I want my activation function select maximal value, generated by `N` filters of `M x M` convolution. This layer would convert `X` channel image to 1-channel one.

How to do that?

First I wrote

```
classifier.add(Conv2D(3, (5, 5), activation='linear')
classifier.add(MaxPooling2D(pool_size=1, strides=1))

```

but then thought it doesn't return 1-channel image, but returns 3 channel.

How to do the job?