Semantic segmentation aims at densely assigning semantic category label to each pixel given an image. Though the remarkable progresses have been dominated by deep neural networks trained on large-scale labeled dataset Chen et al. (2017a). The segmentation model trained on the labeled data in source domain usually cannot generalize well to the unseen data in target domain. For example, the model trained on the data from one city or computer-generated scene Richter et al. (2016); Ros et al. (2016) may fail to yield accurate pixel-level predictions for the scenes of another city or real scene. The main reason lies in the different data distribution between such source and target domains, typically known as domain discrepancy Shimodaira (2000).
To address this issue, domain adaptive semantic segmentation methods have been proposed in which they align data distribution between the source and target domains by adopting a domain discriminator Hoffman et al. (2016); Tsai et al. (2018). Formally, these methods aim to minimize an adversarial loss Goodfellow et al. (2014) to reduce the domain discrepancy at image-level Wu et al. (2018); Hoffman et al. (2018); Chang et al. (2019), feature-level Hoffman et al. (2016)
, and category probability-levelZou et al. (2018); Li et al. (2019); Tsai et al. (2018) distributions without forgetting semantic segmentation ability on the source domain. However, their accuracy is still limited when aligning multi-modal data distribution Arora et al. (2017), which cannot guarantee that the target samples from different categories are properly separated as in Fig. 1 (a).
To tackle this limitation, category-level domain adaptation methods Chen et al. (2017b); Du et al. (2019) have been proposed for semantic segmentation in which they minimize the class-specific domain discrepancy across the source and target domains. Together with supervision from the source domain, this enforces the segmentation network to learn discriminative representation for different classes on both domains. They utilize a category classifier trained on the source domain to generate pseudo class labels on the target domain. It results in inaccurate labels for domain adaptation that misleads the domain alignment and accumulates errors as in Fig. 1 (b). It also contains a class imbalance problem Zou et al. (2018), where the network works well for majority categories with a large number of pixels (e.g. road and building), while not suitable for minority categories with a small number of pixels (e.g. traffic sign).
To overcome this limitation, we present cross-domain grouping and alignment for domain adaptive semantic segmentation. As illustrated in Fig. 1 (c), the key idea of our method is to apply an intermediate grouping module to replace the category classifier, allowing to align samples of source and target domains at each group to be similar without using error-prone category classifier. To make the grouping module help with domain adaptation, we propose several losses in a manner that the category distribution of each group between different domains should be consistent, while the category distribution of different groups in the same domain should be orthogonal. Furthermore, we present a group-level class equivalence scheme in order to align all the categories regardless of the number of pixels. The proposed method is extensively evaluated through an ablation study and comparison with state-of-the-art methods on various domain adaptive semantic segmentation benchmarks, including GTA5 Cityscapes and SYNTHIA Cityscapes.
2 Related Work
2.1 Semantic Segmentation
Numerous methods have been proposed to assign class labels in pixel level for input images. Long et al. Long and Darrell (2015) first transformed a classification convolutional neural network (CNN) Krizhevsky et al. (2012); karen Simonyan and Zisserman (2015); He et al. (2016) to a fully-convolutional network (FCN) for semantic segmentation. Following the line of FCN-based methods, several methods utilized dilated convolutions to enlarge the receptive field Yu and Koltun (2015) and reason about spatial relationship Chen et al. (2017a). Recently, Zhao et al. Zhao et al. (2017) presented pyramid pooling module to encode the global and local context. Although these methods yielded impressive results in semantic segmentation, they still relied on large datasets with dense pixel-level class labels, which is expensive and laborious. An alternative is to utilize synthetic data Richter et al. (2016); Ros et al. (2016) which can make unlimited amounts of labels available. Nevertheless, synthetic data still suffer from a substantially different data distribution from real data, which results in a dramatic performance drop when applying the trained model to real scenes.
2.2 Domain Adaptive Semantic Segmentation
Due to the obvious mismatch between synthetic and real data, unsupervised domain adaptation (UDA) is studied to minimize the domain discrepancy by aligning the feature distribution between source and target data. As a pioneering work, Ganin et al. Ganin and Lempitsky (2015) introduced the domain adversarial network to transfer the feature distribution, and Tzeng et al. Tzeng et al. (2017) proposed adversarial discriminative alignment.
For pixel-level classification, numerous approaches Wu et al. (2018); Hoffman et al. (2018); Chang et al. (2019) utilized image-level adaptation methods which translate source image to have the texture appearance of target image, while preserving the structure information of the source image for adapting cross-domain knowledge. In contrast, several methods Zou et al. (2018); Li et al. (2019, 2020) adopted the iterative self-training approach to alternatively select unlabelled target samples with higher class probability and utilized them as a pseudo ground-truth. The feature-level adaptation methods align the intermediate feature distribution via adversarial framework. Hoffman et al. Hoffman et al. (2016) introduced a feature-level adaptation method to align the intermediate feature distribution for the global and local alignment. Tsai et al. Tsai et al. (2018) adopted output-level adaptation for structured output space, since it contains similar spatial structure with semantic segmentation. However, these methods aim to align overall data distribution without taking into account the inter-class variation.
To solve this problem, several methods Chen et al. (2017b); Du et al. (2019) introduced category-level adversarial learning to align the data distributions independently for each class. Similarly, other works Tsai et al. (2019); Huang et al. (2020) discovered patch-level adaptation methods by using multiple modes of patch-wise output distribution to differentiate the feature representation of patches. However, inaccurate domain alignment occurs because these methods rely heavily on category or patch classifiers trained in the source domain. The most similar to our work is Wang et al. Wang et al. (2020), which group the category classes into several groups for domain adaptive semantic segmentation. While they divide stuff and things (i.e. disconnected regions), our cross-domain grouping module divides the categories into multiple groups that the grouping network and segmentation network can be trained in a joint and boosting manner.
2.3 Unsupervised Deep Clustering
A variety of approaches have applied deep clustering algorithms that simultaneously discover groups in training data and perform representation learning. Chang et al. Chang et al. (2017) proposed to cast the clustering problem into pairwise classification using CNN. Caron et al. Caron et al. (2018) proposed a learning procedure that alternates between clustered images in the representation space and trains a model that assigns images to their clusters. Other approaches Joulin et al. (2012); Tao et al. (2017) localized the salient and common objects by clustering pixels in multiple images. Similarly, Collins et al. Collins et al. (2018)
proposed deep feature factorization (DFF) to group the common part segments between images through non-negative matrix factorization (NMF)Ding et al. (2005) on CNN features. This paper follows such a strategy to group semantic consistent data representation across the source and target domains.
3 Proposed Method
3.1 Problem Statement and Overview
Let us denote the source and target images as , where only the source data is annotated with per-pixel semantic categories as . We seek to train a semantic segmentation network , which outputs pixel-wise class probability on both source and target domains reliably, with height , width , and the number of classes
, respectively. Our goal is to train the segmentation network that yields to align probability distribution of the source and target domainsand so that the network can correctly predict the pixel-level labels even for the target data , following recent study Tsai et al. (2018) of adaptation in the output probability space, which shows better performance than adaptation in the intermediate feature space.
Conventionally two types of domain adaptation approaches have been proposed: global domain adaptation and category-level domain adaptation. The former aims to align the global domain differences, while the latter aims to minimize class specific domain discrepancy for each category. However, global domain adaptation does not take into account the inter-class variations, and category-level domain adaptation rely solely on category classifier. To this end, we propose a novel method by clustering the samples as groups across the source and target domains. Concretely, we cluster the probability distribution into groups using cross-domain grouping module, followed by group-level domain alignment. By setting greater than 1, domain alignment of complicated data distribution can be solved by an alignment of simple data distributions which is the challenge in global domain adaptation. By setting less than , the domain misalignment in category-level can be mitigated without using a category classifier trained in the source domain. In the following, we introduce our overall network architecture (Section 3.2), several constraints for cross-domain grouping (Section 3.3), and cross-domain alignment (Section 3.4).
3.2 Network Architecture
Fig. 2 illustrates our overall framework. Our network consists of three major components: 1) the semantic segmentation network , 2) the cross-domain grouping module to cluster sub-spaces based on the output probability distribution, and 3) the discriminator for group-level domain adaptation. In the following sections, we denote source and target domains as unless otherwise stated.
pre-trained on ImageNetDeng et al. (2009) dataset. The source and target images are fed into the segmentation network , outputting pixel-wise class probability distribution . Note that et al. (2018).
Cross-Domain grouping network.
Our cross-domain grouping network is formulated as two convolutions. We design each convolution withgrouping scores, followed by softmax function to output group probability . We then apply element-wise multiplication between and each channel dimension in , obtaining group-specific feature . The cross-domain grouping network can be easily replaced with other learnable clustering methods.
For group-level domain alignment, we fed into the discriminator . Following Li et al. Li et al. (2019), we set the discriminator using five
convolutional layers of stride 2, where the number of channels isto form the network. We use a leaky ReLU Maas et al. (2013) parameterized by 0.2 which is utilized for each convolutional layer except the last one.
3.3 Losses for Cross-Domain Grouping
Perhaps one of the most straightforward ways of grouping is to utilize existing clustering methods, e.g
. k-meansCoates and Ng (2012) or non-negative matrix factorization (NMF) Collins et al. (2018). These strategies, however, are not learnable, and thus, they cannot weave the advantages of category-level domain information. Unlike these, we present a learnable clustering module with two loss functions to take advantage of the category-level domain adaptation methods. We discuss in more detail about the effectiveness of our grouping compared to non-learnable models Coates and Ng (2012); Collins et al. (2018) in Section 4.2. In the following, we present each loss function in detail.
Our first insight about grouping is that the category distribution of each group between the source and target domains has to be consistent so that the clustered group can benefit from the category-level domain adaptation method. To this end, we first estimate the class distribution by using average pooling layer on each group-level feature such that , where each elements in indicates the probability distribution of containing a particular categories at group . We then encourage a semantic consistency among class distribution by utilizing l2-norm as follows:
Minimizing loss (1) has two desirable effects. First, it encourages the difference of each class distribution of group to be similar, and it also provides the supervisory signals for aligning the probability distribution of group-level features.
The semantic consistency constraint in (1) encourages the class distribution of group across the source and target domains to be consistent. This, however, does not guarantee that class distribution is different for each group. In other words, we cannot divide the multi-modal complex distribution into several simple distributions. To this end, we draw the second insight by introducing orthogonality constraint such that, any two class distribution and
, should be orthogonal each other. It can be realized that their cosine similarity (2) is 0 since are non-negative value. We define the cosine similarity with l2-norm as follows:
We then formulate an orthogonal loss for training such that
where we apply a loss function on each domain . By forcing the cross-domain grouping module to make each group to be orthogonal, it can divide a multi-modal complex distribution into the simple class distributions.
3.4 Losses for Cross-Domain Alignment
In this section, we present a group-level adversarial learning framework as an alternative to global domain adaptation and category-level domain adaptation.
To achieve group-level domain alignment, a straight forward method is to use independent discriminators, similar to conventional category-level domain alignment methods Chen et al. (2017b); Du et al. (2019). However, we simultaneously update grouping module while training the overall network, thus cluster assignment may not be consistent at each training iteration. To this end, we adopt conditional adversarial learning framework following Long et al. (2018), by combining group-level feature with as a condition as follows:
where represent outer product operation. Note that using group-level feature only as input to the discriminator is equivalent to global alignment, while we give a condition by using cross-covariance between and as input. This leads to discriminative domain alignment according to the different groups.
Group-level class equivalence.
For group-level adversarial learning, the existence of particular classes across different domains is desirable. However, since the number of pixels for particular classes are dominant in each image, it can cause class imbalance problem. Thus adaptation model tends to be biased towards majority classes and ignore minority classes Zou et al. (2018). To alleviate this, we propose group level class equivalence following Zhao et al. Zhao et al. (2018)
. We first apply max pooling layer for each group level featuresuch that each element of is a maximum score for each category corresponding to group . We then utilize maximum classification score in the source domain as a pseudo-label, where we aim to train maximum classification score in target domain to be similar. To this end, we apply multi-class binary cross-entropy loss for each class as follows :
where Iverson bracket indicator function · evaluates to 1 when it is true and 0 otherwise, and denotes category. Note that we merely exclude too low probability with the threshold parameter .
The overall loss function of our approach can be written as
where is the supervised cross-entropy loss for semantic segmentation network on the source data, and and are balancing parameter for different losses. We then solve the following minmax problem for optimizing and .
4.1 Experimental Setting
The proposed method was implemented in PyTorch libraryPaszke et al. (2017) and simulated on a PC with a single RTX Titan GPU. We utilize BDL Li et al. (2019) as our baseline model following conventional work Wang et al. (2020) et al. (1998), where the learning rate is set to . For grouping network, we utilize SGD, with learning rate as . Both learning rates decreased with “poly” learning rate policy with power fixed to 0.9 and momentum as 0.9. For discriminator training, we use Adam Kingma and Ba (2014) optimizer with an initial learning rate . We jointly train our segmentation network, grouping network, and discriminator using (7) for a total of iterations. We randomly paired source and target images in each iteration. Through the cross-validation using grid-search in log-scale, we set the hyper-parameters and as 0.001, 0.001, 0.001, 0.0001 and 0.05, respectively.
For experiments, we use the GTA5 Richter et al. (2016) and SYNTHIA Ros et al. (2016) as source dataset. GTA5 dataset Richter et al. (2016) contains 24,966 images with 19141052 resolution. We resize images to 1280 760 following other work Tsai et al. (2018). For SYNTHIA Ros et al. (2016), we use SYNTHIA-RAND-CITYSCAPES dataset with 9,400 images with 1280760 resolution. We use Cityscapes Cordts et al. (2016) as target dataset, which consists of 2,975, 500 and 1,525 images with training, validation and test set. We train our network with training set, while evaluation is done using validation set. We resize images to 1024 512 for both training and testing as Li et al. (2019). We evaluate the class-level intersection over union (IoU) and mean IoU (mIoU) Everingham et al. (2015).
We first visualize each group through cross-domain grouping in Fig. 3. Clustered groups for various showed that our networks clustered semantically consistent regions across the source and target domains through (1). Also, clustered regions along different indicate that our network effectively divided regions into different group using (2).
We further compare our grouping network with k-means clustering algorithm Coates and Ng (2012) and deep feature factorization Collins et al. (2018) which is not trainable methods. As shown in Fig. 4, our method can better capture the object boundaries and semantic consistent objects across source and target domains compared to other non-learnable methods. We further visualize each clustered group through cross-domain grouping with an evolving number of iteration. As the number of iterations increases, cross-domain grouping and group-level domain alignment share complementary information, which decomposes the data distribution and aligns domains for each grouped sub-spaces in a joint and boosting manner.
In Fig. 5, we show the t-SNE visualization van der Maaten and Hinton (2008) of the output probability distribution of our method compares to the non-adapted method and baseline Li et al. (2019). The result shows that our method effectively aligns the distribution of the source and target domains, while others failed to reduce the domain discrepancy. Furthermore, we observe that our model successfully grouped minority categories (i.e. traffic signs in yellow) while others failed. It indicates that the loss (5) can solve class imbalance problem.
4.3 Ablation Study
Fig. 6 shows the result of ablation experiments with different number of groups . Note that the results with are equivalent to global domain adaptation as a baseline. The result shows that ours with various number of consistently outperform the baseline, which shows the effectiveness of our group-level domain alignment. The performance has improved as increased from , and after achieving the best performance at the rest showed no significant difference. The lower performance with the larger number of indicates that over-clustered samples can actually degrade performance as conventional category-level adaptation methods. Since the result with has shown the best performance on both GTA5 Cityscapes and SYNTHIA Cityscapes, we set as for all experiments.
Table 1 shows the result of ablation experiments to validate the effects of proposed loss functions. It verifies the effectiveness of each loss function, including group-level domain adaptation, group-level semantic consistency, group-level orthogonality, and group-level class equivalence. The full usage of our proposed loss functions yields the best results. We also find that adding group-level orthogonality leads to a large improvement in the performance, which demonstrates that we effectively divide the multi-modal complex distribution into simple distributions for group-level domain alignment.
4.4 Comparison with state-of-the-art methods
In the following, we evaluated our method on GTA5 Cityscapes in comparison to the state-of-the-art methods including without adaptation, global adaptation Tsai et al. (2018), image-level adaptation Wu et al. (2018); Chang et al. (2019); Li et al. (2019) and category-level domain alignment Luo et al. (2019); Du et al. (2019); Vu et al. (2019); Tsai et al. (2019); Huang et al. (2020); Wang et al. (2020). As shown in Table 2, our method outperforms all other models on the categories “car, truck, bus, motor, and bike” which share similar appearance. Specifically, we observe that our model achieves performance improvement on the categories “pole and traffic sign”. It demonstrates that our group-level class equivalence effectively solves the class imbalance problem.
We further compared our method to the state-of-the-art methods Luo et al. (2019); Tsai et al. (2018); Du et al. (2019); Li et al. (2019); Huang et al. (2020); Wang et al. (2020) on SYNTHIA Cityscapes, where 13 common classes between SYNTHIA Ros et al. (2016) and Cityscapes Cordts et al. (2016) datasets are evaluated. As shown in Table 3, our method outperforms conventional methods. We can observe that our model achieve similar improvements in GTA5 Cityscapes scenario. Compared to baseline Li et al. (2019), our model achieves large improvement in the performance “traffic sign and traffic light”.
We have introduced cross-domain grouping and alignment for domain adaptive semantic segmentation. The key idea is to apply an intermediate grouping module such that multi-modal data distribution can be divided into several simple distributions. We then apply group-level domain alignment across source and target domains, where the grouping network and segmentation network can be trained in a joint and boosting manner using semantic consistency and orthogonality constraints. To solve the class imbalance problem, we have further introduced a group-level class equivalence constraint, resulting state-of-the-art performance on domain adaptive semantic segmentation. We believe our approach will facilitate further advances in unsupervised domain adaptation on various computer vision tasks.
This research was supported by R&D program for Advanced Integrated-intelligence for Identification (AIID) through the National Research Foundation of KOREA (NRF) funded by Ministry of Science and ICT (NRF-2018M3E3A1057289).
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