Deep Learning: UNIT -II: CNN : 3.pooling layers

 3. Pooling Layer

Apart from Conv2D, we also used another specific module called MaxPooling2D in our model, which had no learnable parameters.

Let's try to understand what is Pooling and what is the significance of a Pooling Module.

The pooling layer helps in reducing the height and width of the image. Thus, it helps in reducing the number of parameters and also make invariant to the position in the input. In other words, it induces "compositionality" in our model architecture.

There are two kinds of pooling :

  1. Max pooling : The filter of size (๐‘“,๐‘“)( is slided over the input with a stride and we take the maximum in that region. It is used in between the Conv Layers.
  2. Average pooling : The filter of size (๐‘“,๐‘“) is slided over the input with a sride and we take the average in that region. This type of pooling is generally used just before the Fully connected layers for feature extraction!

The above figure illustrates the max pooling using a filter (2,2) dimension and having a stride of 2.




The above figure illustrates the average pooling using a filter (2,2) dimension and having a stride of 2.

Does pooling layers add any learnable weights to the network?

  • No, since notice that no kernel/filter matrix is considered.
  • Hence no parameters are needed while computing pooling

What is the Output shape after pooling layer?

ยท      The output shape is given by: 



where n is the input size, f is the pooling filter size and s is the stride size.

 

How Pooling works in Keras?

layers.MaxPooling2D()

layers.AveragePooling2D()

Both these Pooling layers have 2 important arguments:

  1. pool_size: This is the size of the window on which pooling will be applied. By default this size is 2. Can be an integer or a tuple of 2 integers. It can also be thought of as a non-learnable filter which either calculates max over the region or average out the values.
  2. strides: This is same as the one in Conv2D layer. Default is None, which means that the stride size is same as the pool size.

 

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