cad-3d-reconstruction

CAD 3D Reconstruction: 3DGS Feasibility + BrepGaussian Implementation

📄 Start here: METADOC.md

METADOC.md is the full, verified project report — every approach, every result, all figures, and concrete recommendations. All of its numbers were re-checked by re-running the tools and opening every output file.

It corrects this README and FEASIBILITY_REPORT.md on two headline points:

1. MASt3R did not successfully reconstruct the parts. It produced point clouds, but the views were never fused onto a shared object — each view stayed a separate depth sheet (measured cross-view overlap: 0.00–0.30).
2. The BrepGaussian-adapted pipeline produced no usable B-Rep: 0 surfaces for two parts, 1 plane for the other two, and 0 edges/corners/curves everywhere. It also ran at 500 Gaussians / 200+100 iterations, not the 10k / 2k+1k stated below.

See §8 of the metadoc for the full list.

This repository contains the complete pipeline for reconstructing 3D models of mechanical CAD parts from multi-view images, including:

1. 3DGS Feasibility Analysis — testing whether 3D Gaussian Splatting works for CAD parts
2. MASt3R Reconstruction — camera pose estimation + dense point cloud + mesh extraction
3. BrepGaussian Implementation — adapted pipeline for CAD B-rep reconstruction

Dataset

Input: acd23/cad-test-images

• 4 mechanical CAD parts: bracket, connector_module, flange_housing, timing_pulley
• 4-5 view images per part (no camera poses)
• Textureless surfaces (CAD renders/screenshots)

Key Findings

COLMAP fails on this dataset

Standard 3DGS needs COLMAP for pose estimation. COLMAP fails completely — CAD surfaces are textureless, yielding no SIFT feature matches. See FEASIBILITY_REPORT.md for details.

MASt3R solves the pose estimation problem

We use MASt3R (3D foundation model) which works on unposed sparse images without texture. All 4 parts successfully reconstructed:

• 200k-600k points per part
• Poisson + ball-pivoting mesh extraction
• COLMAP-format poses exported for downstream use

BrepGaussian adapted pipeline

The BrepGaussian paper (CVPR 2026, arXiv 2602.21105) is implemented with adaptations:

• MASt3R poses (instead of COLMAP)
• Pure-PyTorch renderer (instead of custom CUDA rasterizer)
• OpenCV edges + watershed masks (instead of NEF rendering + fine-tuned SAM)
• Official B-Rep fitting module (unchanged)

Repository Structure

├── code/                          # All pipeline scripts
│   ├── prepare_images.py          # Image preprocessing (RGBA→RGB, resize)
│   ├── run_colmap.py              # COLMAP SfM (fails on textureless CAD)
│   ├── pipeline_mast3r.py         # MASt3R reconstruction + mesh extraction
│   ├── render_previews.py         # Matplotlib-based preview rendering
│   ├── pipeline_brepgaussian.py   # BrepGaussian adapted pipeline
│   └── simple_renderer.py         # Pure-PyTorch differentiable splat renderer
├── input_images/                  # Prepared input images per part
├── mast3r_results/                # MASt3R outputs per part
│   └── <part>/
│       ├── pointcloud.ply         # Dense point cloud
│       ├── <part>_poisson.ply     # Poisson mesh
│       ├── <part>_poisson.obj     # Poisson mesh (OBJ)
│       ├── <part>_ballpivoting.ply # Ball-pivoting mesh
│       ├── scene.glb              # GLB visualization
│       ├── colmap_sparse/         # COLMAP-format poses
│       │   ├── cameras.txt
│       │   ├── images.txt
│       │   ├── points3D.txt
│       │   └── transforms.json
│       ├── previews/              # Rendered preview images
│       └── metadata.json
├── brepgaussian_results/          # BrepGaussian adapted outputs
│   └── <part>/
│       ├── edge_*.png             # Edge maps (Canny)
│       ├── mask_*.png             # Instance masks (watershed)
│       ├── merged.pcd             # Labeled point cloud
│       ├── gaussian_splat.ply     # Trained Gaussian model
│       ├── brep_output/           # B-Rep fitting results
│       └── metadata.json
├── brep_example_output/           # Official BrepGaussian example output
├── FEASIBILITY_REPORT.md          # Full feasibility analysis
└── README.md

How to View Results

Point Clouds & Meshes

• .ply and .obj files can be opened in MeshLab, Blender, or online viewers
• .glb files can be opened in online GLB viewers
• Preview PNGs in mast3r_results/<part>/previews/ show rendered views

Poses

• colmap_sparse/transforms.json — NeRF/3DGS convention (cam2world matrices)
• colmap_sparse/cameras.txt, images.txt, points3D.txt — COLMAP text format

Methods

MASt3R

• Paper: MASt3R: Joint Image Matching for 3D Reconstruction
• Repo: https://github.com/naver/mast3r
• Model: naver/MASt3R_ViTLarge_BaseDecoder_512_catmlpdpt_metric
• License: CC BY-NC-SA 4.0 (non-commercial)

BrepGaussian

• Paper: BrepGaussian: CAD reconstruction from Multi-View Images with Gaussian Splatting (CVPR 2026)
• Repo: https://github.com/yjx2851/BrepGaussian

Mesh Extraction

• Poisson reconstruction: Open3D
• Ball pivoting: Open3D

Limitations

• 4-5 views is at the edge of feasibility; capture 8-12+ for production
• MASt3R confidence values are low for textureless surfaces (but geometry is valid)
• BrepGaussian adapted pipeline uses reduced iterations (2k+1k vs 30k+15k in paper)
• Custom CUDA rasterizer not available (no nvcc); pure-PyTorch renderer used instead

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Usage

This repository contains 3D data and scripts, not a loadable model — there are no weights and AutoModel will not work on it. Browse mast3r_results/, brepgaussian_brep_dense/ and brep_example_output/ for the geometry, and see METADOC.md for what each file is worth and how to view it.