融合超分辨率重建的YOLOv5松枯死木识别模型

    Recognition of dead pine trees using YOLOv5 by super-resolution reconstruction

    • 摘要: 为解决山地地形起伏大、无人机飞行高度高导致图像中尺度小且纹理模糊的松枯死木识别困难问题,该研究提出了一种在特征层级进行超分辨率重建的YOLOv5松枯死木识别算法。在YOLOv5网络中添加选择性核特征纹理迁移模块生成有细节纹理的高清检测特征图,自适应改变感受野的机制分配权重,将更多注意力集中在纹理细节,提升了小目标和模糊目标的识别精度。同时,使用前景背景平衡损失函数抑制背景噪声干扰,增加正样本的梯度贡献,改善正负样本分布不平衡问题。试验结果表明,改进后算法在交并比(intersection over union, IoU)阈值取0.5时的平均精度均值(mean average precision, mAP50)为92.7%,mAP50~95(以步长0.05从0.5到0.95间取IoU阈值下的平均mAP)为62.1%,APsmall(小目标平均精度值)为53.2%,相比于原算法mAP50提高了3.2个百分点,mAP50~95提升了8.3个百分点,APsmall提升了15.8个百分点。不同算法对比分析表明,该方法优于Faster R-CNN、YOLOv4、YOLOX、MT-YOLOv6、QueryDet、DDYOLOv5等深度学习算法,mAP50分别提高了16.7、15.3、2.5、2.8、12.3和1.2个百分点。改进后松枯死木识别算法具有较高精度,有效缓解了小目标与纹理模糊目标识别困难问题,为后续疫木清零提供技术支持。

       

      Abstract: Pine wilt disease has posed a significant threat to forest ecosystems, due to its highly contagious and destructive nature. The critical step in the prevention and control of pine wilt is eliminating the disease sources, which requires the accurate recognition and removal of dead pine trees. However, small or blurred targets are captured, such as overexposure, backlight, and samples occluded by foliage in practical applications. The reason is that the UAVs have to fly high for capture, due to the geography of the hilly and mountain areas. In this study, a novel You Look Only Once v5 (YOLOv5) algorithm was proposed for the recognition of dead pine trees. The super-resolution reconstruction was performed at the feature level, in order to overcome the challenge of recognizing such targets. The YOLOv5 structure was redesigned in two aspects. Firstly, the Selective Kernel Feature Texture Transfer (SKFTT) module was adopted to create the high-resolution detection feature maps with detailed textures, where improved detection accuracy was obtained for the small targets and blurred targets. Specifically, the feature maps with high texture were selected from the backbone network, whereas, the feature maps with high semantics were selected from the feature fusion network. These feature maps were then sent to the texture extractor and content extractor. A selective feature fusion module was used to fuse the critical information about different scales using their weights. Secondly, the Foreground Background Loss function (FB Loss) was introduced to attenuate useless features, while enhancing the gradient contribution of positive samples, and balancing the distribution of positive and negative samples, in order to supervise the reconstruction of high-resolution feature maps. Furthermore, the dataset was obtained to validate the effectiveness of the improved model from the approximately 15 400 hectares of forest land located in Fuzhou and Minhou City, Fujian Province, China. The UAV images were subsequently cropped and screened to obtain about 29 250 labelled samples for further experiments. A series of ablation tests and visualizations were conducted on the testing datasets to verify the effectiveness. Experimental results showed that the mean Average Precision (mAP50) of the improved model was 92.7%, mAP50~95 was 62.1%, and APsmall was 53.2%. Compared with the baseline model, the improved model was achieved in the increases of 3.2, 8.3, and 15.8 percentage points in the mAP50, mAP50~95, and mAPsmall, respectively. The mAP50 of the improved model was 16.7, 15.3, 2.5, 2.8, 12.3, and 1.2 percentage points higher than that of the Faster R-CNN, YOLOv4, YOLOX, MT-YOLOv6, QueryDet, and DDYOLOv5 networks, respectively. In addition, the improved model was achieved in the frames per second FPS of 37, which fully met the detection requirements of dead pine trees. Visualization results showed that the improved model can be expected to serve as the recognition of occlusion, overexposure, and backlight targets. The feature maps of the small target detection layer were visualized with different super-resolution algorithms to facilitate observation. The comparison revealed that the texture was improved with the apparently clear boundary shape. In conclusion, the detecting challenge of small targets and blurred targets can be effectively alleviated using the improved dead pine tree detection algorithm, due to the high accuracy. Therefore, the improved detection algorithm is conducive to the efficient removal and comprehensive prevention/control of diseased trees. The improved model can greatly contribute to accelerating the “digital forest prevention” process in precision agriculture.

       

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