# Object Detection Network Based on Feature Fusion and Attention Mechanism
## 1 Introduction  
Recently, object detection accuracy has been improved by using deep CNN (Convolutional Neural Network) nets [1]. There are two types of object detection methods: Region-based object detection and regression-based object detection. Representative algorithms of regression-based object detection are R-CNN (region with CNN features) [2], SPP-net [3], Fast R-CNN [4], and Faster R-CNN [5]. Faster R-CNN is an end-to-end object detection network because it combines region proposal and detection into a unified network. The object detection networks have three essential components called the feature extraction net, region proposal network, and classification and regression. Object detection always uses deep CNN nets (VGG-16(very deep convolutional networks)) [1], ResNet [6]), to extract CNN features due to the discriminative representations of CNN features. Deep CNN features contain high-level semantic information, which can better detect objects. However, deep CNN features lose a lot of detailed texture information because of high abstraction. Object detection is a more challenging task when objects are small. Object detection is difficult to achieve the desired effect due to illumination variations, occlusions, and a complex background. As mentioned above, features are crucial for object detection. Object detection often uses the last CNN feature maps for region proposal net. In [7], the author combined high layer information with low layer information for semantic segmentation. HyperNet [8], which combines the different CNN features, achieved thedesired effect accuracy of object detection. The proper fusion of CNN features is more suitable for proposal generation and detection. Recently, many studies have hoped to add attention mechanisms to deep networks. The attention model in deep learning actually simulates the attention model of the human brain. In 2015, [PERSON] et al. [9] introduced the attention in image caption. The visual attention mechanism can quickly locate the region of interest in the complex scene, finding the target of interest, and selectively ignoring the region of no interest. The visual attention mechanism is divided into two ways: Bottom-up model, which is also called hard attention, and top-down model, which is called soft attention. [PERSON] et al. [10] applied hard attention to image caption. However, more research and applications are still more inclined to use soft attention because it can be directly derived for gradient back propagation. The paper [11] proposed the soft attention model and applied it to machine translation. The traditional attention mechanism is soft attention [11; 12], which is obtained by deterministic score calculation to obtain the coded hidden state after the attainment. Soft attention is parameterized and it can be guided and embedded in the model for direct training. The gradient can be passed back through the attention mechanism module to other parts of the model. Object detection is difficult due to the complex background. Pixels are treated the same in the CNN feature maps and the region proposal net. The object is easily disturbed by the background, leading to inaccurate detection. Object detection networks need to enhance the impact of areas of interest and weaken interference from an unrelated background. So, we will apply the attention to the target detection depth network. In this paper, we use the faster R-CNN object detection network as a framework. Deep network VGG-16 is still used in the feature extraction part. We propose a new object detection network that fixes the disadvantages of feature fusion and the attention mechanism in the faster R-CNN in case of background interference and small target problems. The improved object detection network (we called this AF R-CNN) is also an end-to-end network. Our main contributions are two-fold:  
1. Feature fusion: We fuse the 3, 4, 5 layers of CNN feature maps by maximum pooling conv-3 and deconvoluting conv-5, which was extracted from VGG-16.