CNN-based object detection methods are directly applicable to static images. Moreover, many annotated video datasets of the target object are required to train both the object detectors and visual trackers. The proposed algorithms are tested using a drone detection task and the experimental results demonstrated their effectiveness. YOLO v4 algorithm is built on the darknet framework. The model is majorly based on the following three techniques:
A. Residual Blocks (ResNet Architecture)
A residual block is a stack of layers set in such a way that the output of a layer is taken and added to another layer deeper in the block.In this, each image/frame of a video is divided into multiple grids, and each grid detects the objects within them. If an object center appears within a certain grid cell, then that cell will be responsible for its detection.
B. Bounding Box Regression
A bounding box is an outline that highlights an object in an image. Every bounding box in the image consists of the following attributes: height, Width, Bounding Box Centre and Class. YOLO uses a single bounding box regression to predict all of these attributes. In the image above, it represents the probability of that object appearing in the bounding box. Bounding-box regression is a popular technique to refine or predict localization boxes in recent object detection approaches. Typically, bounding-box regressors are trained to regress from either region proposals or fixed anchor boxes to nearby bounding boxes of a pre-defined target object classes.
C. Intersection over Union (IoU)
It’s a term used to describe the extent of overlap of two boxes. From the image shown in Fig. 3, the green box is the correct box and the red box is the predicted box. The aim of all algorithms is to reduce the overlap as much as possible, to produce higher accuracy. The IOU of two boxes can have any values between 0 and 1. In case there are 2 boxes that do not intersect, the area of their intersection would be 0, and therefore the IOU would also be 0. In case there are 2 boxes that completely overlap, the area of the intersection would be equal to the area of their union, and therefore the IOU would be 1.