python - what is the difference between Flatten() and GlobalAveragePooling2D() in keras

Question

I want to pass the output of ConvLSTM and Conv2D to a Dense Layer in Keras, what is the difference between using global average pooling and flatten Both is working in my case.

model.add(ConvLSTM2D(filters=256,kernel_size=(3,3)))
model.add(Flatten())
# or model.add(GlobalAveragePooling2D())
model.add(Dense(256,activation='relu'))

Answer 1

That both seem to work doesn't mean they do the same.

Flatten will take a tensor of any shape and transform it into a one dimensional tensor (plus the samples dimension) but keeping all values in the tensor. For example a tensor (samples, 10, 20, 1) will be flattened to (samples, 10 * 20 * 1).

GlobalAveragePooling2D does something different. It applies average pooling on the spatial dimensions until each spatial dimension is one, and leaves other dimensions unchanged. In this case values are not kept as they are averaged. For example a tensor (samples, 10, 20, 1) would be output as (samples, 1, 1, 1), assuming the 2nd and 3rd dimensions were spatial (channels last).