# Adding a dense layer after a conv2d layer in a convolutional autoencoder

I am trying to implement a convolutional autoencoder with a dense layer at the bottleneck do to some dimensional reduction. I have seen two approaches for this which arent particularly scalable. The first was to introduce 2 dense layers (one at the bottle neck and one before & after that has the same number of nodes as the conv2d layer that precedes the dense layer in the encoder section:

input_image_shape=(200,200,3)
encoding_dims = 20

encoder = Sequential()

#The decoder
decoder = Sequential()


Its easy to see why this approach blows up as there are two densely connected layers with (32*100*100) nodes each or more or in that ballpark which is nuts.

Another approach I have found which makes sense for b/w images such as the mnist stuff is to introduce an arbitrary number of encoding dimensions and reshape it (https://medium.com/analytics-vidhya/building-a-convolutional-autoencoder-using-keras-using-conv2dtranspose-ca403c8d144e). The following chunk of code is copied from the link, I claim no credit for it:

#ENCODER
inp = Input((28, 28,1))
e = Conv2D(32, (3, 3), activation='relu')(inp)
e = MaxPooling2D((2, 2))(e)
e = Conv2D(64, (3, 3), activation='relu')(e)
e = MaxPooling2D((2, 2))(e)
e = Conv2D(64, (3, 3), activation='relu')(e)
l = Flatten()(e)
l = Dense(49, activation='softmax')(l)
#DECODER
d = Reshape((7,7,1))(l)
d = Conv2DTranspose(64,(3, 3), strides=2, activation='relu', padding='same')(d)
d = BatchNormalization()(d)
d = Conv2DTranspose(64,(3, 3), strides=2, activation='relu', padding='same')(d)
d = BatchNormalization()(d)
d = Conv2DTranspose(32,(3, 3), activation='relu', padding='same')(d)
decoded = Conv2D(1, (3, 3), activation='sigmoid', padding='same')(d)


The question is then is there a more rigorous way/ accepted of adding a conv2d layer after a 2d layer?

Hi for me this worked perfectly. I encoded with conv2d and dense and then I flatten I and reshape in the decoder after the dense layer so the encoder and decoder are symmetrical. The only difference is that in my case I use images (224,224,1)

        # create encoder
# 28,28 -> 1st conv2d (filter 3x3,relu activation, padding, strides == 'jumps')
self.encoder = tf.keras.Sequential([layers.Input(shape=(224,224,1)),