» Code examples / Computer Vision / Image similarity estimation using a Siamese Network with a triplet loss

# Image similarity estimation using a Siamese Network with a triplet loss

Authors: Hazem Essam and Santiago L. Valdarrama
Date created: 2021/03/25
Description: Training a Siamese Network to compare the similarity of images using a triplet loss function.

## Introduction

A Siamese Network is a type of network architecture that contains two or more identical subnetworks used to generate feature vectors for each input and compare them.

Siamese Networks can be applied to different use cases, like detecting duplicates, finding anomalies, and face recognition.

This example uses a Siamese Network with three identical subnetworks. We will provide three images to the model, where two of them will be similar (anchor and positive samples), and the third will be unrelated (a negative example.) Our goal is for the model to learn to estimate the similarity between images.

For the network to learn, we use a triplet loss function. You can find an introduction to triplet loss in the FaceNet paper by Schroff et al,. 2015. In this example, we define the triplet loss function as follows:

`L(A, P, N) = max(‖f(A) - f(P)‖² - ‖f(A) - f(N)‖² + margin, 0)`

This example uses the Totally Looks Like dataset by Rosenfeld et al., 2018.

## Setup

``````import matplotlib.pyplot as plt
import numpy as np
import os
import random
import tensorflow as tf
from pathlib import Path
from tensorflow.keras import applications
from tensorflow.keras import layers
from tensorflow.keras import losses
from tensorflow.keras import optimizers
from tensorflow.keras import metrics
from tensorflow.keras import Model
from tensorflow.keras.applications import resnet

target_shape = (200, 200)
``````

We are going to load the Totally Looks Like dataset and unzip it inside the `~/.keras` directory in the local environment.

The dataset consists of two separate files:

• `left.zip` contains the images that we will use as the anchor.
• `right.zip` contains the images that we will use as the positive sample (an image that looks like the anchor).
``````cache_dir = Path(Path.home()) / ".keras"
anchor_images_path = cache_dir / "left"
positive_images_path = cache_dir / "right"
``````
``````!gdown --id 1jvkbTr_giSP3Ru8OwGNCg6B4PvVbcO34
!gdown --id 1EzBZUb_mh_Dp_FKD0P4XiYYSd0QBH5zW
!unzip -oq left.zip -d \$cache_dir
!unzip -oq right.zip -d \$cache_dir
``````
``````zsh:1: command not found: gdown
unzip:  cannot find or open left.zip, left.zip.zip or left.zip.ZIP.
unzip:  cannot find or open right.zip, right.zip.zip or right.zip.ZIP.
``````

## Preparing the data

We are going to use a `tf.data` pipeline to load the data and generate the triplets that we need to train the Siamese network.

We'll set up the pipeline using a zipped list with anchor, positive, and negative filenames as the source. The pipeline will load and preprocess the corresponding images.

``````def preprocess_image(filename):
"""
Load the specified file as a JPEG image, preprocess it and
resize it to the target shape.
"""

image = tf.image.decode_jpeg(image_string, channels=3)
image = tf.image.convert_image_dtype(image, tf.float32)
image = tf.image.resize(image, target_shape)
return image

def preprocess_triplets(anchor, positive, negative):
"""
Given the filenames corresponding to the three images, load and
preprocess them.
"""

return (
preprocess_image(anchor),
preprocess_image(positive),
preprocess_image(negative),
)
``````

Let's setup our data pipeline using a zipped list with an anchor, positive, and negative image filename as the source. The output of the pipeline contains the same triplet with every image loaded and preprocessed.

``````# We need to make sure both the anchor and positive images are loaded in
# sorted order so we can match them together.
anchor_images = sorted(
[str(anchor_images_path / f) for f in os.listdir(anchor_images_path)]
)

positive_images = sorted(
[str(positive_images_path / f) for f in os.listdir(positive_images_path)]
)

image_count = len(anchor_images)

anchor_dataset = tf.data.Dataset.from_tensor_slices(anchor_images)
positive_dataset = tf.data.Dataset.from_tensor_slices(positive_images)

# To generate the list of negative images, let's randomize the list of
# available images and concatenate them together.
rng = np.random.RandomState(seed=42)
rng.shuffle(anchor_images)
rng.shuffle(positive_images)

negative_images = anchor_images + positive_images
np.random.RandomState(seed=32).shuffle(negative_images)

negative_dataset = tf.data.Dataset.from_tensor_slices(negative_images)
negative_dataset = negative_dataset.shuffle(buffer_size=4096)

dataset = tf.data.Dataset.zip((anchor_dataset, positive_dataset, negative_dataset))
dataset = dataset.shuffle(buffer_size=1024)
dataset = dataset.map(preprocess_triplets)

# Let's now split our dataset in train and validation.
train_dataset = dataset.take(round(image_count * 0.8))
val_dataset = dataset.skip(round(image_count * 0.8))

train_dataset = train_dataset.batch(32, drop_remainder=False)
train_dataset = train_dataset.prefetch(8)

val_dataset = val_dataset.batch(32, drop_remainder=False)
val_dataset = val_dataset.prefetch(8)
``````

Let's take a look at a few examples of triplets. Notice how the first two images look alike while the third one is always different.

``````def visualize(anchor, positive, negative):
"""Visualize a few triplets from the supplied batches."""

def show(ax, image):
ax.imshow(image)
ax.get_xaxis().set_visible(False)
ax.get_yaxis().set_visible(False)

fig = plt.figure(figsize=(9, 9))

axs = fig.subplots(3, 3)
for i in range(3):
show(axs[i, 0], anchor[i])
show(axs[i, 1], positive[i])
show(axs[i, 2], negative[i])

visualize(*list(train_dataset.take(1).as_numpy_iterator())[0])
``````

## Setting up the embedding generator model

Our Siamese Network will generate embeddings for each of the images of the triplet. To do this, we will use a ResNet50 model pretrained on ImageNet and connect a few `Dense` layers to it so we can learn to separate these embeddings.

We will freeze the weights of all the layers of the model up until the layer `conv5_block1_out`. This is important to avoid affecting the weights that the model has already learned. We are going to leave the bottom few layers trainable, so that we can fine-tune their weights during training.

``````base_cnn = resnet.ResNet50(
weights="imagenet", input_shape=target_shape + (3,), include_top=False
)

flatten = layers.Flatten()(base_cnn.output)
dense1 = layers.Dense(512, activation="relu")(flatten)
dense1 = layers.BatchNormalization()(dense1)
dense2 = layers.Dense(256, activation="relu")(dense1)
dense2 = layers.BatchNormalization()(dense2)
output = layers.Dense(256)(dense2)

embedding = Model(base_cnn.input, output, name="Embedding")

trainable = False
for layer in base_cnn.layers:
if layer.name == "conv5_block1_out":
trainable = True
layer.trainable = trainable
``````

## Setting up the Siamese Network model

The Siamese network will receive each of the triplet images as an input, generate the embeddings, and output the distance between the anchor and the positive embedding, as well as the distance between the anchor and the negative embedding.

To compute the distance, we can use a custom layer `DistanceLayer` that returns both values as a tuple.

``````class DistanceLayer(layers.Layer):
"""
This layer is responsible for computing the distance between the anchor
embedding and the positive embedding, and the anchor embedding and the
negative embedding.
"""

def __init__(self, **kwargs):
super().__init__(**kwargs)

def call(self, anchor, positive, negative):
ap_distance = tf.reduce_sum(tf.square(anchor - positive), -1)
an_distance = tf.reduce_sum(tf.square(anchor - negative), -1)
return (ap_distance, an_distance)

anchor_input = layers.Input(name="anchor", shape=target_shape + (3,))
positive_input = layers.Input(name="positive", shape=target_shape + (3,))
negative_input = layers.Input(name="negative", shape=target_shape + (3,))

distances = DistanceLayer()(
embedding(resnet.preprocess_input(anchor_input)),
embedding(resnet.preprocess_input(positive_input)),
embedding(resnet.preprocess_input(negative_input)),
)

siamese_network = Model(
inputs=[anchor_input, positive_input, negative_input], outputs=distances
)
``````

## Putting everything together

We now need to implement a model with custom training loop so we can compute the triplet loss using the three embeddings produced by the Siamese network.

Let's create a `Mean` metric instance to track the loss of the training process.

``````class SiameseModel(Model):
"""The Siamese Network model with a custom training and testing loops.

Computes the triplet loss using the three embeddings produced by the
Siamese Network.

The triplet loss is defined as:
L(A, P, N) = max(‖f(A) - f(P)‖² - ‖f(A) - f(N)‖² + margin, 0)
"""

def __init__(self, siamese_network, margin=0.5):
super(SiameseModel, self).__init__()
self.siamese_network = siamese_network
self.margin = margin
self.loss_tracker = metrics.Mean(name="loss")

def call(self, inputs):
return self.siamese_network(inputs)

def train_step(self, data):
# GradientTape is a context manager that records every operation that
# you do inside. We are using it here to compute the loss so we can get
# the gradients and apply them using the optimizer specified in
# `compile()`.
loss = self._compute_loss(data)

# Storing the gradients of the loss function with respect to the
# weights/parameters.

# Applying the gradients on the model using the specified optimizer
)

# Let's update and return the training loss metric.
self.loss_tracker.update_state(loss)
return {"loss": self.loss_tracker.result()}

def test_step(self, data):
loss = self._compute_loss(data)

# Let's update and return the loss metric.
self.loss_tracker.update_state(loss)
return {"loss": self.loss_tracker.result()}

def _compute_loss(self, data):
# The output of the network is a tuple containing the distances
# between the anchor and the positive example, and the anchor and
# the negative example.
ap_distance, an_distance = self.siamese_network(data)

# Computing the Triplet Loss by subtracting both distances and
# making sure we don't get a negative value.
loss = ap_distance - an_distance
loss = tf.maximum(loss + self.margin, 0.0)
return loss

@property
def metrics(self):
# We need to list our metrics here so the `reset_states()` can be
# called automatically.
return [self.loss_tracker]
``````

## Training

We are now ready to train our model.

``````siamese_model = SiameseModel(siamese_network)
siamese_model.fit(train_dataset, epochs=10, validation_data=val_dataset)
``````
``````Epoch 1/10
151/151 [==============================] - 277s 2s/step - loss: 0.5014 - val_loss: 0.3719
Epoch 2/10
151/151 [==============================] - 276s 2s/step - loss: 0.3884 - val_loss: 0.3632
Epoch 3/10
151/151 [==============================] - 287s 2s/step - loss: 0.3711 - val_loss: 0.3509
Epoch 4/10
151/151 [==============================] - 295s 2s/step - loss: 0.3585 - val_loss: 0.3287
Epoch 5/10
151/151 [==============================] - 299s 2s/step - loss: 0.3420 - val_loss: 0.3301
Epoch 6/10
151/151 [==============================] - 297s 2s/step - loss: 0.3181 - val_loss: 0.3419
Epoch 7/10
151/151 [==============================] - 290s 2s/step - loss: 0.3131 - val_loss: 0.3201
Epoch 8/10
151/151 [==============================] - 295s 2s/step - loss: 0.3102 - val_loss: 0.3152
Epoch 9/10
151/151 [==============================] - 286s 2s/step - loss: 0.2905 - val_loss: 0.2937
Epoch 10/10
151/151 [==============================] - 270s 2s/step - loss: 0.2921 - val_loss: 0.2952

<tensorflow.python.keras.callbacks.History at 0x7fc69064bd10>
``````

## Inspecting what the network has learned

At this point, we can check how the network learned to separate the embeddings depending on whether they belong to similar images.

We can use cosine similarity to measure the similarity between embeddings.

Let's pick a sample from the dataset to check the similarity between the embeddings generated for each image.

``````sample = next(iter(train_dataset))
visualize(*sample)

anchor, positive, negative = sample
anchor_embedding, positive_embedding, negative_embedding = (
embedding(resnet.preprocess_input(anchor)),
embedding(resnet.preprocess_input(positive)),
embedding(resnet.preprocess_input(negative)),
)
``````

Finally, we can compute the cosine similarity between the anchor and positive images and compare it with the similarity between the anchor and the negative images.

We should expect the similarity between the anchor and positive images to be larger than the similarity between the anchor and the negative images.

``````cosine_similarity = metrics.CosineSimilarity()

positive_similarity = cosine_similarity(anchor_embedding, positive_embedding)
print("Positive similarity:", positive_similarity.numpy())

negative_similarity = cosine_similarity(anchor_embedding, negative_embedding)
print("Negative similarity", negative_similarity.numpy())
``````
``````Positive similarity: 0.9940324
Negative similarity 0.9918252
``````

## Summary

1. The `tf.data` API enables you to build efficient input pipelines for your model. It is particularly useful if you have a large dataset. You can learn more about `tf.data` pipelines in tf.data: Build TensorFlow input pipelines.

2. In this example, we use a pre-trained ResNet50 as part of the subnetwork that generates the feature embeddings. By using transfer learning, we can significantly reduce the training time and size of the dataset.

3. Notice how we are fine-tuning the weights of the final layers of the ResNet50 network but keeping the rest of the layers untouched. Using the name assigned to each layer, we can freeze the weights to a certain point and keep the last few layers open.

4. We can create custom layers by creating a class that inherits from `tf.keras.layers.Layer`, as we did in the `DistanceLayer` class.

5. We used a cosine similarity metric to measure how to 2 output embeddings are similar to each other.

6. You can implement a custom training loop by overriding the `train_step()` method. `train_step()` uses `tf.GradientTape`, which records every operation that you perform inside it. In this example, we use it to access the gradients passed to the optimizer to update the model weights at every step. For more details, check out the Intro to Keras for researchers and Writing a training loop from scratch.