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Three Ways to Improve Semantic Segmentation with Self-Supervised Depth Estimation

This is the official pytorch implementation of our paper Three Ways to Improve Semantic Segmentation with Self-Supervised Depth Estimation.

Training deep networks for semantic segmentation requires large amounts of labeled training data, which presents a major challenge in practice, as labeling segmentation masks is a highly labor-intensive process. To address this issue, we present a framework for semi-supervised semantic segmentation, which is enhanced by self-supervised monocular depth estimation from unlabeled images.

In particular, we propose three key contributions:

  1. We transfer knowledge from features learned during self-supervised depth estimation to semantic segmentation.
  2. We implement a strong data augmentation by blending images and labels using the structure of the scene.
  3. We utilize the depth feature diversity as well as the level of difficulty of learning depth in a student-teacher framework to select the most useful samples to be annotated for semantic segmentation.

We validate the proposed model on the Cityscapes dataset, where all three modules demonstrate significant performance gains, and we achieve state-of-the-art results for semi-supervised semantic segmentation.

Below, you can see the qualitative results of our model trained with only 100 annotated semantic segmentation samples.

example input output gif

If you find this code useful in your research, please consider citing:

@inproceedings{hoyer2021three,
  title={Three Ways to Improve Semantic Segmentation with Self-Supervised Depth Estimation},
  author={Hoyer, Lukas and Dai, Dengxin and Chen, Yuhua and Köring, Adrian and Saha, Suman and Van Gool, Luc},
  booktitle={IEEE Conference on Computer Vision and Pattern Recognition (CVPR)},
  year={2021}
}

Setup Environment

To install all requirements, you can run:

pip install -r requirements.txt -f https://download.pytorch.org/whl/torch_stable.html

Please download Cityscapes from https://www.cityscapes-dataset.com/downloads/. We require following packages: gtFine_trainvaltest.zip, leftImg8bit_trainvaltest.zip, and leftImg8bit_sequence_trainvaltest.zip. For performance reasons, we work on a downsampled copy of Cityscapes. Please refer to data_preprocessing/prepare_cityscapes.py for more information.

Before continuing, you should have following folder structure prepared:

CITYSCAPES_DIR/
- gtFine/
- leftImg8bit_small/
- leftImg8bit_sequence_small/

You can setup the paths for data and logging in the machine config configs/machine_confg.py. In the following, we assume that you have called your machine ws.

Pretrain Self-supervised Depth on Cityscapes

To run the two phases of the pretraining (first 300k iterations with frozen encoder and 50k iterations with ImageNet feature distance loss), you can execute:

python train.py --machine ws --config configs/cityscapes_monodepth_highres_dec5_crop.yml
python train.py --machine ws --config configs/cityscapes_monodepth_highres_dec6_crop.yml

For more information on the configuration, see the corresponding yaml files. The trained models are available on Google Drive. The download information is provided in models/utils.py.

Run Semi-Supervised Experiments

The semi-supervised experiments can be executed using:

python run_experiments.py --machine ws --exp EXP_ID

The EXP_ID corresponds to the experiment defined in experiments.py. Following experiments are relevant for the paper:

  • Experiment 210: All configurations that are only based on transfer learning.
  • Experiment 211: Unsupervised data selection for annotation configurations.
  • Experiment 212: Configurations that involve multi-task learning.

To use the labels selected by the unsupervised data selection for the other experiments, please copy the content of nlabelsXXX_subset.json from the log directory to loader/preselected_labels.py. For better reproducibility, we have stored our results there as well. Table 3 is generated using experiment 210 with config sel_{pres_method}_scratch.

Be aware that running all experiments takes multiple weeks on a single GPU. For that reason, we have commented out all but one subset size and seed as well as minor ablations.

Framework Structure

Experiments and Configurations
  • configs/machine_config.yml: Definition of data and log paths for different machines.
  • configs/cityscapes_monodepth*: Configurations for monodepth pretraining on Cityscapes.
  • configs/cityscapes_joint.yml: Base configuration for all semi-supervised segmentation experiments.
  • experiments.py: Generation of derivative configurations from cityscapes_joint.yml for the different experiments.
  • run_experiments.py: Execution of experiments defined in experiments.py.
Training Logic
  • train.py: Training script for a specific configuration. It contains the main training logic for self-supervised depth estimation, semi-supervised semantic segmentation, and DepthMix.
  • label_selection.py: Logic for unsupervised data selection for annotation.
  • monodepth_loss.py: Loss for self-supervised depth estimation.
Models
  • models/joint_segmentation_depth.py: Combined model for depth estimation, pose prediction, and semantic segmentation.
  • models/joint_segmentation_depth_decoder.py: Segmentation decoders for transfer learning from self-supervised depth and multi-task learning.
  • models/depth_decoder.py: Multi-scale depth decoder.
  • models/monodepth_layers.py: Operations necessary for self-supervised depth estimation.
Data Loading
  • loader/sequence_segmentation_loader.py: Base class for loading image sequences with segmentation labels.
  • loader/cityscapes_loader.py: Implementation for loading Cityscapes.
  • loader/depth_estimator.py: Generate depth estimates from pretrained self-supervised depth model and store them that they can be loaded by sequence_segmentation_loader as pseudo depth label.
  • loader/preselected_labels.py: A selection of annotated samples obtained with label_selection.py

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An implementation of our work "Three Ways to Improve Semantic Segmentation with Self-Supervised Depth Estimation"

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