Synthetic-to-Real Domain Transfer with Joint Image Translation and Discriminative Learning for Pedestrian Re-Identification.

This reposity has the code of my master's thesis project for the M.Sc. degree in Data Science and Engineering at Politecnico di Torino. It extends my previous internship as both works were completed at the Links Foundation. The code is based on this framework. The objective was to generalize from our dataset GTASynthReid (examples in the image below) to real-world person re-identification data using generative methods. We employed the contrastive unpaired translation (CUT) framework for image translation with joint discriminative learning for synth-to-real adaptation.

In this task, we want to match the same pedestrian across multiple camera views. Traditionally, researchers used real-world data by training on some of the available identities and testing on the remaining ones. For the evaluation, we have to extract a feature vector for all the query and gallery images with our learned model. Then, for each query, we rank by similarity all of the gallery images and evaluate the ranking with metrics such as CMC and MaP. In a real-world scenario, we would like to apply our trained model to different environments. Research has shown that testing on a different dataset leads to poor results. Recent methods adopt source-to-target techniques that bridge the gap between two or more datasets. However, collecting data for this kind of task is error-prone, time-consuming and has brought to light ethical concerns. Synthetic datasets are helpful in these areas, although they introduce all the complexity related to synth-to-real adaptation. We created a dataset with the video game GTAV and experimented with generative methods for image translation.
We adapted to real-world datasets achieving a better performance than the direct transfer baseline and some earlier adaptation methods. We also generated images closer in style to the target ones.

Our network is based on CUT and includes the domain mapping, relationship preservation and discriminative modules. It has a single GAN since it does not need cycle consistency. Discriminative learning is embedded in the image translation process.

The synth-to-real domain mapper is a GAN with an encoder-decoder generator (see picture) and a PatchGan discriminator. We exploited the L2 loss for the adversarial training. We also engineered an adversarial feature matching loss asking for similar features at certain discriminator layers. From the generator's perspective, we want this to happen between translated and target images. From the discriminator's perspective, instead, between translated and source images.

However, this is not enough to produce images that are more similar to the target ones, as the following image shows (left is source, right is translation). We need to better constrain and guide the generative process.

We adopted CUT to guide the generative process instead of cycle consistency models, avoiding bijective mappings. Intuitively, this method asks corresponding patches of the input and output images to be similar after the translation (see image below). The patches are the output feature vectors along the channel dimension of a given encoder's layer l (receptive field arithmetic). Then, these feature vectors are further processed by smaller multilayer perceptrons (H in image). Finally, we have to perform a patch classification. This process is repeated along different layers of the generator's encoder.

We still need to explain how we integrated the re-identification task with image translation. One option would be to train a network for the re-identification on the translated images after having translated the whole source data. However, this would add another training stage and thus slow the entire process. We instead processed with a Resnet the features coming from the last layer of the generator's encoder. These features are responsible for the translation and hold some information about the source identities. They are not yet specialized to discriminate among different pedestrians. For this reason, we trained the Resnet with cross-entropy loss for classification on the source identities using as inputs the features with target-related information.

For the inference stage, we just need to pass the test images into the encoder and re-identification network to extract the feature vectors needed for the ranking. We evaluated the quality of both the re-identification and translation.

We compared our method with similar works (both real-to-real and synth-to-real adaptation) improving some of the results obtained by cyclegans. The most recent approaches that use cycle consistency or far more training images achieve better results than ours. In the table below we report the performance through CMC (rank 1) and MaP on the Market dataset in the following image translation settings: GTASynthReid->Market, GTASynthReid->Duke and GTASynthReid->CHUK03. The baseline is a ResNet trained for direct transfer on the source identities. For additional results, comparisons and ablation studies check chapter 5.2 of my thesis.

We wanted to measure whether our generated images would at least feel more similar to the target ones in terms of style. To do so, we measured the Fréchet Inception Distance before and after the translation for each target dataset, as the table below shows.

In the following picture there are some examples of translated pedestrians to the three real-world datasets (Market, Duke, CUHK03).

Recent works obtained better performance by using thousands of synthetic pedestrians. One could try to add more characters to GTAV and see whether this would improve our results. Another important direction is the disentanglement between pose and appearance with cycle consistency which led generative methods to better results. However, such models are heavy in memory and more research is needed to develop better solutions.

@article{cut,
  title={Contrastive Learning for Unpaired Image-to-Image Translation},
  author={Taesung Park and Alexei A. Efros and Richard Zhang and Jun-Yan Zhu},
  booktitle={European Conference on Computer Vision},
  year={2020}
}

@article{torchreid,
  title={Torchreid: A Library for Deep Learning Person Re-Identification in Pytorch},
  author={Zhou, Kaiyang and Xiang, Tao},
  journal={arXiv preprint arXiv:1910.10093},
  year={2019}
}

@inproceedings{zhou2019osnet,
  title={Omni-Scale Feature Learning for Person Re-Identification},
  author={Zhou, Kaiyang and Yang, Yongxin and Cavallaro, Andrea and Xiang, Tao},
  booktitle={ICCV},
  year={2019}
}

@article{zhou2021osnet,
  title={Learning Generalisable Omni-Scale Representations for Person Re-Identification},
  author={Zhou, Kaiyang and Yang, Yongxin and Cavallaro, Andrea and Xiang, Tao},
  journal={TPAMI},
  year={2021}
}