What's pooling in Convolutional Neural Networks (CNN)?

Pooling is a technique to downsample the feature map. It allows layers which receive relatively undistorted versions of the input to learn low level features such as lines, while layers deeper in the model can learn more abstract features such as texture.

Max pooling is a technique where the maximum value of a receptive field is passed on in the next feature map. The most commonly used receptive field is 2 x 2 with a stride of 2, which means the feature map is downsampled from N x N to N/2 x N/2. Receptive fields larger than 3 x 3 are rarely employed as too much information is lost.

Other pooling techniques include:

  • Average pooling, the output is the average value of the receptive field.
  • Min pooling, the output is the minimum value of the receptive field.
  • Global pooling, where the receptive field is set to be equal to the input size, this means the output is equal to a scalar and can be used to reduce the dimensionality of the feature map.

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