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An Official Repo of CVPR '20 "MSeg: A Composite Dataset for Multi-Domain Segmentation"

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This is the code for the paper:

MSeg: A Composite Dataset for Multi-domain Semantic Segmentation (CVPR 2020, Official Repo) [PDF]
John Lambert*, Zhuang Liu*, Ozan Sener, James Hays, Vladlen Koltun
Presented at CVPR 2020. Link to MSeg Video (3min.)

This repo is the first of 4 repos that introduce our work. It provides utilities to download the MSeg dataset (which is nontrivial), and prepare the data on disk in a unified taxonomy.

Three additional repos are also provided: -

: provides HRNet-W48 Training (sufficient to train a winning entry on the WildDash benchmark) -

: provides Panoptic-FPN and Mask-RCNN training, based on Detectron2 (will be introduced in January 2021) -
: utilities to perform large-scale Mechanical Turk re-labeling

Install the MSeg module:

  • mseg
    can be installed as a python package using
    pip install -e /path_to_root_directory_of_the_repo/

Make sure that you can run

import mseg
in python, and you are good to go!

Download MSeg

The MSeg Taxonomy

We provide comprehensive class definitions and examples here. We provide here a master spreadsheet mapping all training datasets to the MSeg Taxonomy, and the MSeg Taxonomy to test datasets. Please consult to learn what each of the dataset taxonomy names means.

Citing MSeg

If you find this code useful for your research, please cite:

author = {Lambert, John and Liu, Zhuang and Sener, Ozan and Hays, James and Koltun, Vladlen},
title = {{MSeg}: A Composite Dataset for Multi-domain Semantic Segmentation},
booktitle = {Computer Vision and Pattern Recognition (CVPR)},
year = {2020}

Repo Structure

  • download_scripts
    : code and instructions to download the entire MSeg dataset
  • mseg
    : Python module, including
    • dataset_apis
    • dataset_lists
      : ordered classnames for each dataset, and corresponding relative rgb/label file paths
    • label_preparation
      : code for remapping to
      format, and for relabeling masks in place
    • relabeled_data
      : MSeg data, annotated by Mechanical Turk workers, and verified by co-authors
    • taxonomy
      : on-the-fly mapping to a unified taxonomy during training, and linear mapping to evaluation taxonomies
    • utils
      : library functions for mask and image manipulation, filesystem, tsv/csv reading, and multiprocessing
  • tests
    : unit tests on all code

Data License

Creative Commons License
This work is licensed under a Creative Commons Attribution 4.0 International License.

Frequently Asked Questions (FAQ)

Q: Do the weights include the model structure or it's just the weights? If the latter, which model do these weights refer to? Under the

directory, there are several model implementations.

A: The pre-trained models follow the HRNet-W48 architecture. The model structure is defined in the code here. The saved weights provide a dictionary between keys (unique IDs for each weight identifying the corresponding layer/layer type) and values (the floating point weights).

Q: How is testing performed on the test datasets? In the paper you talk about "zero-shot transfer" -- how this is performed? Are the test dataset labels also mapped or included in the unified taxonomy? If you remapped the test dataset labels to the unified taxonomy, are the reported results the performances on the unified label space, or on each test dataset's original label space? How did you you obtain results on the WildDash dataset - which is evaluated by the server - when the MSeg taxonomy may be different from the WildDash dataset.

A: Regarding "zero-shot transfer", please refer to section "Using the MSeg taxonomy on a held-out dataset" on page 6 of our paper. This section describes how we hand-specify mappings from the unified taxonomy to each test dataset's taxonomy as a linear mapping (implemented here in mseg-api). All results are in the test dataset's original label space (i.e. if WildDash expects class indices in the range [0,18] per our names_list, our testing script uses the

functionality to produce indices in that range, remapping probabilities.

Q: Why don't indices in

match the training indices in individual datasets? For example, for the road class: In idd-39, road has index 0, but in idd-39-relabeled, road has index 19. It is index 7 in cityscapes-34. The cityscapes-19-relabeled index road is 11. As far as I can tell, ultimately the 'MSeg_Master.tsv' file provides the final mapping to the MSeg label space. But here, the road class seems to have an index of 98, which is neither 19 nor 11.

A: Indeed, unified taxonomy class index 98 represents "road". But we use the TaxonomyConverter to accomplish the mapping on the fly from idd-39-relabeled to the unified/universal taxonomy (we use the terms "unified" and "universal" interchangeably). This is done by adding a transform in the training loop that calls

on the fly. You can see how that transform is implemented here in


Q: When testing, but there are test classes that are not in the unified taxonomy (e.g. Parking, railtrack, bridge etc. in WildDash), how do you produce predictions for that class? I understand you map the predictions with a binary matrix. But what do you do when there's no one-to-one correspondence?

A: WildDash v1 uses the 19-class taxonomy for evaluation, just like Cityscapes. So we use the following script to remap the 34-class taxonomy to 19-class taxonomy for WildDash for testing inference and submission. You can see how Cityscapes evaluates just 19 of the 34 classes here in the evaluation script and in the taxonomy definition. However, bridge and rail track are actually included in our unified taxonomy, as you’ll see in MSeg_master.tsv.

Q: How are datasets images read in for training/inference? Should I use the


A: The

are not for training or inference. They are purely for generating the MSeg dataset labels on disk. We read in the datasets using
and then remap them to the universal space on the fly.

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