Precise RoI Pooling with coordinate gradient support, proposed in the paper "Acquisition of Localization Confidence for Accurate Object Detection" (https://arxiv.org/abs/1807.11590).
Modern CNN-based object detectors rely on bounding box regression and non-maximum suppression to localize objects. While the probabilities for class labels naturally reflect classification confidence, localization confidence is absent. This makes properly localized bounding boxes degenerate during iterative regression or even suppressed during NMS. In the paper we propose IoU-Net learning 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. Extensive experiments on the MS-COCO dataset show the effectiveness of IoU-Net, as well as its compatibility with and adaptivity to several state-of-the-art object detectors.READ FULL TEXT VIEW PDF
Non-maximum suppression (NMS) is essential for state-of-the-art object
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Many recently developed object detectors focused on coarse-to-fine frame...
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Object detection has been applied in a wide variety of real world scenar...
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Precise RoI Pooling with coordinate gradient support, proposed in the paper "Acquisition of Localization Confidence for Accurate Object Detection" (https://arxiv.org/abs/1807.11590).
Classification and localization are solved differently in such detection pipeline. Specifically, given a proposal, while the probability for each class label naturally acts as an “classification confidence” of the proposal, the bounding box regression module finds the optimal transformation for the proposal to best fit the ground-truth. However, the “localization confidence” is absent in the loop.
This brings about two drawbacks. (1) First, the suppression of duplicated detections is ignorant of the localization accuracy while the classification scores are typically used as the metric for ranking the proposals. In Figure 1(a), we show a set of cases where the detected bounding boxes with higher classification confidences contrarily have smaller overlaps with the corresponding ground-truth. Analog to Gresham’s saying that bad money drives out good, the misalignment between classification confidence and localization accuracy may lead to accurately localized bounding boxes being suppressed by less accurate ones in the NMS procedure. (2) Second, the absence of localization confidence makes the widely-adopted bounding box regression less interpretable. As an example, previous works  report the non-monotonicity of iterative bounding box regression. That is, bounding box regression may degenerate the localization of input bounding boxes if applied for multiple times (shown as Figure 1(b)).
In this paper we introduce IoU-Net, which predicts the IoU between detected bounding boxes and their corresponding ground-truth boxes, making the networks aware of the localization criterion analog to the classification module. This simple coefficient provides us with new solutions to the aforementioned problems:
IoU is a natural criterion for localization accuracy. We can replace classification confidence with the predicted IoU as the ranking keyword in NMS. This technique, namely IoU-guided NMS, help to eliminate the suppression failure caused by the misleading classification confidences.
We present an optimization-based bounding box refinement procedure on par with the traditional regression-based methods. During the inference, the predicted IoU is used as the optimization objective, as well as an interpretable indicator of the localization confidence. The proposed Precise RoI Pooling layer enables us to solve the IoU optimization by gradient ascent. We show that compared with the regression-based method, the optimization-based bounding box refinement empirically provides a monotonic improvement on the localization accuracy. The method is fully compatible with and can be integrated into various CNN-based detectors [16, 3, 10].
First of all, we explore two drawbacks in object localization: the misalignment between classification confidence and localization accuracy and the non-monotonic bounding box regression. A standard FPN  detector is trained on MS-COCO trainval35k as the baseline and tested on minival for the study.
With the objective to remove duplicated bounding boxes, NMS has been an indispensable component in most object detectors since . NMS works in an iterative manner. At each iteration, the bounding box with the maximum classification confidence is selected and its neighboring boxes are eliminated using a predefined overlapping threshold. In Soft-NMS  algorithm, box elimination is replaced by the decrement of confidence, leading to a higher recall. Recently, a set of learning-based algorithms have been proposed as alternatives to the parameter-free NMS and Soft-NMS.  calculates an overlap matrix of all bounding boxes and performs affinity propagation clustering to select exemplars of clusters as the final detection results.  proposes the GossipNet, a post-processing network trained for NMS based on bounding boxes and the classification confidence.  proposes an end-to-end network learning the relation between detected bounding boxes. However, these parameter-based methods require more computational resources which limits their real-world application.
In the widely-adopted NMS approach, the classification confidence is used for ranking bounding boxes, which can be problematic. We visualize the distribution of classification confidences of all detected bounding boxes before NMS, as shown in Figure 2(a). The x-axis is the IoU between the detected box and its matched ground-truth, while the y-axis denotes its classification confidence. The Pearson correlation coefficient indicates that the localization accuracy is not well correlated with the classification confidence.
We attribute this to the objective used by most of the CNN-based object detectors in distinguishing foreground (positive) samples from background (negative) samples. A detected bounding box is considered positive during training if its IoU with one of the ground-truth bounding box is greater than a threshold . This objective can be misaligned with the localization accuracy. Figure 1(a) shows cases where bounding boxes having higher classification confidence have poorer localization.
Recall that in traditional NMS, when there exists duplicated detections for a single object, the bounding box with maximum classification confidence will be preserved. However, due to the misalignment, the bounding box with better localization will probably get suppressed during the NMS, leading to the poor localization of objects. Figure 3 quantitatively shows the number of positive bounding boxes after NMS. The bounding boxes are grouped by their IoU with the matched ground-truth. For multiple detections matched with the same ground-truth, only the one with the highest IoU is considered positive. Therefore, No-NMS could be considered as the upper-bound for the number of positive bounding boxes. We can see that the absence of localization confidence makes more than half of detected bounding boxes with IoU being suppressed in the traditional NMS procedure, which degrades the localization quality of the detection results.
In general, single object localization can be classified into two categories: bounding box-based methods and segment-based methods. The segment-based methods[19, 20, 13, 10] aim to generate a pixel-level segment for each instance but inevitably require additional segmentation annotation. This work focuses on the bounding box-based methods.
Single object localization is usually formulated as a bounding box regression task. The core idea is that a network directly learns to transform (i.e., scale or shift) a bounding box to its designated target. In [9, 8]linear regression or fully-connected layer is applied to refine the localization of object proposals generated by external pre-processing modules (e.g., Selective Search  or EdgeBoxes ). Faster R-CNN  proposes region proposal network (RPN) in which only predefined anchors are used to train an end-to-end object detector. [14, 32] utilize anchor-free, fully-convolutional networks to handle object scale variation. Meanwhile, Repulsion Loss is proposed in  to robustly detect objects with crowd occlusion. Due to its effectiveness and simplicity, bounding box regression has become an essential component in most CNN-based detectors.
A broad set of downstream applications such as tracking and recognition will benefit from accurately localized bounding boxes. This raises the demand for improving localization accuracy. In a series of object detectors [31, 7, 6, 21], refined boxes will be fed to the bounding box regressor again and go through the refinement for another time. This procedure is performed for several times, namely iterative bounding box regression. Faster R-CNN  first performs the bounding box regression twice to transform predefined anchors into final detected bounding boxes.  proposes a group recursive learning approach to iteratively refine detection results and minimize the offsets between object proposals and the ground-truth considering the global dependency among multiple proposals. G-CNN is proposed in  which starts with a multi-scale regular grid over the image and iteratively pushes the boxes in the grid towards the ground-truth. However, as reported in , applying bounding box regression more than twice brings no further improvement.  attribute this to the distribution mismatch in multi-step bounding box regression and address it by a resampling strategy in multi-stage bounding box regression.
We experimentally show the performance of iterative bounding box regression based on FPN and Cascade R-CNN frameworks. The Average Precision (AP) of the results after each iteration are shown as the blue curves in Figure 4(a) and Figure 4(b), respectively. The AP curves in Figure 4 show that the improvement on localization accuracy, as the number of iterations increase, is non-monotonic for iterative bounding box regression. The non-monotonicity, together with the non-interpretability, brings difficulties in applications. Besides, without localization confidence for detected bounding boxes, we can not have fine-grained control over the refinement, such as using an adaptive number of iterations for different bounding boxes.
To quantitatively analyze the effectiveness of IoU prediction, we first present the methodology adopted for training an IoU predictor in Section 3.1. In Section 3.2 and Section 3.3, we show how to use IoU predictor for NMS and bounding box refinement, respectively. Finally in Section 3.4 we integrate the IoU predictor into existing object detectors such as FPN .
Shown in Figure 5
, the IoU predictor takes visual features from the FPN and estimates the localization accuracy (IoU) for each bounding box. We generate bounding boxes and labels for training the IoU-Net by augmenting the ground-truth, instead of taking proposals from RPNs. Specifically, for all ground-truth bounding boxes in the training set, we manually transform them with a set of randomized parameters, resulting in a candidate bounding box set. We then remove from this candidate set the bounding boxes having an IoU less thanwith the matched ground-truth. We uniformly sample training data from this candidate set w.r.t. the IoU. This data generation process empirically brings better performance and robustness to the IoU-Net. For each bounding box, the features are extracted from the output of FPN with the proposed Precise RoI Pooling layer (see Section 3.3). The features are then fed into a two-layer feedforward network for the IoU prediction. For a better performance, we use class-aware IoU predictors.
The IoU predictor is compatible with most existing RoI-based detectors. The accuracy of a standalone IoU predictor can be found in Figure 2. As the training procedure is independent of specific detectors, it is robust to the change of the input distributions (e.g., when cooperates with different detectors). In later sections, we will further demonstrate how this module can be jointly optimized in a full detection pipeline (i.e., jointly with RPNs and R-CNN).
We resolve the misalignment between classification confidence and localization accuracy with a novel IoU-guided NMS procedure, where the classification confidence and localization confidence (an estimation of the IoU) are disentangled. In short, we use the predicted IoU instead of the classification confidence as the ranking keyword for bounding boxes. Analog to the traditional NMS, the box having the highest IoU with a ground-truth will be selected to eliminate all other boxes having an overlap greater than a given threshold . To determine the classification scores, when a box eliminates box , we update the classification confidence of box by . This procedure can also be interpreted as a confidence clustering: for a group of bounding boxes matching the same ground-truth, we take the most confident prediction for the class label. A psuedo-code for this algorithm can be found in Algorithm 1.
IoU-guided NMS resolves the misalignment between classification confidence and localization accuracy. Quantitative results show that our method outperforms traditional NMS and other variants such as Soft-NMS . Using IoU-guided NMS as the post-processor further pushes forward the performance of several state-of-the-art object detectors.
The problem of bounding box refinement can formulated mathematically as finding the optimal st:
where is the detected bounding box, is a (targeting) ground-truth bounding box and transform is a bounding box transformation function taking as parameter and transform the given bounding box. is a criterion measuring the distance between two bounding boxes. In the original Fast R-CNN  framework, is chosen as an smooth-L1 distance of coordinates in log-scale, while in , is chosen as the between two bounding boxes.
Regression-based algorithms directly estimate the optimal solution
with a feed-forward neural network. However, iterative bounding box regression methods are vulnerable to the change in the input distribution and may result in non-monotonic localization improvement, as shown in Figure 4. To tackle these issues, we propose an optimization-based bounding box refinement method utilizing IoU-Net as a robust localization accuracy (IoU) estimator. Furthermore, IoU estimator can be used as an early-stop condition to implement iterative refinement with adaptive steps.
IoU-Net directly estimates . While the proposed Precise RoI Pooling layer enables the computation of the gradient of IoU w.r.t. bounding box coordinates111We prefer Precise RoI-Pooling layer to RoI-Align layer  as Precise RoI-Pooling layer is continuously differentiable w.r.t. the coordinates while RoI-Align is not., we can directly use gradient ascent method to find the optimal solution to Equation 1. Shown in Algorithm 2, viewing the estimation of the IoU as an optimization objective, we iteratively refine the bounding box coordinates with the computed gradient and maximize the IoU between the detected bounding box and its matched ground-truth. Besides, the predicted IoU is an interpretable indicator of the localization confidence on each bounding box and helps explain the performed transformation.
In the implementation, shown in Algorithm 2 Line 6, we manually scale up the gradient w.r.t. the coordinates with the size of the bounding box on that axis (e.g., we scale up with ). This is equivalent to perform the optimization in log-scaled coordinates () as in . We also employ a one-step bounding box regression for an initialization of the coordinates.
Precise RoI Pooling. We introduce Precise RoI Pooling (PrRoI Pooling, for short) powering our bounding box refinement222The code is released at: https://github.com/vacancy/PreciseRoIPooling. It avoids any quantization of coordinates and has a continuous gradient on bounding box coordinates. Given the feature map before RoI/PrRoI Pooling (e.g. from Conv4 in ResNet-50), let be the feature at one discrete location
on the feature map. Using bilinear interpolation, the discrete feature map can be considered continuous at any continuous coordinates:
where is the interpolation coefficient. Then denote a bin of a RoI as , where and are the continuous coordinates of the top-left and bottom-right points, respectively. We perform pooling (e.g., average pooling) given and feature map by computing a two-order integral:
For a better understanding, we visualize RoI Pooling, RoI Align  and our PrRoI Pooing in Figure 6: in the traditional RoI Pooling, the continuous coordinates need to be quantized first to calculate the sum of the activations in the bin; to eliminate the quantization error, in RoI Align, continuous points are sampled in the bin, denoted as , and the pooling is performed over the sampled points. Contrary to RoI Align where is pre-defined and not adaptive w.r.t. the size of the bin, the proposed PrRoI pooling directly compute the two-order integral based on the continuous feature map.
Moreover, based on the formulation in Equation 3, is differentiable w.r.t. the coordinates of . For example, the partial derivative of w.r.t. could be computed as:
The partial derivative of w.r.t. other coordinates can be computed in the same manner. Since we avoids any quantization, is continuously differentiable.
The IoU predictor can be integrated into standard FPN pipelines for end-to-end training and inference. For clarity, we denote backbone
as the CNN architecture for image feature extraction andhead as the modules applied to individual RoIs.
Shown in Figure 5, the IoU-Net uses ResNet-FPN  as the backbone, which has a top-down architecture to build a feature pyramid. FPN extracts features of RoIs from different levels of the feature pyramid according to their scale. The original RoI Pooling layer is replaced by the Precise RoI Pooling layer. As for the network head, the IoU predictor works in parallel with the R-CNN branch (including classification and bounding box regression) based on the same visual feature from the backbone.
We initialize weights from pre-trained ResNet models on ImageNet
. All new layers are initialized with a zero-mean Gaussian with standard deviation 0.01 or 0.001. We use smooth-L1 loss for training the IoU predictor. The training data for the IoU predictor is separately generated as described in Section3.1 within images in a training batch. IoU labels are normalized s.t. the values are distributed over .
Input images are resized to have 800px along the short axis and a maximum of 1200px along the long axis. The classification and regression branch take 512 RoIs per image from RPNs. We use a batch size 16 for the training. The network is optimized for 160k iterations, with a learning rate of 0.01 which is decreased by a factor of 10 after 120k iterations. We also warm up the training by setting the learning rate to 0.004 for the first 10k iteration. We use a weight decay of 1e-4 and a momentum of 0.9.
During inference, we first apply bounding box regression for the initial coordinates. To speed up the inference, we first apply IoU-guided NMS on all detected bounding boxes. 100 bounding boxes with highest classification confidence are further refined using the optimization-based algorithm. We set as the step size, as the early-stop threshold, as the localization degeneration tolerance and as the number of iterations.
We perform experiments on the 80-category MS-COCO detection dataset . Following [1, 16], the models are trained on the union of 80k training images and 35k validation images (trainval35k) and evaluated on a set of 5k validation images (minival). To validate the proposed methods, in both Section 4.1 and 4.2, a standalone IoU-Net (without R-CNN modules) is trained separately with the object detectors. IoU-guided NMS and optimization-based bounding box refinement, powered by the IoU-Net, are applied to the detection results.
Table 1 summarizes the performance of different NMS methods. While Soft-NMS preserve more bounding boxes (there is no real “suppression”), IoU-guided NMS improves the results by improving the localization of the detected bounding boxes. As a result, IoU-guided NMS performs significantly better than the baselines on high IoU metrics (e.g., AP).
We delve deeper into the behavior of different NMS algorithms by analyzing their recalls at different IoU threshold. The raw detected bounding boxes are generated by a ResNet50-FPN without any NMS. As the requirement of localization accuracy increases, the performance gap between IoU-guided NMS and other methods goes larger. In particular, the recall at matching IoU drops to 18.7% after traditional NMS, while the IoU-NMS reaches 28.9% and the No-NMS “upper bound” is 39.7%.
The proposed optimization-based bounding box refinement is compatible with most of the CNN-based object detectors [16, 3, 10], as shown in Table 2. Applying the bounding box refinement after the original pipelines with the standalone IoU-Net further improve the performance by localizing object more accurately. The refinement further improves by and the overall AP by even for Cascade R-CNN which has a three-stage bounding box regressor.
IoU-Net can be end-to-end optimized in parallel with object detection frameworks. We find that adding IoU predictor to the network helps the network to learn more discriminative features which improves the overall AP by 0.6 and 0.4 percent for ResNet50-FPN and ResNet101-FPN respectively. The IoU-guided NMS and bounding box refinement further push the performance forward. We achieve AP with ResNet101-FPN compared to the baseline (improved by ). The inference speed is demonstrated in Table 3, showing that IoU-Net improves the detection performance with tolerable computation overhead.
We mainly attribute the inferior results on AP in Table 3
to the IoU estimation error. When the bounding boxes have a lower IoU with the ground-truth, they have a larger variance in appearance. Visualized in Figure2(b), the IoU estimation becomes less accurate for boxes with lower IoU. This degenerates the performance of the downstream refinement and suppression. We empirically find that this problem can be partially solved by techniques such as sampling more bounding boxes with lower IoU during the training.
In this paper, a novel network architecture, namely IoU-Net, is proposed for accurate object localization. By learning to predict the IoU with matched ground-truth, IoU-Net acquires “localization confidence” for the detected bounding box. This empowers an IoU-guided NMS procedure where accurately localized bounding boxes are prevented from being suppressed. The proposed IoU-Net is intuitive and can be easily integrated into a broad set of detection models to improve their localization accuracy. Experimental results on MS-COCO demonstrate its effectiveness and potential in practical applications.
This paper points out the misalignment of classification and localization confidences in modern detection pipelines. We also formulate an novel optimization view on the problem of bounding box refinement, and the proposed solution surpasses the regression-based methods. We hope these novel viewpoints provide insights to future works on object detection, and beyond.
Bell, S., Lawrence Zitnick, C., Bala, K., Girshick, R.: Inside-outside net: Detecting objects in context with skip pooling and recurrent neural networks. In: Proceedings of the IEEE Conference on Computer Vision and Pattern Recognition. pp. 2874–2883 (2016)