Deep learning object detectors achieve state-of-the-art accuracy at the expense of high computational overheads, impeding their utilization on embedded systems such as drones. A primary source of these overheads is the exhaustive classification of typically 10^4-10^5 regions per image. Given that most of these regions contain uninformative background, the detector designs seem extremely superfluous and inefficient. In contrast, biological vision systems leverage selective attention for fast and efficient object detection. Recent neuroscientific findings shedding new light on the mechanism behind selective attention allowed us to formulate a new hypothesis of object detection efficiency and subsequently introduce a new object detection paradigm. To that end, we leverage this knowledge to design a novel region proposal network and empirically show that it achieves high object detection performance on the COCO dataset. Moreover, the model uses two to three orders of magnitude fewer computations than state-of-the-art models and consequently achieves inference speeds exceeding 500 frames s, thereby making it possible to achieve object detection on embedded systems.
Both one-stage and two-stage object detection methods typically evaluate @math candidate regions per image; densely covering many different spatial positions, scales, and aspect ratios. In the current state-of-the-art one-stage detector, RetinaNet @cite_33 , evaluation ( predicting the probability of object presence) of each of these regions is carried by a classification subnet, which is a fully-convolutional neural network comprising five convolutional layers, each with typically 256 filters and each followed by ReLU activations. Input images into these networks are typically re-scaled to @math pixels, from which @math candidate regions are individually evaluated. Therefore, significant computational costs are incurred. Consequently, the improved speed of one-stage detectors comes at a significant computational cost, which makes them impractical for embedded systems. Moreover, due to the extreme background-to-object class-imbalance in typical images, the exhaustive region classification design seems extremely superfluous and inefficient.
@cite_33
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The highest accuracy object detectors to date are based on a two-stage approach popularized by R-CNN, where a classifier is applied to a sparse set of candidate object locations. In contrast, one-stage detectors that are applied over a regular, dense sampling of possible object locations have the potential to be faster and simpler, but have trailed the accuracy of two-stage detectors thus far. In this paper, we investigate why this is the case. We discover that the extreme foreground-background class imbalance encountered during training of dense detectors is the central cause. We propose to address this class imbalance by reshaping the standard cross entropy loss such that it down-weights the loss assigned to well-classified examples. Our novel Focal Loss focuses training on a sparse set of hard examples and prevents the vast number of easy negatives from overwhelming the detector during training. To evaluate the effectiveness of our loss, we design and train a simple dense detector we call RetinaNet. Our results show that when trained with the focal loss, RetinaNet is able to match the speed of previous one-stage detectors while surpassing the accuracy of all existing state-of-the-art two-stage detectors. Code is at: this https URL