What's the Focus layer?

[maykulkarni]

I see a focus layer after the input

class Focus(nn.Module):
    # Focus wh information into c-space
    def __init__(self, c1, c2, k=1):
        super(Focus, self).__init__()
        self.conv = Conv(c1 * 4, c2, k, 1)

    def forward(self, x):  # x(b,c,w,h) -> y(b,4c,w/2,h/2)
        return self.conv(torch.cat([x[..., ::2, ::2],
                                    x[..., 1::2, ::2],
                                    x[..., ::2, 1::2],
                                    x[..., 1::2, 1::2]], 1))

Which transforms:

[[[[ 0,  1,  2,  3],
   [ 4,  5,  6,  7],
   [ 8,  9, 10, 11],
   [12, 13, 14, 15]]]]

to 

[[[[ 0,  2],
   [ 8, 10]],

  [[ 4,  6],
  [12, 14]],

  [[ 1,  3],
   [9, 11]],

  [[5,  7],
  [13, 15]]]]

Which sort of seems like a downsample, but why not use DownSample directly? What does this accomplish and is there any literature I can read about this technique?

[github-actions]

Hello @maykulkarni, thank you for your interest in our work! Please visit our Custom Training Tutorial to get started, and see our Jupyter Notebook , Docker Image, and Google Cloud Quickstart Guide for example environments.

If this is a bug report, please provide screenshots and minimum viable code to reproduce your issue, otherwise we can not help you.

If this is a custom model or data training question, please note that Ultralytics does not provide free personal support. As a leader in vision ML and AI, we do offer professional consulting, from simple expert advice up to delivery of fully customized, end-to-end production solutions for our clients, such as:

• Cloud-based AI systems operating on hundreds of HD video streams in realtime.
• Edge AI integrated into custom iOS and Android apps for realtime 30 FPS video inference.
• Custom data training, hyperparameter evolution, and model exportation to any destination.

For more information please visit https://www.ultralytics.com.

[bonlime]

check TResNet paper. p2. They call it SpaceToDepth

[seekFire]

@maykulkarni I think It is the inverse operation of pixelshuffle

[violet17]

What's the difference between Focus layer and reorg layer in YOLOv1?

[wilile26811249]

@violet17 Focus is equal to the reorg layer.