary of Changes

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This pull request significantly enhances COLMAP's 3D reconstruction capabilities by adding a robust mesh simplification feature. It allows users to efficiently reduce the complexity of high-resolution meshes, making them more manageable for visualization or further processing, without sacrificing significant visual fidelity. The new functionality is seamlessly integrated into both the command-line tool and the Python API, providing flexible access to this powerful optimization.

Highlights

New Mesh Simplification Feature: Introduced a new mesh_simplifier command and corresponding C++ and Python API for reducing the face count of dense meshes using Quadric Error Metric (QEM) decimation.
QEM Algorithm Implementation: The core mesh simplification logic is implemented based on the Garland and Heckbert (1997) Quadric Error Metric, including quadric computation, edge collapse, and face flip prevention.
Configurable Options: The mesh simplifier provides configurable options such as target_face_ratio, max_error, boundary_weight, interpolate_colors, and num_threads for fine-grained control over the simplification process.
Documentation and Examples: Updated the command-line interface (CLI) and tutorial documentation to include the new mesh_simplifier command, its usage, and examples, along with a new bibliography entry.

Changelog

doc/bibliography.rst
- Added a new bibliography entry for the Garland and Heckbert 1997 paper on surface simplification.
doc/cli.rst
- Documented the new colmap mesh_simplifier command, including its usage example and detailed descriptions of its options.
- Added mesh_simplifier to the list of available COLMAP commands.
doc/tutorial.rst
- Updated the tutorial to mention optional mesh simplification using Quadric Error Metric (QEM) decimation.
- Added a note suggesting the use of mesh_simplifier for reducing mesh size for faster visualization or processing.
src/colmap/controllers/option_manager.cc
- Integrated MeshSimplificationOptions into the OptionManager by adding its shared pointer, initializing it in the constructor, and implementing the AddMeshSimplificationOptions method.
src/colmap/controllers/option_manager.h
- Declared the MeshSimplificationOptions struct, its shared pointer, and the AddMeshSimplificationOptions method within the OptionManager class.
src/colmap/exe/colmap.cc
- Registered the new mesh_simplifier command with its corresponding RunMeshSimplifier function, making it accessible from the COLMAP executable.
src/colmap/exe/mvs.cc
- Implemented the RunMeshSimplifier command-line function, which handles parsing input/output paths and simplification options, reads the input PLY mesh, performs simplification, and writes the result.
src/colmap/exe/mvs.h
- Declared the RunMeshSimplifier function for the MVS executable.
src/colmap/mvs/CMakeLists.txt
- Configured CMake to build the new mesh_simplification.h and mesh_simplification.cc source files.
- Added mesh_simplification_test.cc to the build system for unit testing.
src/colmap/mvs/mesh_simplification.cc
- Implemented the core mesh simplification algorithm using Quadric Error Metric (QEM) decimation, including functions for computing quadrics, edge collapse candidates, and preventing face flips.
- Added helper functions for sorted insertion and erasure in adjacency lists.
src/colmap/mvs/mesh_simplification.h
- Defined the MeshSimplificationOptions struct with parameters for target face ratio, max error, boundary weight, color interpolation, and number of threads.
- Declared the SimplifyMesh function, which performs QEM-based mesh decimation.
src/colmap/mvs/mesh_simplification_test.cc
- Added comprehensive unit tests for the MeshSimplificationOptions validation and the SimplifyMesh functionality, covering various mesh types (tetrahedron, grid, wavy grid) and simplification scenarios, including max error and boundary preservation.
src/pycolmap/pipeline/meshing.cc
- Exposed the MeshSimplificationOptions struct to the Python API, allowing configuration of simplification parameters.
- Added a simplify_mesh function to the Python API, enabling Python users to perform mesh simplification on PLY files.

Activity

The author ahojnnes initiated this pull request to introduce mesh simplification functionality.
The pull request includes visual comparisons of meshes before and after simplification, demonstrating the effectiveness of the new feature at different reduction levels (16M, 4M, and 0.4M faces).

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