YOLOv5 version v1 is published on Jun 26, 2020. The latest version is v7, which is published on Nov 22, 2022. Yet, it seems that there is no paper or tech report provided talking about it.
(One way is to read the codes directly, which is cumbersome. Or finding blogs/vlogs which explains about it. To me, one of the famous blogs to explain YOLOv5 is from Zhihu, which is in Chinese language.)
(Thus, I decide to write one on my own so that I can easily review it again later. To do this, I’ve found some papers that enhance YOLOv5. Based on these papers, I summarize the basic YOLOv5 as below.)
The image was processed through a input layer (input) and sent to the backbone for feature extraction.
The backbone obtains feature maps of different sizes, and then fuses these features through the feature fusion network (neck) to finally generate three feature maps P3, P4, and P5 (in the YOLOv5, the dimensions are expressed with the size of 80×80, 40×40 and 20×20) to detect small, medium, and large objects in the picture, respectively.
After the three feature maps were sent to the prediction head (head), the confidence calculation and bounding-box regression were executed for each pixel in the feature map using the preset prior anchor, so as to obtain a multi-dimensional array (BBoxes) including object class, class confidence, box coordinates, width, and height information.
By setting the corresponding thresholds (confthreshold, objthreshold) to filter the useless information in the array, and performing a non-maximum suppression (NMS) process, the final detection information can be output.
The basic idea of FPN is to up-sampling the output feature map (C3, C4, and C5) generated by multiple convolution down sampling operations from the feature extraction network to generate multiple new feature maps (P3, P4, and P5) for detecting different scales targets.
4. YOLOv5: Head
YOLOv5: Head
Bounding Box Regression
The coordinate value of the upper left corner of the feature map is set to (0, 0).
rx and ry are the unadjusted coordinates of the predicted center point.
gx, gy, gw, gh represent the information of the adjusted prediction box.
pw and ph are for the information of the prior anchor.
sx and sy represent the offsets calculated by the model.
The process of adjusting the center coordinate and size of the preset prior anchor to the center coordinate and size of the final prediction box.
5. YOLOv5: Model Variants
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Five versions of YOLOv5
There are 5 versions of YOLOv5, namely YOLOv5x, YOLOv5l, YOLOv5m, YOLOv5s, and YOLOv5n.
There are also 5 corresponding larger versions, YOLOv5x6, YOLOv5l6, YOLOv5m6, YOLOv5s6, and YOLOv5n6.
