Seeing without Pixels: Perception from Camera Trajectories

Seeing without Pixels: Perception from Camera Trajectories

Zihui Xue, Kristen Grauman, Dima Damen, Andrew Zisserman, Tengda Han

CVPR 2026

Project page | arXiv | Data and checkpoints | Citation

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Can we understand a video from camera motion alone, without seeing any pixels?

Our paper shows that camera trajectories carry surprisingly rich semantic information. CamFormer takes only the camera's path through 3D space and learns to align it with natural-language descriptions, making it possible to retrieve what is happening in a video from motion alone.

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This repository contains the released code for CamFormer pretraining and 5-way multiple-choice retrieval evaluation on:

• Ego-Exo4D, using egocentric Aria camera trajectories
• DynPose-100K, using exocentric camera trajectories
• Nymeria, for egocentric zero-shot transfer

The code operates on camera-pose trajectories only; no video frames are used.

Method

CamFormer is a four-layer Transformer over camera poses. It maps a pose sequence to a single motion embedding and trains it against the frozen CLIP text encoder with a contrastive trajectory-text loss. For egocentric data, the model can encode a longer temporal context around each labeled action, then pool only the labeled sub-window. This helps disambiguate short or visually sparse camera motions.

Setup

Create the conda environment:

conda env create -f environment.yml
conda activate camformer

Or install into an existing Python 3.9+ environment:

pip install -r requirements.txt

The CLIP text encoder is installed from GitHub, so git must be available. If the pinned PyTorch wheels do not match your CUDA version, install torch and torchvision from pytorch.org first, then install the rest of requirements.txt.

Data

The metadata CSVs are included in data_files/. Large derived artifacts are hosted separately:

• precomputed retrieval features, for reproducing results without a GPU
• pretrained CamFormer checkpoints
• camera-pose trajectory archives for training or checkpoint evaluation

See DATA.md for the download link, archive list, expected directory layout, and environment variables.

Fastest Reproduction

To reproduce retrieval numbers without downloading trajectories, download camformer_retrieval_features.zip from DATA.md, unzip it in this repository, and run:

python eval_retrieval.py retrieval_features/egoexo4d
python eval_retrieval.py retrieval_features/dynpose_original
python eval_retrieval.py retrieval_features/dynpose_vipe
python eval_retrieval.py retrieval_features/nymeria_a
python eval_retrieval.py retrieval_features/nymeria_b
python eval_retrieval.py retrieval_features/nymeria_c
python eval_retrieval.py retrieval_features/nymeria_d

The main metric is Motion->Text MCQ acc.

Evaluation From Checkpoints

To run the model yourself, download camformer_checkpoints.zip and the pose archive for the dataset you want to evaluate. Set the environment variables described in DATA.md, then run one of the commands below.

Ego-Exo4D, using Aria ground-truth poses:

python train.py --dataset egoexo4d_pretrain_longseq --test --scenario all \
    --pose_encoding rel9d_grav --take_duration 8 --sample_dur \
    --num_gpus 1 --batch_size 1000 \
    --init_ckpt checkpoints/egoexo4d_dur8.pt

DynPose-100K, using original dataset poses:

python train.py --dataset dynpose_pretrain --test \
    --pose_source original --pose_encoding rel9d \
    --num_gpus 1 --batch_size 1000 \
    --init_ckpt checkpoints/dynpose100k_original.pt

DynPose-100K, using ViPE-estimated poses:

python train.py --dataset dynpose_pretrain --test \
    --pose_source vipe --pose_encoding rel9d \
    --num_gpus 1 --batch_size 1000 \
    --init_ckpt checkpoints/dynpose100k_vipe.pt

Nymeria zero-shot transfer, using the Ego-Exo4D long-context checkpoint:

python train.py --dataset nymeria_pretrain --test \
    --text_column a --pose_encoding rel9d_grav \
    --num_gpus 1 --batch_size 1000 \
    --init_ckpt checkpoints/egoexo4d_dur16.pt

For Nymeria, --text_column selects the narration type:

• a: body posture
• b: hands and arms motion
• c: legs and feet motion
• d: focus of attention

Each --test run prints the directory where it saved frames.pt and text.pt. Pass that directory to eval_retrieval.py.

Pretraining

Training requires the corresponding pose archives from DATA.md. The common settings are:

• --pose_encoding rel9d_grav: relative 9D pose plus gravity in camera coordinates, used for egocentric checkpoints
• --pose_encoding rel9d: relative 9D pose without gravity, used for DynPose-100K
• --take_duration: number of seconds of context around each labeled action
• --sample_dur: randomly vary the context duration during training
• --pose_source: DynPose-100K pose source, either original or vipe
• --use_pi3_pose: use Pi3-estimated Ego-Exo4D poses instead of Aria ground truth

Ego-Exo4D, Aria ground-truth poses:

python train.py --dataset egoexo4d_pretrain_longseq \
    --pose_encoding rel9d_grav --take_duration 8 --sample_dur

Ego-Exo4D, Pi3-estimated poses:

python train.py --dataset egoexo4d_pretrain_longseq \
    --use_pi3_pose --pose_encoding rel9d_grav \
    --take_duration 8 --sample_dur

DynPose-100K, original poses:

python train.py --dataset dynpose_pretrain \
    --pose_source original --pose_encoding rel9d

DynPose-100K, ViPE-estimated poses:

python train.py --dataset dynpose_pretrain \
    --pose_source vipe --pose_encoding rel9d

Training writes logs under ~/data/logs/<dataset>/<job_name>/. W&B logging is enabled by default; run wandb login first, or set WANDB_MODE=offline if you want local-only runs.

Data Preparation

The released metadata and pose archives are enough for training and evaluation. If you want to rebuild the Ego-Exo4D long-sequence files from the raw Ego-Exo4D release, place the raw data under ~/data/egoexo4d and run:

python -m data_prep.step1_extract_segments --mode train
python -m data_prep.step2_clean --mode train
python -m data_prep.step3_build_longseq --mode train --duration 30

python -m data_prep.step1_extract_segments --mode val
python -m data_prep.step2_clean --mode val
python -m data_prep.step3_build_longseq --mode val --duration 30

Then set EGOEXO4D_PRETRAIN_TRAJ_DIR to the generated camera_motion_cache/egoexo4d_pretrain directory.

Citation

If you use this code or data release, please cite:

@inproceedings{xue2026seeing,
  title={Seeing without Pixels: Perception from Camera Trajectories},
  author={Xue, Zihui and Grauman, Kristen and Damen, Dima and Zisserman, Andrew and Han, Tengda},
  booktitle={Proceedings of the IEEE/CVF Conference on Computer Vision and Pattern Recognition (CVPR)},
  year={2026}
}