An Exploration of Embodied Visual Exploration

This repository contains the code to run experiments from our IJCV publication:

An Exploration of Embodied Visual Exploration
Santhosh K. Ramakrishnan, Dinesh Jayaraman, Kristen Grauman
International Journal of Computer Vision, 2021

Abstract

Embodied computer vision considers perception for robots in general, unstructured environments. Of particular importance is the embodied visual exploration problem: how might a robot equipped with a camera scope out a new environment? Despite the progress thus far, many basic questions pertinent to this problem remain unanswered: (i) What does it mean for an agent to explore its environment well? (ii) Which methods work well, and under which assumptions and environmental settings? (iii) Where do current approaches fall short, and where might future work seek to improve? Seeking answers to these questions, we perform a thorough empirical study of four state-of-the-art paradigms on two photorealistic simulated 3D environments. We present a taxonomy of key exploration methods and a standard framework for benchmarking visual exploration algorithms. Our experimental results offer insights, and suggest new performance metrics and baselines for future work in visual exploration.

Simulation environments

This codebase supports experiments on Active Vision dataset and Matterport3D. The corresponding simulators can be installed from the following directories:

• Active Vision simulator: environments/gym-avd
• Matterport3D simulator (via habitat-api): environments/habitat

Installation instructions

1. Clone this github repository and add to path.

cd exploring_exploration
export PYTHONPATH=$PWD:$PYTHONPATH
export EXPLORING_EXPLORATION=$PWD

2. Install the 3D simulation environments. Instructions can be found here:

   • Active Vision simulator - $EXPLORING_EXPLORATION/environments/gym-avd/README.md
   • Habitat simulator - $EXPLORING_EXPLORATION/environments/habitat/README.md

3. Install other dependencies.

cd $EXPLORING_EXPLORATION
pip install requirements.txt

4. Install OpenAI baselines

cd $EXPLORING_EXPLORATION
git clone [email protected]:openai/baselines.git
cd baselines
git checkout 7bfbcf177eca8f46c0c0bfbb378e044539f5e061
pip install -e .
cp ../baselines.patch .
git apply baselines.patch

5. Install astar_pycpp for fast planning.

cd $EXPLORING_EXPLORATION/exploring_exploration/models/
git clone [email protected]:srama2512/astar_pycpp.git
cd astar_pycpp
git checkout exploration_study
make

Downloading pre-trained models

We provide pre-trained models for different baselines and paradigms on both AVD and MP3D.

cd $EXPLORING_EXPLORATION
mkdir pretrained_models
cd pretrained_models
wget https://dl.fbaipublicfiles.com/exploring-exploration/avd_pretrained_models.tar.gz
wget https://dl.fbaipublicfiles.com/exploring-exploration/mp3d_pretrained_models.tar.gz

Evaluating on visitation metrics

We evaluate exploration using three visitation metrics: the amount of area / landmarks / objects visited during exploration. The evaluate_visitation.py script evaluates performance on the visitation metrics.

Evaluation on AVD

cd $EXPLORING_EXPLORATION
export model_path=<PATH TO MODEL>

python -W ignore evaluate_visitation.py \
     --seed 123 \
     --num-steps 200 \
     --env-name avd-pose-v0 \
     --eval-split test \
     --num-processes 16 \
     --num-pose-refs 10 \
     --load-path $model_path \
     --eval-episodes 100 \
     --interval_steps 200 \
     --actor-type learned_policy \
     --visualize-policy False \
     --log-dir visitation_results

Evaluation on MP3D

export GLOG_minloglevel=2
export MAGNUM_LOG=quiet

cd $EXPLORING_EXPLORATION
export model_path=<PATH TO MODEL>

python -W ignore evaluate_visitation.py \
     --seed 123 \
     --num-steps 1000 \
     --env-name habitat \
     --eval-split test \
     --num-processes 1 \
     --num-pose-refs 20 \
     --load-path $model_path \
     --eval-episodes 10 \
     --interval_steps 1000 \
     --actor-type learned_policy \
     --habitat-config-file configs/exploration/ppo_pose_test.yaml \
     --visualize-policy False \
     --log-dir visitation_results

Here, the metrics are computed after exploring for 1000 steps on each episode. The average results over all episodes are logged in visitation_results/eval_log.txt. Detailed statistics on a per-episode basis are stored in visitation_results/statistics.json.

Note

• Performance can be measured at intermediate time-steps by setting the appropriate values for --interval-steps. For example, setting --interval-steps 50 100 150 200 for AVD would additionally record the performance at 50, 100, 150 and 200 steps.
• The number of evaluation episodes can be controlled by varying --eval-episodes. Evaluating on MP3D takes ~0.6 minutes per episode on a Quadro GP100. Evaluating on the full test set (1000 episodes) would take ~12 hours.

Evaluating on view localization

The view localization task requires the agent to estimate the pose of a set of reference views in the environment using the information gathered during exploration. See the supplementary for more details. We provide pre-trained pose estimation models for AVD and MP3D which can be used for evaluation in $EXPLORING_EXPLORATION/pretrained_models/*/pose_estimation_nets.

Evaluation on AVD

cd $EXPLORING_EXPLORATION
export model_path=<PATH TO MODEL>
export retrieval_net_path=<PATH TO RETRIEVAL NET>
export pairwise_pose_net_path=<PATH TO PAIRWISE POSE PREDICTOR>

python -W ignore evaluate_pose_estimation.py \
     --num-steps 200 \
     --map-size 31 \
     --env-name avd-pose-v0 \
     --map-scale 500.0 \
     --vote-kernel-size 3 \
     --num-processes 16 \
     --num-pose-refs 10 \
     --load-path $model_path \
     --eval-split test \
     --seed 123 \
     --pretrained-rnet $retrieval_net_path \
     --pretrained-posenet $pairwise_pose_net_path \
     --eval-episodes 100 \
     --interval_steps 200 \
     --actor-type learned_policy \
     --pose-predictor-type ransac \
     --ransac-niter 10 \
     --log-dir pose_estimation_results \
     --visualize-policy False

Evaluation on MP3D

export GLOG_minloglevel=2
export MAGNUM_LOG=quiet

cd $EXPLORING_EXPLORATION
export model_path=<PATH TO MODEL>
export retrieval_net_path=<PATH TO RETRIEVAL NET>
export pairwise_pose_net_path=<PATH TO PAIRWISE POSE PREDICTOR>

python -W ignore evaluate_pose_estimation.py \
     --num-steps 1000 \
     --map-size 101 \
     --env-name habitat \
     --map-scale 0.5 \
     --vote-kernel-size 5 \
     --num-processes 1 \
     --num-pose-refs 20 \
     --load-path $model_path \
     --eval-split test \
     --seed 123 \
     --pretrained-rnet $retrieval_net_path \
     --pretrained-posenet $pairwise_pose_net_path \
     --eval-episodes 10 \
     --interval_steps 1000 \
     --actor-type learned_policy \
     --pose-predictor-type ransac \
     --ransac-niter 10 \
     --use-classification True \
     --num-classes 15 \
     --visualize-policy False \
     --use-multi-gpu True \
     --habitat-config-file configs/pose_estimation/ppo_pose_test.yaml \
     --log-dir pose_estimation_results

Note

• Evaluating on MP3D takes ~1 minute per episode on a Quadro GP100. Evaluating on the full test set (1000 episodes) would take ~16 hours.

Evaluating on reconstruction

The reconstruction task requires the agent to accurately reconstruct the concepts present at a set of reference poses in the environment. See the supplementary. We provide pre-trained reconstruction task-heads for AVD and MP3D which can be used for evaluation in $EXPLORING_EXPLORATION/pretrained_models/*/pretrained_reconstruction/ckpt.pth. We also provide the concept clusters extracted for both datasets here.

Evaluation on AVD

cd $EXPLORING_EXPLORATION
export model_path=<PATH TO MODEL>
export reconstruction_head_path=<PATH TO RECONSTRUCTION HEAD MODEL>
export clusters_path=reconstruction_data_generation/avd/imagenet_clusters/clusters_00030_data.h5

python -W ignore evaluate_reconstruction.py \
     --num-steps 200 \
     --env-name avd-recon-v0 \
     --load-path $model_path \
     --num-processes 16 \
     --seed 123 \
     --num-pose-refs 50 \
     --eval-split test \
     --clusters-path $clusters_path \
     --n-transformer-layers 2 \
     --load-path-rec $reconstruction_head_path \
     --eval-episodes 100 \
     --interval_steps 200 \
     --actor-type learned_policy \
     --visualize-policy False \
     --log-dir reconstruction_results

Evaluation on MP3D

export GLOG_minloglevel=2
export MAGNUM_LOG=quiet

cd $EXPLORING_EXPLORATION
export model_path=<PATH TO MODEL>
export reconstruction_head_path=<PATH TO RECONSTRUCTION HEAD MODEL>
export clusters_path=reconstruction_data_generation/mp3d/imagenet_clusters/clusters_00050_data.h5

python -W ignore evaluate_reconstruction.py \
     --num-steps 1000 \
     --env-name habitat \
     --load-path $model_path \
     --num-processes 1 \
     --seed 123 \
     --num-pose-refs 100 \
     --eval-split test \
     --clusters-path $clusters_path \
     --n-transformer-layers 2 \
     --load-path-rec $reconstruction_head_path \
     --eval-episodes 10 \
     --interval_steps 1000 \
     --actor-type learned_policy \
     --visualize-policy False \
     --habitat-config-file configs/reconstruction_exploration/ppo_pose_test.yaml \
     --use-multi-gpu True \
     --log-dir reconstruction_results

Note

• Evaluating on MP3D takes ~0.8 minute per episode on a Quadro GP100. Evaluating on the full test set (1000 episodes) would take ~13 hours.

Training on Active Vision simulator

Imitation learning pre-training

A simple exploration policy can be trained by imitating an oracle agent that sequentially visits a set of pre-defined locations in the environment using the shortest paths.

cd $EXPLORING_EXPLORATION
mkdir -p trained_models/imitation_learning/avd

python -W ignore -u pretrain_imitation.py  \
     --lr 1e-4 \
     --seed 123 \
     --num-processes 16 \
     --num-steps 200 \
     --num-rl-steps 50 \
     --env-name avd-pose-landmarks-oracle-v0 \
     --save-interval 100 \
     --eval-interval 100 \
     --num-episodes 16000 \
     --save-dir trained_models/imitation_learning/avd/ \
     --log-dir trained_models/imitation_learning/avd/ \
     --agent-start-action-prob 0.0 \
     --agent-end-action-prob 0.3 \
     --agent-action-prob-schedule 100 \
     --agent-action-prob-factor 0.1 \
     --use-inflection-weighting True

Different environments correspond to different oracles:

• oracle-random: set env-name to avd-pose-random-oracle-v0
• oracle-landmarks: set env-name to avd-pose-landmarks-oracle-v0
• oracle-objects: set env-name to avd-pose-objects-oracle-v0

In practice, we find that oracle-landmarks performs well across most metrics.

Coverage + Novelty exploration training

For training the coverage and novelty agents, we use the train_exploration.py script. For example, the command to train an area-coverage agent:

cd $EXPLORING_EXPLORATION
mkdir -p trained_models/area_coverage/avd

python -W ignore -u train_exploration.py \
     --lr 3e-5 \
     --seed 123 \
     --num-processes 32 \
     --num-steps 200 \
     --num-rl-steps 50 \
     --env-name avd-pose-landmarks-oracle-v0 \
     --save-interval 100 \
     --eval-interval 100 \
     --num-episodes 64000 \
     --save-dir trained_models/area_coverage/avd \
     --log-dir trained_models/area_coverage/avd \
     --pretrained-il-model '' \
     --use-gae \
     --ppo-epoch 4 \
     --num-mini-batch 4 \
     --area-reward-scale 0.3 \
     --smooth-coverage-reward-scale 0.0 \
     --novelty-reward-scale 0.0

Smooth coverage and novelty agents can be trained by setting the corresponding reward coefficients to a non-zero value and zeroing out the rest.

Curiosity-based exploration training

For training the curiosity agent, we use the train_curiosity_exploration.py script:

cd $EXPLORING_EXPLORATION
mkdir -p trained_models/curiosity/avd

python -W ignore -u train_curiosity_exploration.py \
     --lr 1e-4 \
     --seed 123 \
     --num-processes 32 \
     --num-steps 200 \
     --num-rl-steps 50 \
     --save-interval 100 \
     --eval-interval 100 \
     --num-episodes 64000 \
     --env-name avd-pose-landmarks-oracle-v0 \
     --save-dir trained_models/curiosity/avd \
     --log-dir trained_models/curiosity/avd \
     --pretrained-il-model '' \
     --use-gae \
     --ppo-epoch 4 \
     --num-mini-batch 4 \
     --reward-scale 1e-3 \
     --icm-embedding-type policy-lstm \
     --normalize-icm-rewards True

Reconstruction-based exploration training

For training the reconstruction agent, there are two phases. The first phase is a pre-training of the reconstruction task-head.

cd $EXPLORING_EXPLORATION
mkdir -p trained_models/reconstruction/avd/pretraining

python -W ignore pretrain_reconstruction.py \
     --lr 1e-4 \
     --seed 123 \
     --num-processes 16 \
     --num-steps 200 \
     --num-rl-steps 200 \
     --env-name avd-recon-v0 \
     --save-interval 100 \
     --eval-interval 100 \
     --num-episodes 64000 \
     --save-dir trained_models/reconstruction/avd/pretraining \
     --log-dir trained_models/reconstruction/avd/pretraining \
     --num-pose-refs 50 \
     --clusters-path reconstruction_data_generation/avd/imagenet_clusters/clusters_00030_data.h5 \
     --rec-loss-fn-J 3 \
     --n-transformer-layers 2

For convenience, we provide a pre-trained reconstruction task-head in $EXPLORING_EXPLORATION/pretrained_models/avd/pretrained_reconstruction/ckpt.pth. The second phase is the training of the exploration policy. The exploration agent is rewarded for maximizing the reconstruction performance. Note, we keep the pre-trained reconstruction task-head frozen for this stage.

cd $EXPLORING_EXPLORATION
mkdir -p trained_models/reconstruction/avd/exploration_policy

python -W ignore train_reconstruction_exploration.py \
     --lr 3e-5 \
     --seed 123 \
     --num-processes 32 \
     --num-steps 200 \
     --num-rl-steps 50 \
     --env-name avd-recon-v0 \
     --save-interval 100 \
     --eval-interval 5 \
     --num-episodes 64000 \
     --save-dir trained_models/reconstruction/avd/exploration_policy \
     --log-dir trained_models/reconstruction/avd/exploration_policy \
     --load-path-rec pretrained_models/avd/pretrained_reconstruction/ckpt.pth \
     --pretrained-il-model '' \
     --num-pose-refs 50 \
     --use-gae \
     --ppo-epoch 4 \
     --num-mini-batch 4 \
     --rec-reward-scale 1e-1 \
     --clusters-path reconstruction_data_generation/avd/imagenet_clusters/clusters_00030_data.h5 \
     --n-transformer-layers 2 \
     --rec-reward-interval 1

Training on Habitat simulator

Imitation learning pre-training

Command to train an exploration policy by imitating an oracle shortest-path follower:

export GLOG_minloglevel=2
export MAGNUM_LOG=quiet

cd $EXPLORING_EXPLORATION
mkdir -p trained_models/imitation_learning/mp3d

python -W ignore -u pretrain_imitation.py  \
     --lr 1e-4 \
     --seed 123 \
     --num-processes 8 \
     --num-steps 500 \
     --num-rl-steps 100 \
     --env-name habitat \
     --save-interval 200 \
     --eval-interval 200 \
     --save-unique True \
     --num-episodes 16000 \
     --save-dir trained_models/imitation_learning/mp3d/ \
     --log-dir trained_models/imitation_learning/mp3d/ \
     --habitat-config-file configs/pretrain_imitation/ppo_pose_train_oracle_random.yaml \
     --eval-habitat-config-file configs/exploration/ppo_pose_val.yaml \
     --agent-start-action-prob 0.0 \
     --agent-end-action-prob 0.5 \
     --agent-action-prob-schedule 1000 \
     --agent-action-prob-factor 0.1 \
     --agent-action-duration 1 \
     --use-inflection-weighting True

Different oracles can be selected by varying the ORACLE_TYPE variable in the configuration file. The following config files correspond to different oracles:

• oracle-random: configs/pretrain_imitation/ppo_pose_train_oracle_random.yaml
• oracle-landmarks: configs/pretrain_imitation/ppo_pose_train_oracle_landmarks.yaml
• oracle-objects: configs/pretrain_imitation/ppo_pose_train_oracle_objects.yaml

Coverage + Novelty exploration training

For training the coverage and novelty agents, we use the train_exploration.py script. For example, the command to train an area-coverage agent:

export GLOG_minloglevel=2
export MAGNUM_LOG=quiet

cd $EXPLORING_EXPLORATION
mkdir -p trained_models/area_coverage/mp3d

python -W ignore -u train_exploration.py \
     --lr 1e-1 \
     --seed 123 \
     --num-processes 8 \
     --num-steps 500 \
     --num-rl-steps 100 \
     --env-name habitat \
     --save-interval 200 \
     --eval-interval 200 \
     --num-episodes 16000 \
     --save-unique True \
     --save-dir trained_models/area_coverage/mp3d \
     --log-dir trained_models/area_coverage/mp3d \
     --habitat-config-file configs/exploration/ppo_pose_train.yaml \
     --eval-habitat-config-file configs/exploration/ppo_pose_val.yaml \
     --pretrained-il-model '' \
     --use-gae \
     --ppo-epoch 4 \
     --num-mini-batch 2 \
     --area-reward-scale 1e-3 \
     --smooth-coverage-reward-scale 0.0 \
     --novelty-reward-scale 0.0

Smooth coverage and novelty agents can be trained by setting the corresponding reward coefficients to a non-zero value and zeroing out the rest.

Curiosity-based exploration training

For training the curiosity agent, we use the train_curiosity_exploration.py script:

export GLOG_minloglevel=2
export MAGNUM_LOG=quiet

cd $EXPLORING_EXPLORATION
mkdir -p trained_models/curiosity/mp3d

python -W ignore train_curiosity_exploration.py \
     --lr 1e-4 \
     --seed 123 \
     --num-processes 8 \
     --num-steps 500 \
     --num-rl-steps 100 \
     --env-name habitat \
     --save-interval 200 \
     --eval-interval 200 \
     --num-episodes 16000 \
     --save-unique True \
     --save-dir trained_models/curiosity/mp3d \
     --log-dir trained_models/curiosity/mp3d \
     --habitat-config-file configs/exploration/ppo_pose_train.yaml \
     --eval-habitat-config-file configs/exploration/ppo_pose_val.yaml \
     --pretrained-il-model '' \
     --use-gae \
     --ppo-epoch 4 \
     --num-mini-batch 2 \
     --reward-scale 1e-3 \
     --icm-embedding-type policy-lstm \
     --normalize-icm-rewards True

Reconstruction-based exploration training

For training the reconstruction agent, there are two phases. The first phase is a pre-training of the reconstruction task-head.

export GLOG_minloglevel=2
export MAGNUM_LOG=quiet

cd $EXPLORING_EXPLORATION
mkdir -p trained_models/reconstruction/mp3d/pretraining

python pretrain_reconstruction.py \
     --lr 3e-5 \
     --seed 123 \
     --num-processes 8 \
     --num-steps 500 \
     --num-rl-steps 500 \
     --env-name habitat \
     --save-interval 200 \
     --eval-interval 200 \
     --save-unique True \
     --num-episodes 16000 \
     --save-dir trained_models/reconstruction/mp3d/pretraining \
     --log-dir trained_models/reconstruction/mp3d/pretraining \
     --habitat-config-file configs/pretrain_reconstruction/ppo_pose_train.yaml \
     --eval-habitat-config-file configs/pretrain_reconstruction/ppo_pose_val.yaml \
     --num-pose-refs 100 \
     --clusters-path reconstruction_data_generation/mp3d/imagenet_clusters/clusters_00050_data.h5 \
     --rec-loss-fn-K 3 \
     --n-transformer-layers 2 \
     --use-multi-gpu True

For convenience, we provide a pre-trained reconstruction task-head in $EXPLORING_EXPLORATION/pretrained_models/mp3d/pretrained_reconstruction/ckpt.pth. The second phase is the training of the exploration policy. The exploration agent is rewarded for maximizing the reconstruction performance given a frozen, pre-trained reconstruction task-head.

export GLOG_minloglevel=2
export MAGNUM_LOG=quiet

cd $EXPLORING_EXPLORATION
mkdir -p trained_models/reconstruction/mp3d/exploration_policy

python train_reconstruction_exploration.py \
     --lr 1e-5 \
     --seed 123 \
     --num-processes 8 \
     --num-steps 500 \
     --num-rl-steps 100 \
     --env-name habitat \
     --habitat-config-file configs/reconstruction_exploration/ppo_pose_train.yaml \
     --eval-habitat-config-file configs/reconstruction_exploration/ppo_pose_val.yaml \
     --save-interval 200 \
     --eval-interval 200 \
     --save-unique True \
     --num-episodes 16000 \
     --save-dir trained_models/reconstruction/mp3d/exploration_policy \
     --log-dir trained_models/reconstruction/mp3d/exploration_policy \
     --load-path-rec pretrained_models/mp3d/pretrained_reconsturction/ckpt.pth \
     --pretrained-il-model '' \
     --num-pose-refs 100 \
     --use-gae \
     --ppo-epoch 4 \
     --num-mini-batch 2 \
     --rec-reward-scale 1.0 \
     --clusters-path reconstruction_data_generation/mp3d/imagenet_clusters/clusters_00050_data.h5 \
     --n-transformer-layers 2 \
     --rec-reward-interval 5

Data generation for reconstruction task

For training reconstruction-based exploration agents and evaluating on the reconstruction task, we need to first extract image clusters that represent concepts.

Concepts generation for AVD

1. Uniformly sample images from all environments.

cd $EXPLORING_EXPLORATION/reconstruction_data_generation/avd
python -W ignore gather_uniform_points.py

2. Extract imagenet features for the images, cluster them and save cluster statistics.

python generate_imagenet_clusters.py \
        --dataset-root avd/uniform_samples \
        --num-clusters 30 \
        --save-dir avd/imagenet_clusters

Concepts generation for MP3D

1. Uniformly sample images from all environments.

cd $EXPLORING_EXPLORATION/reconstruction_data_generation/mp3d
chmod +x extract_data_script.sh && ./extract_data_script.sh

2. Extract imagenet features for the images, cluster them and save cluster statistics.

python generate_imagenet_clusters.py \
        --image-size 84 \
        --dataset-root mp3d/uniform_samples \
        --num-clusters 50 \
        --save-dir mp3d/imagenet_clusters

Note

• The clusters can be visualized as follows:

Concepts generation for AVD

1. Uniformly sample images from all environments.

cd $EXPLORING_EXPLORATION/reconstruction_data_generation/avd
python -W ignore gather_uniform_points.py

2. Extract imagenet features for the images, cluster them and save cluster statistics.

python generate_imagenet_clusters.py \
        --dataset-root avd/uniform_samples \
        --num-clusters 30 \
        --save-dir avd/imagenet_clusters

Concepts generation for MP3D

1. Uniformly sample images from all environments.

cd $EXPLORING_EXPLORATION/reconstruction_data_generation/mp3d
chmod +x extract_data_script.sh && ./extract_data_script.sh

2. Extract imagenet features for the images, cluster them and save cluster statistics.

python generate_imagenet_clusters.py \
        --image-size 84 \
        --dataset-root mp3d/uniform_samples \
        --num-clusters 50 \
        --save-dir mp3d/imagenet_clusters

Note

• The clusters can be visualized as follows:

  # For AVD
  cd $EXPLORING_EXPLORATION/reconstruction_data_generation/avd/imagenet_clusters
  tensorboard --logdir=.
  
  # For MP3D
  cd $EXPLORING_EXPLORATION/reconstruction_data_generation/mp3d/imagenet_clusters
  tensorboard --logdir=.
  

• To ensure reproducibility, we have provided the clusters we generated here. Copying them to reconstruction_data_generation/avd/imagenet_clusters/clusters_00030_data.h5 and reconstruction_data_generation/mp3d/imagenet_clusters/clusters_00030_data.h5 will ensure that the script re-uses the same clusters for generating the visualizations.

Acknowledgements

The training code is based on the PyTorch RL library by Ilya Kostrikov. The Matterport3D simulation relies heavily on habitat-lab and habitat-sim. The Active Vision simulation borrows ideas from the Vision-and-Language simulator.

Citation

@article {ramakrishnan2021exploration,
    author = {Ramakrishnan, Santhosh K. and Jayaraman, Dinesh and Grauman, Kristen},
    title = {An Exploration of Embodied Visual Exploration},
    year = {2021},
    doi = {10.1007/s11263-021-01437-z},
    URL = {https://doi.org/10.1007/s11263-021-01437-z},
    journal = {International Journal of Computer Vision}
}

License

This project is released under the CC-BY-NC 4.0 license, as found in the LICENSE file.