There has been significant progress in object detection in recent years [11, 44, 2, 26, 13, 3], powered by the availability of challenging and diverse object detection datasets, e.g. PASCAL VOC , COCO , KITTI , WiderFace , etc. However, existing detectors are usually domain-specific, e.g. trained and tested on a single dataset. This is partly due to the fact that object detection datasets are diverse and there is a nontrivial domain shift between them. As shown in Figure 1, detection tasks can vary in terms of categories (human face, horse, medical lesion, etc.), camera viewpoints (images taken from aircrafts, autonomous vehicles, etc.), image styles (comic, clipart, watercolor, medical), etc. In general, high detection performance requires a detector specialized on the target dataset.
This poses a significant problem for practical applications, which are not usually restricted to any one of the domains of Figure 1. Hence, there is a need for systems capable of detecting objects regardless of the domain in which images are collected. A simple solution is to design a specialized detector for each domain of interest, e.g. use detectors trained on datasets, and load the detector specialized to the domain of interest at each point in time. This, however, may be impractical, for two reasons. First, in most applications involving autonomous systems the domain of interest can change frequently and is not necessarily known a priori. Second, the overall model size increases linearly with the number of domains . A recent trend, known as general AI, is to request that a single universal model solves multiple tasks [21, 25, 62], or the same task over multiple domains [40, 1]
. However, existing efforts in this area mostly address image classification, rarely targeting the problem of object detection. The fact that modern object detectors are complex systems, composed of a backbone network, proposal generator, bounding box regressor, classifier, etc., makes the design of a universal object detector much more challenging than a universal image classifier.
In this work, we consider the design of an object detector capable of operating over multiple domains. We begin by establishing a new universal object detection benchmark, denoted as UODB, consisting of 11 diverse object detection datasets (see Figure 1). This is significantly more challenging than the Decathlon 
benchmark for multi-domain recognition. To the best of our knowledge, we are the first to attack universal object detection using deep learning. We expect this new benchmark will encourage more efforts in the area. We then propose a number of architectures, shown in Figure2, to address the universal/multi-domain detection problem.
The two architecture on the left of Figure 2 are multi-domain detectors, which require prior knowledge of the domain of interest. The two architectures on the right are universal detectors, with no need for such knowledge. When operating on an unknown domain, the multi-domain detector have to repeat the inference process with different sets of domain-specific parameters, while the universal detector performs inference only once. The detector of Figure 2 (a) is a bank of domain-specific detectors, with no sharing of parameters/computations. Multi-domain learning (MDL) [20, 35, 24, 59, 19, 5] improves on this, by sharing parameters across various domains, and adding small domain-specific layers. In [40, 1], expensive convolutional layers are shared and complemented with light-weight domain-specific adaptation layers. Inspired by these, we propose a new class of light adapters for detection, based on the squeeze and excitation (SE) mechanism of , and denoted SE adapters. This leads to the multi-domain detector of Figure 2 (b), where domain-specific SE adapters are introduced throughout the network to compensate for domain shift. On UODB, this detector outperforms that of Figure 2 (a) with 5 times fewer parameters.
In contrast, the universal detector of Figure 2 (c) shares all parameters/computations (other than output layers) across domains. It consists of a single network, which is always active. This is the most efficient solution in terms of parameter sharing, but it is difficult for a single model to cover many domains with nontrivial domain shifts. Hence, this solution underperforms the multi-domain detector of Figure 2 (b). To overcome this problem, we propose the domain-attentive universal detector of Figure 2 (d). This leverages a novel domain attention (DA) module, in which a bank of the new universal SE adapters (active at all times) is first added, and a feature-based attention mechanism is then introduced to achieve domain sensitivity. This module learns to assign network activations to different domains, through the universal SE adapter bank, and soft-routs their responses by the domain-attention mechanism. This enables the adapters to specialize on individual domains. Since the process is data-driven, the number of domains does not have to match the number of datasets and datasets can span multiple domains. This allows the network to leverage shared knowledge across domains, which is not available in the common single-domain detectors. Our experiments, on the newly established UODB, show that this data-driven form of parameter/computation sharing enables substantially better multi-domain detection performance than the remaining architectures of Figure 2.
2 Related Work
Object Detection: The two stage detection framework of the R-CNN , Fast R-CNN  and Faster R-CNN  detectors has achieved great success in recent years. Many works have expanded this base architecture. For example, MS-CNN  and FPN  built a feature pyramid to effectively detect objects of various scales; the R-FCN  proposed a position-sensitive pooling to achieve further speed-ups; and the Cascade R-CNN  introduced a multi-stage cascade for high quality object detection. In parallel, single-stage object detectors, such as YOLO  and SSD , became popular for their fairly good performance and high speed. However, none of these detectors could reach high detection performance on more than one dataset/domain without finetuning. In the pre-deep learning era,  proposed a universal DPM  detector, by adding dataset specific biases to the DPM. But this solution is limited since DPM is not comparable to deep learning detectors.
Multi-Task Learning: Multi-task learning (MTL) investigates how to jointly learn multiple tasks simultaneously, assuming a single input domain. Various multi-task networks [25, 62, 13, 28, 50, 63] have been proposed for joint solution of tasks such as object recognition, object detection, segmentation, edge detection, human pose, depth, action recognition, etc., by leveraging information sharing across tasks. However, the sharing is not always beneficial, sometimes hurting performance [7, 22]. To address this,  proposed a cross-stitch unit, which combines tasks of different types, eliminating the need to search through several architectures on a per task basis.  studied the common structure and relationships of several different tasks.
Multi-Domain Learning/Adaptation: Multi-domain learning (MDL) addresses the learning of representations for multiple domains, known a priori [20, 36]. It uses a combination of parameters that are shared across domains and domain-specific parameters. The latter are adaptation parameters, inspired by works on domain adaptation [38, 30, 46, 31], where a model learned from a source domain is adapted to a target domain.  showed that multi-domain learning is feasible by simply adding domain-specific BN layers to an otherwise shared network.  learned multiple visual domains with residual adapters, while  empirically studied efficient parameterizations. However, they build on BN layers and are not suitable for detection, due to the batch constraints of detector training. Instead, we propose an alternative SE adapters, inspired by “Squeeze-and-Excitation” , to solve this problem.
Attention Module:  proposed a self-attention module for machine translation, and similarly,  proposed a non-local network for video classification, based on a spacetime dependency/attention mechanism. 
focused on channel relationships, introducing the SE module to adaptatively recalibrate channel-wise feature responses, which achieved good results on ImageNet recognition. In this work, we introduce a domain attention module inspired by SE to make data-driven domain assignments of network activations, for the more challenging problem of universal object detection.
3 Multi-domain Object Detection
The problem of multi-domain object detection is to detect objects on various domains.
3.1 Universal Object Detection Benchmark
To train and evaluate universal/multi-domain object detection systems, we established a new universal object detection benchmark (UODB) of datasets: Pascal VOC , WiderFace , KITTI , LISA , DOTA , COCO , Watercolor , Clipart , Comic , Kitchen  and DeepLesions . This set includes the popular VOC  and COCO , composed of images of everyday objects, e.g. bikes, humans, animals, etc. The 20 VOC categories are replicated on CrossDomain  with three subsets of Watercolor, Clipart and Comic, with objects depicted in watercolor, clipart and comic styles, respectively. Kitchen  consists of common kitchen objects, collected with an hand-held Kinect, while WiderFace  contains human faces, collected on the web. Both KITTI  and LISA  depict traffic scenes, collected with cameras mounted on moving vehicles. KITTI covers the categories of vehicle, pedestrian and cyclist, while LISA is composed of traffic signs. DOTA  is a surveillance-style dataset, containing objects such as vehicles, planes, ships, harbors, etc. imaged from aerial cameras. Finally DeepLesion  is a dataset of lesions on medical CT images. A representative example of each dataset is shown in Figure 1. Some more details are summarized in Table 1. Altogether, UODB covers a wide range of variations in category, camera view, image style, etc, and thus establishes a good suite for the evaluation of universal/multi-domain object detection.
3.2 Single-domain Detector Bank
The Faster R-CNN  is used as the baseline architecture of all detectors proposed in this work. As a single-domain object detector, the Faster R-CNN is implemented in two stages. First, a region proposal network (RPN) produces preliminary class-agnostic detection hypotheses. The second stage processes these with a region-of-interest detection network to output the final detections.
As illustrated in Figure 2 (a), the simplest solution to multi-domain detection is to use an independent detector per dataset. We use this detector bank as a multi-domain detection baseline. This solution is the most expensive, since it implies replicating all parameters of all detectors. Figure 3
shows the statistics (mean and variance) of the convolutional activations of thedetectors on the corresponding dataset. Some observations can be made. First, these statistics vary non-trivially across datasets. While the activation distributions of VOC and COCO are similar, DOTA, DeepLesion and CrossDomain have relatively different distributions. Second, the statistics vary across network layers. Early layers, which are more responsible for correcting domain shift, have more evident differences than latter layers. This tends to hold up to the output layers. These are responsible for the assignment of images to different categories and naturally differ. Interestingly, this behavior also holds for RPN layers, even though they are category-independent. Third, many layers have similar statistics across datasets. This is especially true for intermediate layers, suggesting that they can be shared by at least some domains.
3.3 Adaptive Multi-domain Detector
Inspired by Figure 3, we propose an adaptive multi-domain detector, shown in Figure 2 (b). In this model, the output and RPN layers are domain-specific. The remainder of the network, e.g. all convolutional layers, is shared. However, to allow adaptation to new domains, we introduce some additional domain-specific layers, as is commonly done in MDL [40, 1]. These extra layers should be 1) sufficiently powerful to compensate for domain shift; 2) as light as possible to minimize parameters/computation. The adaptation layers of [40, 1] rely extensively on BN. This is unfeasible for detection, where BN layers have to be frozen, due to the small batch sizes allowable for detector training.
Instead, we have experimented with the squeeze-and-excitation (SE) module  of Figure 4 (a). There are a few reasons for this. First, feature-based attention is well known to be used in mammalian vision as a mechanism to adapt perception to different tasks and environments [61, 37, 52, 18, 60]. Hence, it seems natural to consider feature-based attention mechanisms for domain adaptation. Second, the SE is a module that accounts for interdependencies among channels to modulate channel responses. This can be seen as a a feature-based attention mechanism. Third the SE module has enabled the SENet to achieve state-of-the-art classification on ImageNet. Finally, it is a light-weight module. Even when added to each residual block of the ResNet  it increases the total parameter count by only 10%. This is close to what was reported by  for BN-based adapters. For all these reasons, we adopt the SE module as the atomic adaptation unit, used to build all domain adaptive detectors proposed in this work, and denote it by the SE adapter.
3.4 SE Adapters
(a): a global pooling layer, a fully connected (FC) layer, a ReLU layer, and a second FC layer, implementing the computation
where is a global average pooling operator, and the combination of FC+ReLU+FC layers. The channel dimension reduction factor , in Figure 4, is set as 16 in our experiments. To enable multi-domain object detection, the SE adapter is generalized to the architecture of Figure 4 (b), which is denoted as the SE adapter bank. This consists of adding a SE adapter branch per domain and a domain-switch, which allows the selection of the SE adapter associated with the domain of interest. Note that this architecture assumes this domain to be known a priori. It leads to the multi-domain detector of Figure 2 (b). Compared to Figure 2 (a), this model is up to 5 times smaller, while achieving better overall performance across the 11 datasets.
4 Universal Object detection
The detectors of the previous section require prior knowledge of the domain of interest. This is undesirable for autonomous systems, like robots or self-driving cars, where determining the domain is part of the problem to solve. In this section, we consider the design of universal detectors, which eliminate this problem.
4.1 Universal Detector
The simplest solution to universal detection, shown in Figure 2 (c), is to share a single detector by all tasks. Note that, even for this detector, the output layer has to be task-specific, by definition of the detection problem. We have found that there is also a benefit in using task-specific RPN layers, due to the observations of Figure 3. This is not a problem because the task, namely what classes the system is trying to detect, is always known. Universality refers to the domain of input images that the detector processes, which does not have to be known in the case of Figure 2 (c). Beyond universal, the fully shared detector is the most efficient of all detectors considered in this work, as it has no domain-specific parameters. On the other hand, by forcing the same set of parameters/representations on all domains, it has little flexibility to deal with the statistical variations of Figure 3. In our experiments, this detector usually underperforms the multi-domain detectors of Figure 2 (a) and (b).
4.2 Domain-attentive Universal Detector
Ideally, a universal detector should have some domain sensitivity, and be able to adapt to different domains. While this has a lot in common with multi-domain detection, there are two main differences. First, the domain must be inferred automatically. Second, there is no need to tie domains and tasks. For example, the traffic tasks of Figure 1 operate on a common visual domain, “traffic scenes”, which can have many sub-domains, e.g. due to weather conditions (sunny vs. rainy), environment (city vs. rural ), etc. Depending on the specific operating conditions, any of the tasks may have to be solved in any of the domains. In fact, the domains may not even have clear semantics, i.e. they can be data-driven. In this case, there is no need to request that each detector operates on a single domain, and a soft domain-assignment makes more sense. Given all of this, while domain adaptation can still be implemented with the SE adapter of Figure 4 (a), the hard attention mechanism of Figure 4 (b), which forces the network to fully attend to a single domain, can be suboptimal. To address this limitations, we propose the domain adaptation (DA) module of Figure 5. This has two components, a universal SE adapter bank and a domain attention mechanism, which are discussed next.
4.3 Universal SE Adapter Bank
The universal SE (USE) Adapter Bank, shown in Figure 5, is an SE adapter bank similar to that of Figure 4 (b). The main difference is that there is no domain switching, i.e. the adapter bank is universal. This is implemented by concatenating the outputs of the individual domain adapters to form a universal representation space
where is the number of adapters and the output of each adapter, given by (1). Note that is not necessarily identical to the number of detection tasks. The USE adapter bank can be seen as a non-linear generalization of the filter banks commonly used in signal processing . Each branch (non-linearly) projects the input along a subspace matched to the statistics of a particular domain. The attention component then produces a domain-sensitive set of weights that are used to combine these projections in a data-driven way. In this case, there is no need to know the operating domain in advance. In fact there may not even be a single domain, since an input image can excite multiple SE adapter branches.
4.4 Domain Attention
The attention component, of Figure 5
, produces a domain-sensitive set of weights that are used to combine the SE bank projections. Motivated by the SE module, the domain attention component first applies a global pooling to the input feature map, to remove spatial dimensions, and then a softmax layer (linear layer plus softmax function)
is the matrix of softmax layer weights. The vectoris then used to weigh the USE bank output , to produce a vector of domain adaptive responses
As in the SE module of , is finally used to channel-wise rescale the activations being adapted,
where implements a channel-wise multiplication, and
is the sigmoid function.
In this way, the USE bank captures the feature subspaces of the domains spanned by all datasets, and the DA mechanism soft-routes the USE projections. Both operations are data-driven, and operate with no prior knowledge of the domain. Unlike the hard attention mechanism of Figure 4 (b), this DA module enables information sharing across domains, leading to a more effective representation. In our experiments, the domain-attentive universal detector outperforms the other detectors of Figure 2.
In all experiments, we used a PyTorch implementation of the Faster R-CNN with the SE-ResNet-50 
pretrained on ImageNet, as the backbone for all detectors. Training started with a learning rate of 0.01 for 10 epochs and 0.001 for another 2 epochs on 8 synchronized GPUs, each holding 2 images per iteration. All samples of a batch are from a single (randomly sampled) dataset, and in each epoch, all samples of each dataset are processed only once. As is common for detection, the first convolutional layer, the first residual block and all BN layers are frozen, during training. These settings were used in all experiments, unless otherwise noted. Both multi-domain and universal detectors were trained on all domains of interest simultaneously.
The Faster R-CNN has many hyperparameters. In the literature, where detectors are tested on a single domain, these are tuned to the target dataset, for best performance. This is difficult, and very tedious, to do over the datasets now considered. We use the same hyperparameters across datasets, except when this is critical for performance and relatively easy to do, e.g. the choice of anchors. The main dataset-specific hyperparameters are shown in Table 1.
5.1 Datasets and Evaluation
Our experiments used the new UODB benchmark introduced in Section 3.1. For Watercolor , Clipart , Comic , Kitchen  and DeepLesion , we trained on the official trainval sets and tested on the test set. For Pascal VOC , we trained on VOC2007 and VOC2012 trainval set and tested on VOC2007 test set. For WiderFace , we trained on the train set and tested on the val set. For KITTI , we followed the train/val splitting of  for development and trained on the trainval set for the final results on test set. For LISA , we trained on the train set and tested on the val set. For DOTA , we followed the pre-processing of , trained on train set and tested on val set. For MS-COCO , we trained on COCO 2014 valminusminival and tested on minival, to shorten the experimental period.
All detectors were evaluated on each dataset individually. The Pascal VOC mean average precision (mAP) was used for evaluation in all cases. The average mAPs was used as the overall measure of universal/multi-domain detection performance. The domain attentive universal detector was also evaluated using the official evaluation tool of each dataset, for comparison with the literature.
5.2 Single-domain Detection
(76.4 mAP for ResNet-101). The other entries in the table are incomparable to the literature, where different evaluation metrics/tools are used for different datasets. The detector bank is a fairly strong baseline for multi-domain detection (average mAP of 59.4).
|# adapters||Params||DA index||KITTI||VOC||WiderFace||LISA||Kitchen||COCO||DOTA||DeepLesion||Comic||Clipart||Watercolor||Avg|
5.3 Multi-domain Detection
Table 2 compares the multi-domain object detection performance of all architectures of Figure 2. For simplicity, only five datasets (VOC, KITTI, WiderFace, LISA and Kitchen) were used in this section. The table confirms that the adaptive multi-domain detector of Section 3.3 (“adaptive”) is light-weight, only adding 11M parameters to the Faster R-CNN over the five datasets. Nevertheless, it outperforms the much more expensive single-domain detector bank by 0.7 points. Note that the latter is a strong baseline, showing the multi-domain detector can beat individually trained models with a fraction of the computation. Table 2 also shows that the proposed SE adapter significantly outperforms the BN adapter (BNA) of  and the residual adapter (RA) or , previously proposed for classification. This is not surprising, given the above discussed inadequacy of BN as an adaptation mechanism for object detection.
The universal detector of Figure 2 (c) is even more efficient, adding only 0.5M parameters to the Faster R-CNN, accounting for domain-specific RPN and output layers. However, its performance (“universal” in Table 2) is much weaker than that of the adaptive multi-domain detector (1.7 points). Finally, the domain-attentive universal detector (“universal+DA”) has the best performance. With a 7% parameter increase per domain, i.e. comparable to the multi-domain detector, it outperforms the single-domain bank baseline by 1.6 points. To assess the importance of data-driven domain attention mechanism of Figure 5 (b), we fixed the soft domain assignments, simply averaging the SE adapter responses, during both training and inference. This (denoted “universal+DA”) caused a performance drop of 0.5 point. Finally, Table 2 shows the relative run-times of all methods on the five datasets, when the domain is unknown. It can be seen that “universal+DA” is about 4 faster than the multi-domain detectors (“single-domain” and “adaptive”) and only 1.33 slower than “universal”.
5.4 Effect of the number of SE adapters
For the USE bank of Figure 5 (b), the number of SE adapters does not have to match the number of detection tasks. Table 4 summarizes how the performance of the domain attentive universal detector depends on . For simplicity, we again use 5 datasets in this experiment. For a single adapter, the DA module reduces to the standard SE module, and the domain attentive universal detector to the universal detector. This has the worst performance. Performance improves with the number of adapters. On the other hand, the number of parameters increases linearly with the number of adapters. In these experiments, the best trade-off between performance and parameters is around 5 adapters. This suggests that, while a good rule of thumb is to use “as many adapters as domains”, fewer adapters can be used when complexity is at a premium.
5.5 Results on the full benchmark
Table 3 presents results on the full benchmark. The settings are as above, but we used 10 epochs with learning rate 0.1, and then 4 epochs with 0.01 on 8 GPUs, each holding 2 images. The universal detector performs comparably to the single-domain detector bank, with times fewer parameters. The domain-attentive universal detector (“universal+DA”) improves baseline performance by 4.4 points with a -fold parameter decrease. It has large performance gains (5 points) on DeepLesion, Comic, and Clipart. This is because Comic/Clipart contain underpopulated classes, greatly benefiting from information leveraged from other domains. The large gain of DeepLesion is quite interesting, given the nontrivial domain shift between its medical CT images and the RGB images of the other datasets. The gains are mild for VOC, KITTI, Kitchen, WiderFace and WaterColor (15 points), and none for COCO, LISA and DOTA. In contrast, for the universal detector, joint training is not always beneficial. This shows the importance of domain sensitivity for universal detection.
To investigate what was learned by the domain attention module of Figure 5 (b), we show the soft assignments of each dataset, averaged over its validation set, in Figure 6. Only the first and last blocks of the 4th and 5th residual stages are shown. The fact that some datasets, e.g. VOC and COCO, have very similar assignment distributions, suggests a substantial domain overlap. On the other hand, DOTA and DeepLesion have distributions quite distinct from the remaining. For example, on block “DA_4_1”, DeepLesion fully occupies a single domain. These observations are consistent with Figure 3, indicating that the proposed DA module is able to learn domain-specific knowledge.
A comparison of the first and the last blocks of each residual stage, e.g. “DA_4_1” v.s. “DA_4_6”, shows that the latter are much less domain sensitive than the former, suggesting that they could be made universal. To test this hypothesis, we trained a model with only 6 SE adapters for the 11 datasets, and only in the first and middle blocks, e.g. “DA_4_1” and “DA_4_3”. This model, “universal+DA*”, achieved the best performance with much less parameters than the “universal+DA” detector of 11 adapters. It outperformed the single domain baseline by 4.5 points.
5.6 Official evaluation
Since, to the best of our knowledge, this is the first work to explore universal/multi-domain object detection on 11 datasets, there is no literature for a direct comparison. Instead, we compared the “universal+DA*” detector of Table 3 to the literature using the official evaluation for each dataset. This is an unfair comparison, since the universal detector has to remember 11 tasks. On VOC, we trained two models, with/without COCO. Results are shown in Table (a)a, where all methods were trained on Pascal VOC 07+12 trainval. Note that our Faster R-CNN baseline ( SE-ResNet-50 backbone) is stronger than that of  (ResNet-101). Adding universal domain adapters improved on the baseline by more than 1.1 points. Adding COCO enabled another 3.1 points. Note that, 1) this universal training is different from the training scheme of  (the network trained on COCO then finetuned on VOC), where the final model is only optimized for VOC; and 2) only the 35k images of COCO2014 valminusminival were used.
The baseline was the default Faster R-CNN that initially worked on VOC, with minimum dataset-specific changes, e.g. in Table 1. Table (e)e shows that this performed weakly on KITTI. However, the addition of adapters, enabled a gain of 6.4 points (Moderate setting). This is comparable to detectors optimized explicitly on KITTI, e.g. MS-CNN  and F-PointNet . For WiderFace, which has enough training face instances, the gains of shared knowledge are smaller (see Table (b)b). On the other hand, on DeepLesion and CrossDomain (Clipart, Comic and Watercolor), see Table (c)c and (d)d respectively, the domain attentive universal detector significantly outperformed the state-of-the-art. Overall, these results show that a single detector, which operates on 11 datasets, is competitive with single-domain detectors in highly researched datasets, such as VOC or KITTI, and substantially better than the state-of-the-art in less explored domains. This is achieved with a relatively minor increase in parameters, vastly smaller than that needed to deploy 11 single task detectors.
We have investigated the unexplored and challenging problem of universal/multi-domain object detection. We proposed a universal detector that requires no prior domain knowledge, consisting of a single network that is active for all tasks. The proposed detector achieves domain sensitivity through a novel data-driven domain adaptation module and was shown to outperform multiple universal/multi-domain detectors on a newly established benchmark, and even individual detectors optimized for a single task.
This work was partially funded by NSF awards IIS-1546305 and IIS-1637941, a gift from 12 Sigma Technologies, and NVIDIA GPU donations.
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