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Hydra (closed-loop experiments only)

VLMaP uses Hydra only as a volumetric mapper: a TSDF with a top-k semantic integrator, fed posed depth and ground-truth labels. No scene-graph layers are used. The single file that touches it is langsensor/vlmap/hydra_map.py, which implements the VoxelMap interface in langsensor/vlmap/voxel_map.py; any other mapper that returns, per filled voxel, a TSDF distance and top-k label weights can replace it.

Nothing else in the repository needs Hydra: training, the static benchmark and rendering every figure and table work without it.

What is needed

The Python bindings (hydra_python) with a map.get_filled_semantic_voxels() method, which is not in upstream Hydra. It is on the branch feature/semantic_window_python of MIT-SPARK/Hydra; the paper's sweep used commit 8bd55289.

The bindings link against Hydra's C++ libraries, so build the ROS 2 workspace first. Versions contemporaneous with the sweep (last commit on or before 2026-05-14, unverified as a set):

Package Repository Commit
hydra MIT-SPARK/Hydra 8bd55289 (branch feature/semantic_window_python)
config_utilities MIT-SPARK/config_utilities 629688a
spark_dsg MIT-SPARK/Spark-DSG c6e7430
spatial_hash MIT-SPARK/Spatial-Hash b5be4d4
kimera_pgmo MIT-SPARK/Kimera-PGMO 8fcfd86
kimera_rpgo MIT-SPARK/Kimera-RPGO f1fee09
pose_graph_tools MIT-SPARK/pose_graph_tools 23b7876
teaser_plusplus MIT-SPARK/TEASER-plusplus 52a9c52
ianvs MIT-SPARK/Ianvs e76fc7c

(ROS 2 Jazzy, which also provides GTSAM.)

Build

  1. Set up a ROS 2 workspace for Hydra following the installation instructions of Hydra-ROS, then check out the commits in the table (Hydra on feature/semantic_window_python).

  2. Build it and put its libraries on the loader path:

    cd ~/hydra_ws && colcon build --cmake-args -DCMAKE_BUILD_TYPE=Release
source install/setup.bash

  3. Install the Python bindings (the Hydra repository root is a Python project) into this repository's environment, and check them:

    CMAKE_ARGS="-DCMAKE_PREFIX_PATH=$CMAKE_PREFIX_PATH" pip install ~/hydra_ws/src/hydra
python -c "from hydra_python._hydra_bindings import HydraReconstruction; print('ok')"

install/setup.bash must be sourced in every shell that runs langsensor.vlmap.run.

Troubleshooting

  • Could not find a package configuration file provided by hydra when building the bindings: pass the workspace prefix through CMAKE_ARGS as above; CMAKE_PREFIX_PATH alone does not reach pip's isolated build.
  • Segmentation fault on the first HydraReconstruction.step(): the bindings were built against a different libhydra.so than the one on the loader path (e.g. the workspace was rebuilt from another branch). Rebuild the workspace and the bindings from the same commits.
  • Cannot register already existent type ... errors at start-up are harmless; the paper's sweep printed them too.