Linking Modality Isolation in Heterogeneous Collaborative Perception.（CVPR2026）

Official implementation of Linking Modality Isolation in Heterogeneous Collaborative Perception (paper).

This paper studies modality isolation, a heterogeneous collaborative perception setting where agents with different modalities never co-occur in the same training frame.

CodeAlign links isolated modalities through feature-code-feature translation, aligning heterogeneous features without requiring cross-modal co-occurrence.

Environment

This repository follows the environment setup of HEAL. Please refer to the HEAL installation instructions for dependency installation, CUDA/PyTorch setup, and OpenCOOD-style package setup.

Dataset

Create a dataset/ folder under the repository root. The main experiments use OPV2V and DAIR-V2X-C:

CodeAlign/
└── dataset/
    ├── OPV2V/
    │   ├── train/
    │   ├── validate/
    │   └── test/
    └── my_dair_v2x/
        └── v2x_c/
            └── cooperative-vehicle-infrastructure/
                ├── train.json
                └── val.json

Dataset download references follow HEAL:

Dataset	Download
OPV2V	Follow OpenCOOD. HEAL additionally notes that additional-001.zip is needed for camera modality data.
DAIR-V2X-C	Download from the DAIR-V2X page, and follow the complemented annotation instructions.
OPV2V-H	Optional, available on Hugging Face.
V2XSet	Optional, follow V2X-ViT.
V2X-Sim 2.0	Optional, download from the V2X-Sim page and its accompanying pickle files.

Modality Setting

Modality definitions and scenario assignments are stored in:

opencood/modality_assign/

Important files include:

opencood/modality_assign/10modality_define.yaml
opencood/modality_assign/10modality_define_codebook16.yaml
opencood/modality_assign/10modality_define_dair.yaml
opencood/modality_assign/opv2v_4modality.json
opencood/modality_assign/opv2v_4modality_in_order.json

In the paper setting, m1 is PointPillar LiDAR, m2 is SECOND LiDAR, m3 is VoxelNet LiDAR, m4/m5 are LiDAR variants, and m6/m7 are LSS camera variants.

The opv2v_4modality*.json files assign vehicle IDs to scenario slots. For CodeAlign experiments, the paper-level modality semantics come from 10modality_define*.yaml, and each YAML's heter.mapping_dict maps those scenario slots to the intended paper modalities.

Training

Training is mainly controlled by YAML configs. Standard single-process training:

python opencood/tools/train.py -y ${CONFIG_FILE}

Distributed training:

CUDA_VISIBLE_DEVICES=0,1 python -m torch.distributed.launch \
  --nproc_per_node=2 --use_env \
  opencood/tools/train_ddp.py \
  -y ${CONFIG_FILE}

CodeAlign YAML templates are under:

opencood/hypes_yaml/opv2v/Heter_group/codebook/
opencood/hypes_yaml/dairv2x/Heter/

A CodeAlign YAML generally contains:

root_dir: dataset/OPV2V/train
validate_dir: dataset/OPV2V/validate
test_dir: dataset/OPV2V/test

yaml_parser: load_general_params_heter_group

heter:
  definition_path: opencood/modality_assign/10modality_define.yaml
  assignment_path: opencood/modality_assign/opv2v_4modality.json
  ego_modality: ['m7']
  heter_group: [['m7']]

fusion:
  core_method: intermediateheter

model:
  core_method: heter_codebook_shared_head_c2c
  args:
    only_train_translator: true
    backend_modality: ['m1', 'm6']
    codebook:
      seg_num: 1
      dict_size: 16
      r: 1
    translator:
      core_method: convnext

loss:
  core_method: point_pillar_pyramid_loss

optimizer:
  core_method: Adam

Key CodeAlign-specific parameters:

Parameter	Meaning
model.core_method	Selects the CodeAlign model, e.g. heter_codebook_shared_head, heter_codebook_shared_head_c2c, or heter_codebook_shared_head_c2c_infer.
heter.definition_path	Modality definition file.
heter.assignment_path	Vehicle-to-modality assignment for heterogeneous OPV2V evaluation/training.
heter.ego_modality / heter.heter_group	Defines source modalities used in the current training/evaluation stage.
fix_encoder / fix_backend	Freezes pretrained encoder/backbone or backend modules during codebook/alignment training.
aligner / aligner_model	Defines and optionally loads modality-specific aligners.
codebook	Controls codebook structure, including seg_num, dict_size, and r.
only_train_translator	Enables CodeAlign stage-2 translator-only training.
backend_modality / backend	Specifies target backend modalities and pretrained backend checkpoints.
translator	Defines the FCF translator architecture and feature/code shape.
use_coded_feature, use_codemap, use_d2d	Ablation switches for different translation/representation variants.

The typical CodeAlign training flow is:

Stage 0: single-modality pretraining
  train each modality-specific detector independently
  use the resulting single-modality checkpoints as frozen or initialized backends

Stage 1: code space / group formation
  heter_codebook_shared_head
  train aligner + codebook while keeping pretrained perception backend fixed

Stage 2: feature-code-feature translation
  heter_codebook_shared_head_c2c
  load source aligner and target backend/codebooks
  train translator only

Inference

Fixed-order heterogeneous inference, used for the main OPV2V/DAIR-V2X tables:

CUDA_VISIBLE_DEVICES=0 python opencood/tools/inference_heter_in_order.py \
  --model_dir opencood/logs/path_to_checkpoint \
  --fusion_method intermediate \
  --range 102.4,102.4 \
  --use_cav '[2]'

Late fusion:

CUDA_VISIBLE_DEVICES=0 python opencood/tools/inference_heter_in_order.py \
  --model_dir opencood/logs/path_to_late_checkpoint \
  --fusion_method late \
  --range 102.4,102.4 \
  --use_cav '[2]'

Pair-average heterogeneous inference:

CUDA_VISIBLE_DEVICES=0 python opencood/tools/inference_heter_task_average.py \
  --model_dir opencood/logs/path_to_checkpoint \
  --fusion_method intermediate \
  --range 102.4,102.4

Standard homogeneous or single-model inference:

CUDA_VISIBLE_DEVICES=0 python opencood/tools/inference.py \
  --model_dir opencood/logs/path_to_checkpoint \
  --fusion_method intermediate

Noise robustness for fixed-order heterogeneous inference can be evaluated with:

CUDA_VISIBLE_DEVICES=0 python opencood/tools/inference_heter_in_order.py \
  --model_dir opencood/logs/path_to_checkpoint \
  --fusion_method intermediate \
  --range 102.4,102.4 \
  --use_cav '[2]' \
  --noise 0.2

Acknowledgements

This implementation is based on code from several repositories. We especially thank HEAL for its heterogeneous collaborative perception codebase.

Citation

If you find this project useful, please cite:

@inproceedings{liu2026linking,
  title={Linking Modality Isolation in Heterogeneous Collaborative Perception},
  author={Liu, Changxing and Chao, Zichen and Chen, Siheng},
  booktitle={Proceedings of the IEEE/CVF Conference on Computer Vision and Pattern Recognition},
  year={2026}
}