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Spatial Attention Module (SAM) is a feature extraction module for object detection used in ThunderNet. The ThunderNet SAM explicitly re-weights the feature map before RoI warping over the spatial dimensions. The key idea of SAM is to use the knowledge from RPN to refine the feature distribution of the feature map. RPN is trained to recognize foreground regions under the supervision of ground truths. Therefore, the intermediate features in RPN can be used to distinguish foreground features from background features. SAM accepts two inputs: the intermediate feature map from RPN $\mathcal{F}^{RPN}$ and the thin feature map from the Context Enhancement Module $\mathcal{F}^{CEM}$. The output of SAM $\mathcal{F}^{SAM}$ is defined as: $$ \mathcal{F}^{SAM} = \mathcal{F}^{CEM} * \text{sigmoid}\left(\theta\left(\mathcal{F}^{RPN}\right)\right) $$ Here $\theta\left(·\right)$ is a dimension transformation to match the number of channels in both feature maps. The sigmoid function is used to constrain the values within $\left[0, 1\right]$. At last, $\mathcal{F}^{CEM}$ is re-weighted by the generated feature map for better feature distribution. For computational efficiency, we simply apply a 1×1 convolution as $\theta\left(·\right)$, so the computational cost of CEM is negligible. The Figure to the right shows the structure of SAM. SAM has two functions. The first one is to refine the feature distribution by strengthening foreground features and suppressing background features. The second one is to stabilize the training of RPN as SAM enables extra gradient flow from R-CNN subnet to RPN. As a result, RPN receives additional supervision from RCNN subnet, which helps the training of RPN.

ThunderNet is a two-stage object detection model. The design of ThunderNet aims at the computationally expensive structures in state-of-the-art two-stage detectors. The backbone utilises a ShuffleNetV2 inspired network called SNet designed for object detection. In the detection part, ThunderNet follows the detection head design in Light-Head R-CNN, and further compresses the RPN and R-CNN subnet. To eliminate the performance degradation induced by small backbones and small feature maps, ThunderNet uses two new efficient architecture blocks, Context Enhancement Module (CEM) and Spatial Attention Module (SAM). CEM combines the feature maps from multiple scales to leverage local and global context information, while SAM uses the information learned in RPN to refine the feature distribution in RoI warping.

A TridentNet Block is a feature extractor used in object detection models. Instead of feeding in multi-scale inputs like the image pyramid, in a TridentNet block we adapt the backbone network for different scales. These blocks create multiple scale-specific feature maps. With the help of dilated convolutions, different branches of trident blocks have the same network structure and share the same parameters yet have different receptive fields. Furthermore, to avoid training objects with extreme scales, a scale-aware training scheme is employed to make each branch specific to a given scale range matching its receptive field. Weight sharing is used to prevent overfitting.

Cross-Covariance Image Transformers, or XCiT, is a type of vision transformer that aims to combine the accuracy of conventional transformers with the scalability of convolutional architectures. The self-attention operation underlying transformers yields global interactions between all tokens, i.e. words or image patches, and enables flexible modelling of image data beyond the local interactions of convolutions. This flexibility, however, comes with a quadratic complexity in time and memory, hindering application to long sequences and high-resolution images. The authors propose a “transposed” version of self-attention called cross-covariance attention that operates across feature channels rather than tokens, where the interactions are based on the cross-covariances matrix between keys and queries.

An XCiT Layer is the main building block of the XCiT architecture which uses a cross-covariance attention operator as its principal operation. The XCiT layer consists of three main blocks, each preceded by LayerNorm and followed by a residual connection: (i) the core cross-covariance attention (XCA) operation, (ii) the local patch interaction (LPI) module, and (iii) a feed-forward network (FFN). By transposing the query-key interaction, the computational complexity of XCA is linear in the number of data elements N, rather than quadratic as in conventional self-attention.

Adaptive Non-Maximum Suppression is a non-maximum suppression algorithm that applies a dynamic suppression threshold to an instance according to the target density. The motivation is to find an NMS algorithm that works well for pedestrian detection in a crowd. Intuitively, a high NMS threshold keeps more crowded instances while a low NMS threshold wipes out more false positives. The adaptive-NMS thus applies a dynamic suppression strategy, where the threshold rises as instances gather and occlude each other and decays when instances appear separately. To this end, an auxiliary and learnable sub-network is designed to predict the adaptive NMS threshold for each instance.

Attention-augmented Convolution is a type of convolution with a two-dimensional relative self-attention mechanism that can replace convolutions as a stand-alone computational primitive for image classification. It employs scaled-dot product attention and multi-head attention as with Transformers. It works by concatenating convolutional and attentional feature map. To see this, consider an original convolution operator with kernel size $k$, $F\_{in}$ input filters and $F\_{out}$ output filters. The corresponding attention augmented convolution can be written as" $$\text{AAConv}\left(X\right) = \text{Concat}\left[\text{Conv}(X), \text{MHA}(X)\right] $$ $X$ originates from an input tensor of shape $\left(H, W, F\_{in}\right)$. This is flattened to become $X \in \mathbb{R}^{HW \times F\_{in}}$ which is passed into a multi-head attention module, as well as a convolution (see above). Similarly to the convolution, the attention augmented convolution 1) is equivariant to translation and 2) can readily operate on inputs of different spatial dimensions.

CBNet is a backbone architecture that consists of multiple identical backbones (specially called Assistant Backbones and Lead Backbone) and composite connections between neighbor backbones. From left to right, the output of each stage in an Assistant Backbone, namely higher-level features, flows to the parallel stage of the succeeding backbone as part of inputs through composite connections. Finally, the feature maps of the last backbone named Lead Backbone are used for object detection. The features extracted by CBNet for object detection fuse the high-level and low-level features of multiple backbones, hence improve the detection performance.

CSPResNeXt is a convolutional neural network where we apply the Cross Stage Partial Network (CSPNet) approach to ResNeXt. The CSPNet partitions the feature map of the base layer into two parts and then merges them through a cross-stage hierarchy. The use of a split and merge strategy allows for more gradient flow through the network.

CSPResNeXt Block is an extended ResNext Block where we partition the feature map of the base layer into two parts and then merge them through a cross-stage hierarchy. The use of a split and merge strategy allows for more gradient flow through the network.

Represent and associate with a composite of primitive fields.

CurricularFace, or Adaptive Curriculum Learning, is a method for face recognition that embeds the idea of curriculum learning into the loss function to achieve a new training scheme. This training scheme mainly addresses easy samples in the early training stage and hard ones in the later stage. Specifically, CurricularFace adaptively adjusts the relative importance of easy and hard samples during different training stages.

DetNAS is a neural architecture search algorithm for the design of better backbones for object detection. It is based on the technique of one-shot supernet, which contains all possible networks in the search space. The supernet is trained under the typical detector training schedule: ImageNet pre-training and detection fine-tuning. Then, the architecture search is performed on the trained supernet, using the detection task as the guidance. DetNAS uses evolutionary search as opposed to RL-based methods or gradient-based methods.

Dynamic R-CNN is an object detection method that adjusts the label assignment criteria (IoU threshold) and the shape of regression loss function (parameters of Smooth L1 Loss) automatically based on the statistics of proposals during training. The motivation is that in previous two-stage object detectors, there is an inconsistency problem between the fixed network settings and the dynamic training procedure. For example, the fixed label assignment strategy and regression loss function cannot fit the distribution change of proposals and thus are harmful to training high quality detectors. It consists of two components: Dynamic Label Assignment and Dynamic Smooth L1 Loss, which are designed for the classification and regression branches, respectively. For Dynamic Label Assignment, we want our model to be discriminative for high IoU proposals, so we gradually adjust the IoU threshold for positive/negative samples based on the proposals distribution in the training procedure. Specifically, we set the threshold as the IoU of the proposal at a certain percentage since it can reflect the quality of the overall distribution. For Dynamic Smooth L1 Loss, we want to change the shape of the regression loss function to adaptively fit the distribution change of error and ensure the contribution of high quality samples to training. This is achieved by adjusting the $\beta$ in Smooth L1 Loss based on the error distribution of the regression loss function, in which $\beta$ actually controls the magnitude of the gradient of small errors.

An EESP Unit, or Extremely Efficient Spatial Pyramid of Depth-wise Dilated Separable Convolutions, is an image model block designed for edge devices. It was proposed as part of the ESPNetv2 CNN architecture. This building block is based on a reduce-split-transform-merge strategy. The EESP unit first projects the high-dimensional input feature maps into low-dimensional space using groupwise pointwise convolutions and then learns the representations in parallel using depthwise dilated separable convolutions with different dilation rates. Different dilation rates in each branch allow the EESP unit to learn the representations from a large effective receptive field. To remove the gridding artifacts caused by dilated convolutions, the EESP fuses the feature maps using hierarchical feature fusion (HFF).

FreeAnchor is an anchor supervision method for object detection. Many CNN-based object detectors assign anchors for ground-truth objects under the restriction of object-anchor Intersection-over-Unit (IoU). In contrast, FreeAnchor is a learning-to-match approach that breaks the IoU restriction, allowing objects to match anchors in a flexible manner. It updates hand-crafted anchor assignment to free anchor matching by formulating detector training as a maximum likelihood estimation (MLE) procedure. FreeAnchor targets at learning features which best explain a class of objects in terms of both classification and localization.

GridMask is a data augmentation method that randomly removes some pixels of an input image. Unlike other methods, the region that the algorithm removes is neither a continuous region nor random pixels in dropout. Instead, the algorithm removes a region with disconnected pixel sets, as shown in the Figure. We express the setting as $$ \tilde{\mathbf{x}}=\mathbf{x} \times M $$ where $\mathbf{x} \in R^{H \times W \times C}$ represents the input image, $M \in$ $\{0,1\}^{H \times W}$ is the binary mask that stores pixels to be removed, and $\tilde{\mathbf{x}} \in R^{H \times W \times C}$ is the result produced by the algorithm. For the binary mask $M$, if $M_{i, j}=1$ we keep pixel $(i, j)$ in the input image; otherwise we remove it. GridMask is applied after the image normalization operation. The shape of $M$ looks like a grid, as shown in the Figure . Four numbers $\left(r, d, \delta_{x}, \delta_{y}\right)$ are used to represent a unique $M$. Every mask is formed by tiling the units. $r$ is the ratio of the shorter gray edge in a unit. $d$ is the length of one unit. $\delta\_{x}$ and $\delta\_{y}$ are the distances between the first intact unit and boundary of the image.

Guided Anchoring is an anchoring scheme for object detection which leverages semantic features to guide the anchoring. The method is motivated by the observation that objects are not distributed evenly over the image. The scale of an object is also closely related to the imagery content, its location and geometry of the scene. Following this intuition, the method generates sparse anchors in two steps: first identifying sub-regions that may contain objects and then determining the shapes at different locations.

IoU-Net is an object detection architecture that introduces localization confidence. IoU-Net learns to predict the IoU between each detected bounding box and the matched ground-truth. The network acquires this confidence of localization, which improves the NMS procedure by preserving accurately localized bounding boxes. Furthermore, an optimization-based bounding box refinement method is proposed, where the predicted IoU is formulated as the objective.

Primal Wasserstein Imitation Learning, or PWIL, is a method for imitation learning which ties to the primal form of the Wasserstein distance between the expert and the agent state-action distributions. The reward function is derived offline, as opposed to recent adversarial IL algorithms that learn a reward function through interactions with the environment, and requires little fine-tuning.

This method proposes first discretizing observations and calculating the action distribution distance under comparable cases (intersection states).

PolarMask is an anchor-box free and single-shot instance segmentation method. Specifically, PolarMask takes an image as input and predicts the distance from a sampled positive location (ie a candidate object's center) with respect to the object's contour at each angle, and then assembles the predicted points to produce the final mask. There are several benefits to the system: (1) The polar representation unifies instance segmentation (masks) and object detection (bounding boxes) into a single framework (2) Two modules are designed (i.e. soft polar centerness and polar IoU loss) to sample high-quality center examples and optimize polar contour regression, making the performance of PolarMask does not depend on the bounding box prediction results and more efficient in training. (3) PolarMask is fully convolutional and can be embedded into most off-the-shelf detection methods.

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TridentNet is an object detection architecture that aims to generate scale-specific feature maps with a uniform representational power. A parallel multi-branch architecture is constructed in which each branch shares the same transformation parameters but with different receptive fields. A scale-aware training scheme is used to specialize each branch by sampling object instances of proper scales for training.

U2-Net is a two-level nested U-structure architecture that is designed for salient object detection (SOD). The architecture allows the network to go deeper, attain high resolution, without significantly increasing the memory and computation cost. This is achieved by a nested U-structure: on the bottom level, with a novel ReSidual U-block (RSU) module, which is able to extract intra-stage multi-scale features without degrading the feature map resolution; on the top level, there is a U-Net like structure, in which each stage is filled by a RSU block.

ZoomNet is a 2D human whole-body pose estimation technique. It aims to localize dense landmarks on the entire human body including face, hands, body, and feet. ZoomNet follows the top-down paradigm. Given a human bounding box of each person, ZoomNet first localizes the easy-to-detect body keypoints and estimates the rough position of hands and face. Then it zooms in to focus on the hand/face areas and predicts keypoints using features with higher resolution for accurate localization. Unlike previous approaches which usually assemble multiple networks, ZoomNet has a single network that is end-to-end trainable. It unifies five network heads including the human body pose estimator, hand and face detectors, and hand and face pose estimators into a single network with shared low-level features.

ACKTR, or Actor Critic with Kronecker-factored Trust Region, is an actor-critic method for reinforcement learning that applies trust region optimization using a recently proposed Kronecker-factored approximation to the curvature. The method extends the framework of natural policy gradient and optimizes both the actor and the critic using Kronecker-factored approximate curvature (K-FAC) with trust region.

Bi3D is a stereo depth estimation framework that estimates depth via a series of binary classifications. Rather than testing if objects are at a particular depth D, as existing stereo methods do, it classifies them as being closer or farther than D. It takes the stereo pair and a disparity $d\_{i}$ and produces a confidence map, which can be thresholded to yield the binary segmentation. To estimate depth on $N + 1$ quantization levels we run this network $N$ times and maximize the probability in Equation 8 (see paper). To estimate continuous depth, whether full or selective, we run the SegNet block of Bi3DNet for each disparity level and work directly on the confidence volume.

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Context-Aware Visual Attention-based end-to-end pipeline for Webpage Object Detection (CoVA) aims to learn function f to predict labels y = [$y_1, y_2, ..., y_N$] for a webpage containing N elements. The input to CoVA consists of: 1. a screenshot of a webpage, 2. list of bounding boxes [x, y, w, h] of the web elements, and 3. neighborhood information for each element obtained from the DOM tree. This information is processed in four stages: 1. the graph representation extraction for the webpage, 2. the Representation Network (RN), 3. the Graph Attention Network (GAT), and 4. a fully connected (FC) layer. The graph representation extraction computes for every web element i its set of K neighboring web elements $N_i$. The RN consists of a Convolutional Neural Net (CNN) and a positional encoder aimed to learn a visual representation $v_i$ for each web element i ∈ {1, ..., N}. The GAT combines the visual representation $v_i$ of the web element i to be classified and those of its neighbors, i.e., $v_k$ ∀k ∈ $N_i$ to compute the contextual representation $c_i$ for web element i. Finally, the visual and contextual representations of the web element are concatenated and passed through the FC layer to obtain the classification output.

DIoU-NMS is a type of non-maximum suppression where we use Distance IoU rather than regular DIoU, in which the overlap area and the distance between two central points of bounding boxes are simultaneously considered when suppressing redundant boxes. In original NMS, the IoU metric is used to suppress the redundant detection boxes, where the overlap area is the unique factor, often yielding false suppression for the cases with occlusion. With DIoU-NMS, we not only consider the overlap area but also central point distance between two boxes.

Deformable ConvNets do not learn an affine transformation. They divide convolution into two steps, firstly sampling features on a regular grid $ \mathcal{R} $ from the input feature map, then aggregating sampled features by weighted summation using a convolution kernel. The process can be written as: \begin{align} Y(p_{0}) &= \sum_{p_i \in \mathcal{R}} w(p_{i}) X(p_{0} + p_{i}) \end{align} \begin{align} \mathcal{R} &= {(-1,-1), (-1, 0), \dots, (1, 1)} \end{align} The deformable convolution augments the sampling process by introducing a group of learnable offsets $\Delta p_{i}$ which can be generated by a lightweight CNN. Using the offsets $\Delta p_{i}$, the deformable convolution can be formulated as: \begin{align} Y(p_{0}) &= \sum_{p_i \in \mathcal{R}} w(p_{i}) X(p_{0} + p_{i} + \Delta p_{i}). \end{align} Through the above method, adaptive sampling is achieved. However, $\Delta p_{i}$ is a floating point value unsuited to grid sampling. To address this problem, bilinear interpolation is used. Deformable RoI pooling is also used, which greatly improves object detection. Deformable ConvNets adaptively select the important regions and enlarge the valid receptive field of convolutional neural networks; this is important in object detection and semantic segmentation tasks.

ExtremeNet is a a bottom-up object detection framework that detects four extreme points (top-most, left-most, bottom-most, right-most) of an object. It uses a keypoint estimation framework to find extreme points, by predicting four multi-peak heatmaps for each object category. In addition, it uses one heatmap per category predicting the object center, as the average of two bounding box edges in both the x and y dimension. We group extreme points into objects with a purely geometry-based approach. We group four extreme points, one from each map, if and only if their geometric center is predicted in the center heatmap with a score higher than a pre-defined threshold, We enumerate all $O\left(n^{4}\right)$ combinations of extreme point prediction, and select the valid ones.

Feature Fusion Module v1 is a feature fusion module from the M2Det object detection model, and feature fusion modules are crucial for constructing the final multi-level feature pyramid. They use 1x1 convolution layers to compress the channels of the input features and use concatenation operation to aggregate these feature map. FFMv1 takes two feature maps with different scales in backbone as input, it adopts one upsample operation to rescale the deep features to the same scale before the concatenation operation.