ultralytics/docs/yolov5/tutorials/architecture_description.md

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1. Model Structure

YOLOv5 (v6.0/6.1) consists of:

• Backbone: New CSP-Darknet53
• Neck: SPPF, New CSP-PAN
• Head: YOLOv3 Head

Model structure (yolov5l.yaml):

Some minor changes compared to previous versions:

1. Replace the Focus structure with 6x6 Conv2d(more efficient, refer #4825)
2. Replace the SPP structure with SPPF(more than double the speed)

test code

import time
import torch
import torch.nn as nn


class SPP(nn.Module):
    def __init__(self):
        super().__init__()
        self.maxpool1 = nn.MaxPool2d(5, 1, padding=2)
        self.maxpool2 = nn.MaxPool2d(9, 1, padding=4)
        self.maxpool3 = nn.MaxPool2d(13, 1, padding=6)

    def forward(self, x):
        o1 = self.maxpool1(x)
        o2 = self.maxpool2(x)
        o3 = self.maxpool3(x)
        return torch.cat([x, o1, o2, o3], dim=1)


class SPPF(nn.Module):
    def __init__(self):
        super().__init__()
        self.maxpool = nn.MaxPool2d(5, 1, padding=2)

    def forward(self, x):
        o1 = self.maxpool(x)
        o2 = self.maxpool(o1)
        o3 = self.maxpool(o2)
        return torch.cat([x, o1, o2, o3], dim=1)


def main():
    input_tensor = torch.rand(8, 32, 16, 16)
    spp = SPP()
    sppf = SPPF()
    output1 = spp(input_tensor)
    output2 = sppf(input_tensor)

    print(torch.equal(output1, output2))

    t_start = time.time()
    for _ in range(100):
        spp(input_tensor)
    print(f"spp time: {time.time() - t_start}")

    t_start = time.time()
    for _ in range(100):
        sppf(input_tensor)
    print(f"sppf time: {time.time() - t_start}")


if __name__ == '__main__':
    main()

result:

True
spp time: 0.5373051166534424
sppf time: 0.20780706405639648

2. Data Augmentation

• Mosaic

• Copy paste

• Random affine(Rotation, Scale, Translation and Shear)

• MixUp

• Albumentations

• Augment HSV(Hue, Saturation, Value)

• Random horizontal flip

3. Training Strategies

• Multi-scale training(0.5~1.5x)
• AutoAnchor(For training custom data)
• Warmup and Cosine LR scheduler
• EMA(Exponential Moving Average)
• Mixed precision
• Evolve hyper-parameters

4. Others

4.1 Compute Losses

The YOLOv5 loss consists of three parts:

• Classes loss(BCE loss)
• Objectness loss(BCE loss)
• Location loss(CIoU loss)

4.2 Balance Losses

The objectness losses of the three prediction layers(P3, P4, P5) are weighted differently. The balance weights are [4.0, 1.0, 0.4] respectively.

4.3 Eliminate Grid Sensitivity

In YOLOv2 and YOLOv3, the formula for calculating the predicted target information is:

In YOLOv5, the formula is:

Compare the center point offset before and after scaling. The center point offset range is adjusted from (0, 1) to (-0.5, 1.5). Therefore, offset can easily get 0 or 1.

Compare the height and width scaling ratio(relative to anchor) before and after adjustment. The original yolo/darknet box equations have a serious flaw. Width and Height are completely unbounded as they are simply out=exp(in), which is dangerous, as it can lead to runaway gradients, instabilities, NaN losses and ultimately a complete loss of training. refer this issue

4.4 Build Targets

Match positive samples:

• Calculate the aspect ratio of GT and Anchor Templates

• Assign the successfully matched Anchor Templates to the corresponding cells

• Because the center point offset range is adjusted from (0, 1) to (-0.5, 1.5). GT Box can be assigned to more anchors.