This is part of the CNN Architectures series by Dimitris Katsios. Find all CNN Architectures online:

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XCEPTION

We will use the tensorflow.keras Functional API to build Xception from the original paper: “Xception: Deep Learning with Depthwise Separable Convolutions” by François Chollet. [paper]

In the paper we can read:

[i] “all Convolution and SeparableConvolution layers are followed by batch normalization [7] (not included in the diagram).”

[ii] “All SeparableConvolution layers use a depth multiplier of 1 (no depth expansion).”

We will also use the following Diagram [iii]:

as well the following Table [iv] to check the total number of parameters:

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Network architecture

The model is separated in 3 flows as depicted at [iii]:

• Entry flow
• Middle flow with 8 repetitions of the same block
• Exit flow

According to [i] all Convolution and Separable Convolution layers are followed by batch normalization.

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Workflow

We will:

1. import the neccesary layers
2. write one helper function for the Conv-BatchNorm block and one for the SeparableConv-BatchNorm block according to [i]
3. write one function for each one of the 3 flows according to [iii]
4. use these helper functions to build the model.

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1. Imports

Code:

from tensorflow.keras.layers import Input, Conv2D, SeparableConv2D, \
     Add, Dense, BatchNormalization, ReLU, MaxPool2D, GlobalAvgPool2D

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2.1. Conv-BatchNorm block

The Conv-BatchNorm block will:

• take as inputs:
  • a tensor (x)
  • the number of filters of the Convolution layer (filters)
  • the kernel size of the Convolution layer (kernel_size)
  • the strides of the Convolution layer (strides)
• run:
  • apply a Convolution layer to x
  • apply a Batch Normalization layer to this tensor
• return the tensor

Code:

def conv_bn(x, filters, kernel_size, strides=1):
    x = Conv2D(filters=filters,
               kernel_size=kernel_size,
               strides=strides,
               padding='same',
               use_bias=False)(x)
    x = BatchNormalization()(x)
    return x

Note: We include use_bias=False for the final number of parameters to match the ones written at [iv].

---

2.2. SeparableConv-BatchNorm

The SeparableConv-BatchNorm block has similar structure with the Conv-BatchNorm one

Code:

def sep_bn(x, filters, kernel_size, strides=1):
    x = SeparableConv2D(filters=filters,
                        kernel_size=kernel_size,
                        strides=strides,
                        padding='same',
                        use_bias=False)(x)
    x = BatchNormalization()(x)
    return x

---

3.1. Entry flow

Code:

def entry_flow(x):
    x = conv_bn(x, filters=32, kernel_size=3, strides=2)
    x = ReLU()(x)
    x = conv_bn(x, filters=64, kernel_size=3)
    tensor = ReLU()(x)
  
    x = sep_bn(tensor, filters=128, kernel_size=3)
    x = ReLU()(x)
    x = sep_bn(x, filters=128, kernel_size=3)
    x = MaxPool2D(pool_size=3, strides=2, padding='same')(x)

    tensor = conv_bn(tensor, filters=128, kernel_size=1, strides=2)

    x = Add()([tensor, x])
    x = ReLU()(x)
    x = sep_bn(x, filters=256, kernel_size=3)
    x = ReLU()(x)
    x = sep_bn(x, filters=256, kernel_size=3)
    x = MaxPool2D(pool_size=3, strides=2, padding='same')(x)

    tensor = conv_bn(tensor, filters=256, kernel_size=1, strides=2)

    x = Add()([tensor, x])
    x = ReLU()(x)
    x = sep_bn(x, filters=728, kernel_size=3)
    x = ReLU()(x)
    x = sep_bn(x, filters=728, kernel_size=3)
    x = MaxPool2D(pool_size=3, strides=2, padding='same')(x)

    tensor = conv_bn(tensor, filters=728, kernel_size=1, strides=2)
    x = Add()([tensor, x])

    return x

---

3.2. Middle flow

Code:

def middle_flow(tensor):
    for _ in range(8):
        x = ReLU()(tensor)
        x = sep_bn(x, filters=728, kernel_size=3)
        x = ReLU()(x)
        x = sep_bn(x, filters=728, kernel_size=3)
        x = ReLU()(x)
        x = sep_bn(x, filters=728, kernel_size=3)

        tensor = Add()([tensor, x])

    return tensor

---

3.3. Exit flow

Code:

def exit_flow(tensor):
    x = ReLU()(tensor)
    x = sep_bn(x, filters=728, kernel_size=3)
    x = ReLU()(x)
    x = sep_bn(x, filters=1024, kernel_size=3)
    x = MaxPool2D(3, strides=2, padding='same')(x)

    tensor = conv_bn(tensor, filters=1024, kernel_size=1, strides=2)

    x = Add()([tensor, x])
    x = sep_bn(x, filters=1536, kernel_size=3)
    x = ReLU()(x)
    x = sep_bn(x, filters=2048, kernel_size=3)
    x = ReLU()(x)
    x = GlobalAvgPool2D()(x)
    x = Dense(units=1000, activation='softmax')(x)

    return x

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4. Model code

Code:

input = Input(shape=[299, 299, 3])

x = entry_flow(input)
x = middle_flow(x)
output = exit_flow(x)

from tensorflow.keras import Model 
model = Model(input, output)

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Check number of parameters

We can also check the total number of trainable parameters of the model by calling count_params() on each result element of model.trainable_weights.

According to [iv] there are 22,855,952 trainable parameters at Xception model.

Code:

>>> import numpy as np
>>> import tensorflow.keras.backend as K
>>> np.sum([K.count_params(p) for p in model.trainable_weights])
22855952

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Final code

Code:

from tensorflow.keras.layers import Input, Conv2D, SeparableConv2D, \
     Add, Dense, BatchNormalization, ReLU, MaxPool2D, GlobalAvgPool2D

def conv_bn(x, filters, kernel_size, strides=1):
    x = Conv2D(filters=filters,
               kernel_size=kernel_size,
               strides=strides,
               padding='same',
               use_bias=False)(x)
    x = BatchNormalization()(x)
    return x


def sep_bn(x, filters, kernel_size, strides=1):
    x = SeparableConv2D(filters=filters,
                        kernel_size=kernel_size,
                        strides=strides,
                        padding='same',
                        use_bias=False)(x)
    x = BatchNormalization()(x)
    return x


def entry_flow(x):
    x = conv_bn(x, filters=32, kernel_size=3, strides=2)
    x = ReLU()(x)
    x = conv_bn(x, filters=64, kernel_size=3)
    tensor = ReLU()(x)

    x = sep_bn(tensor, filters=128, kernel_size=3)
    x = ReLU()(x)
    x = sep_bn(x, filters=128, kernel_size=3)
    x = MaxPool2D(pool_size=3, strides=2, padding='same')(x)

    tensor = conv_bn(tensor, filters=128, kernel_size=1, strides=2)

    x = Add()([tensor, x])
    x = ReLU()(x)
    x = sep_bn(x, filters=256, kernel_size=3)
    x = ReLU()(x)
    x = sep_bn(x, filters=256, kernel_size=3)
    x = MaxPool2D(pool_size=3, strides=2, padding='same')(x)

    tensor = conv_bn(tensor, filters=256, kernel_size=1, strides=2)

    x = Add()([tensor, x])
    x = ReLU()(x)
    x = sep_bn(x, filters=728, kernel_size=3)
    x = ReLU()(x)
    x = sep_bn(x, filters=728, kernel_size=3)
    x = MaxPool2D(pool_size=3, strides=2, padding='same')(x)

    tensor = conv_bn(tensor, filters=728, kernel_size=1, strides=2)
    x = Add()([tensor, x])

    return x


def middle_flow(tensor):
    for _ in range(8):
        x = ReLU()(tensor)
        x = sep_bn(x, filters=728, kernel_size=3)
        x = ReLU()(x)
        x = sep_bn(x, filters=728, kernel_size=3)
        x = ReLU()(x)
        x = sep_bn(x, filters=728, kernel_size=3)

        tensor = Add()([tensor, x])

    return tensor


def exit_flow(tensor):
    x = ReLU()(tensor)
    x = sep_bn(x, filters=728, kernel_size=3)
    x = ReLU()(x)
    x = sep_bn(x, filters=1024, kernel_size=3)
    x = MaxPool2D(3, strides=2, padding='same')(x)

    tensor = conv_bn(tensor, filters=1024, kernel_size=1, strides=2)

    x = Add()([tensor, x])
    x = sep_bn(x, filters=1536, kernel_size=3)
    x = ReLU()(x)
    x = sep_bn(x, filters=2048, kernel_size=3)
    x = ReLU()(x)
    x = GlobalAvgPool2D()(x)
    x = Dense(units=1000, activation='softmax')(x)

    return x


input = Input(shape=[299, 299, 3])

x = entry_flow(input)
x = middle_flow(x)
output = exit_flow(x)

from tensorflow.keras import Model 
model = Model(input, output)

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Model diagram