Multi-Class Image Classification Using Transfer Learning With PySpark

In this article, we’ll demonstrate a Computer Vision problem with the power to combined two state-of-the-art technologies: Deep Learning with Apache Spark. We will leverage the power of Deep Learning Pipelines for a Multi-Class image classification problem.

Deep Learning Pipelines is a high-level Deep Learning framework that facilitates common Deep Learning workflows via the Spark MLlib Pipelines API. It currently supports TensorFlow and Keras with the TensorFlow-backend.

The library comes from Databricks and leverages Spark for its two strongest facets:

• In the spirit of Spark and Spark MLlib, It provides easy-to-use APIs that enable deep learning in very few lines of code.
• It uses Spark’s powerful distributed engine to scale out deep learning on massive datasets.

Transfer Learning

Transfer learning is a technique in machine learning in general that focuses on saving knowledge (weights and biases) gained while solving one problem and further applying it to a different but related problem.

Deep Learning Pipelines provides utilities to perform transfer learning on the images, which is one of the fastest ways to start using deep learning. With the concept of a Featurizer, Deep Learning Pipelines enables fast transfer learning on Spark-Cluster. Right now it provides the following neural nets for Transfer Learning:

• InceptionV3
• Xception
• ResNet50
• VGG16
• VGG19

For the demonstration purposes, we’ll work only on the InceptionV3 model. You can read the technical details of this model from here.

The following example combines the InceptionV3 model and multinomial logistic regression in Spark. A utility function from Deep Learning Pipelines called DeepImageFeaturizer automatically peels off the last layer of a pre-trained neural network and uses the output from all the previous layers as features for the logistic regression algorithm.

Data Set

The Bengali script has ten numerical digits (graphemes or symbols indicating the numbers from 0 to 9). Numbers larger than 9 are written in Bengali using a positional base 10 numeral system.

We choose NumtaDB as a source of our dataset. It’s a collection of Bengali Handwritten Digit data. The dataset contains more than 85,000 digits from over 2,700 contributors. But here we’re not planning to work on the whole data set rather than choose randomly 50 images of each class.

Fig 1: Each Folder Contains 50 Images [ Classes (0 to 9) ]

Let’s look below what we’ve inside each above ten folders. I rename each image shown below of its corresponding class label for demonstration purposes.

Fig 2: Bengali Handwritten Digit

At first, we load all the images to SparkData Frame. Then we build the model and train it. After that, we’ll evaluate the performance of our trained model.

Load Images
The data sets (from 0 to 9) contains almost 500 handwritten Bangla digits (50 images each class). Here we manually load each image into spark data-frame with a target column. After loading the whole dataset we split into an 8:2 ratio randomly for the training set and final test set.

Our goal is to train the model with the training data set and finally evaluate the model’s performance with the test dataset.

# necessary import 
from pyspark.sql import SparkSession
from pyspark.ml.image import ImageSchema
from pyspark.sql.functions import lit
from functools import reduce

# create a spark session
spark = SparkSession.builder.appName(‘DigitRecog’).getOrCreate()

# loaded image
zero = ImageSchema.readImages("0").withColumn("label", lit(0))
one = ImageSchema.readImages("1").withColumn("label", lit(1))
two = ImageSchema.readImages("2").withColumn("label", lit(2))
three = ImageSchema.readImages("3").withColumn("label", lit(3))
four = ImageSchema.readImages("4").withColumn("label", lit(4))
five = ImageSchema.readImages("5").withColumn("label", lit(5))
six = ImageSchema.readImages("6").withColumn("label", lit(6))
seven = ImageSchema.readImages("7").withColumn("label", lit(7))
eight = ImageSchema.readImages("8").withColumn("label", lit(8))
nine = ImageSchema.readImages("9").withColumn("label", lit(9))

dataframes = [zero, one, two, three,four,
             five, six, seven, eight, nine]
             
# merge data frame
df = reduce(lambda first, second: first.union(second), dataframes)

# repartition dataframe 
df = df.repartition(200)
# split the data-frame
train, test = df.randomSplit([0.8, 0.2], 42)

Here we can perform various Exploratory Data Analysis on Spark DataFrame. We can also view the schema of the data frame too.

df.printSchema()

root
 |-- image: struct (nullable = true)
 |    |-- origin: string (nullable = true)
 |    |-- height: integer (nullable = false)
 |    |-- width: integer (nullable = false)
 |    |-- nChannels: integer (nullable = false)
 |    |-- mode: integer (nullable = false)
 |    |-- data: binary (nullable = false)
 |-- label: integer (nullable = false)

We can also convert Spark-DataFrame to Pandas-DataFrame using .toPandas().

Model Training

Here we combine the InceptionV3 model and logistic regression in Spark. The DeepImageFeaturizer automatically peels off the last layer of a pre-trained neural network and uses the output from all the previous layers as features for the logistic regression algorithm.

Since logistic regression is a simple and fast algorithm, this transfer learning training can converge quickly.

from pyspark.ml.evaluation import MulticlassClassificationEvaluator
from pyspark.ml.classification import LogisticRegression
from pyspark.ml import Pipeline
from sparkdl import DeepImageFeaturizer

# model: InceptionV3
# extracting feature from images
featurizer = DeepImageFeaturizer(inputCol="image",
                                 outputCol="features",
                                 modelName="InceptionV3")
                                 
# used as a multi class classifier
lr = LogisticRegression(maxIter=5, regParam=0.03, 
                        elasticNetParam=0.5, labelCol="label")
                        
# define a pipeline model
sparkdn = Pipeline(stages=[featurizer, lr])
spark_model = sparkdn.fit(train) # start fitting or training

Evaluation

Now, It’s time to evaluate model performance. We now like to evaluate four evaluation metrics such as F1-score, Precision, Recall, Accuracy on the test data set.

from pyspark.ml.evaluation import MulticlassClassificationEvaluator

# evaluate the model with test set
evaluator = MulticlassClassificationEvaluator() 
tx_test = spark_model.transform(test)

print('F1-Score ', evaluator.evaluate(tx_test, 
                                      {evaluator.metricName: 
                                      'f1'}))
                                      
print('Precision ', evaluator.evaluate(tx_test, 
                                     {evaluator.metricName:
                                     'weightedPrecision'}))
                                       
print('Recall ', evaluator.evaluate(tx_test, 
                                    {evaluator.metricName: 
                                    'weightedRecall'}))
                                    
print('Accuracy ', evaluator.evaluate(tx_test, 
                                      {evaluator.metricName: 
                                      'accuracy'}))

Here we get the result. It’s promising until now.

F1-Score   0.8111782234361806
Precision  0.8422058244785519
Recall     0.8090909090909091
Accuracy   0.8090909090909091

Confusion Metrix

Here, we’ll summarize the performance of a classification model using the confusion matrix.

import matplotlib.pyplot as plt
import numpy as np
import itertools

# function for plotting confusion matrix
def plot_confusion_matrix(cm, classes,
                          normalize=False,
                          title='Confusion matrix',
                          cmap=plt.cm.GnBu):
                          
              plt.imshow(cm, interpolation='nearest', cmap=cmap)
                  plt.title(title)
                  tick_marks = np.arange(len(classes))
                  plt.xticks(tick_marks, classes, rotation=45)
                  plt.yticks(tick_marks, classes)
                  fmt = '.2f' if normalize else 'd'
                  thresh = cm.max() / 2.
                  for i, j in itertools.product(range(cm.shape[0]), 
                      range(cm.shape[1])):
                      plt.text(j, i, format(cm[i, j], fmt),
                               horizontalalignment="center",
                               color="white" if cm[i, j] > thresh else "black")

                  plt.tight_layout()
                  plt.ylabel('True label')
                  plt.xlabel('Predicted label')

For this, we need to convert Spark-DataFrame to Pandas-DataFrame first and then call Confusion Matrix with true and predicted labels.

from sklearn.metrics import confusion_matrix
y_true = tx_test.select("label")
y_true = y_true.toPandas()

y_pred = tx_test.select("prediction")
y_pred = y_pred.toPandas()

cnf_matrix = confusion_matrix(y_true, y_pred,labels=range(10))

Let’s visualize the Confusion Matrix

import seaborn as sns
sns.set_style("darkgrid")
plt.figure(figsize=(7,7))
plt.grid(False)

# call pre defined function
plot_confusion_matrix(cnf_matrix, classes=range(10))

Fig 3: Confusion Matrix for 10 Bengali Digits (0 to 9)

Classification Report

Here we can also get a report on the classification of each class by the evaluation matrix.

from sklearn.metrics import classification_report
target_names = ["Class {}".format(i) for i in range(10)]
print(classification_report(y_true, y_pred, target_names = target_names))

It’ll demonstrate much better the model performance for each class label prediction.

               precision  recall   f1-score   support
     Class 0       1.00      0.92      0.96        13
     Class 1       0.57      1.00      0.73         8
     Class 2       0.64      1.00      0.78         7
     Class 3       0.88      0.70      0.78        10
     Class 4       0.90      1.00      0.95         9
     Class 5       0.67      0.83      0.74        12
     Class 6       0.83      0.62      0.71         8
     Class 7       1.00      0.80      0.89        10
     Class 8       1.00      0.80      0.89        20
     Class 9       0.70      0.54      0.61        13
     
   micro avg       0.81      0.81      0.81       110
   macro avg       0.82      0.82      0.80       110
weighted avg       0.84      0.81      0.81       110

ROC AUC Score

Let’s also find ROC AUC score point of this model. I’ve found the following piece of code from here.

from sklearn.metrics import roc_curve, auc, roc_auc_score
from sklearn.preprocessing import LabelBinarizer

def multiclass_roc_auc_score(y_test, y_pred, average="macro"):
    lb = LabelBinarizer()
    lb.fit(y_test)
    y_test = lb.transform(y_test)
    y_pred = lb.transform(y_pred)
    return roc_auc_score(y_test, y_pred, average=average)
    
print('ROC AUC score:', multiclass_roc_auc_score(y_true,y_pred))

It scores 0.901.

Predicted Samples

Let’s see some of its prediction, comparison with the real label.

# all columns after transformations
print(tx_test.columns)

# see some predicted output
tx_test.select('image', "prediction", "label").show()

And, the result would be as following

['image', 'label', 'features', 'rawPrediction', 'probability', 'prediction']
+------------------+----------+--------+
|       image      |prediction|  label |
+------------------+----------+--------+
|[file:/home/i...|       1.0|    1|
|[file:/home/i...|       8.0|    8|
|[file:/home/i...|       9.0|    9|
|[file:/home/i...|       1.0|    8|
|[file:/home/i...|       1.0|    1|
|[file:/home/i...|       1.0|    9|
|[file:/home/i...|       0.0|    0|
|[file:/home/i...|       2.0|    9|
|[file:/home/i...|       8.0|    8|
|[file:/home/i...|       9.0|    9|
|[file:/home/i...|       0.0|    0|
|[file:/home/i...|       4.0|    0|
|[file:/home/i...|       5.0|    9|
|[file:/home/i...|       1.0|    1|
|[file:/home/i...|       9.0|    9|
|[file:/home/i...|       9.0|    9|
|[file:/home/i...|       1.0|    1|
|[file:/home/i...|       1.0|    1|
|[file:/home/i...|       9.0|    9|
|[file:/home/i...|       3.0|    6|
+--------------------+----------+-----+
only showing top 20 rows

EndNote

Though we’ve used an ImageNet weight our model performs pretty much promising to recognize handwritten digits. Moreover, we didn’t also perform any image processing task for better generalization. Also, the model is trained on very few amounts of data compared to ImageNet dataset.

At a very high level, each Spark apps consists of a driver program that launches various parallel operations on a cluster. The driver program contains our apps’ main function and defines distributed datasets on the cluster, then applies operations to them.

As this is a standalone application where we’ve linked the application to Spark first and then we imported the Spark packages in our program and create a SparkContext using SparkSession. Though we’ve worked on a single machine, we can connect the same shell to a cluster to train data in parallel.

However, you can get the source code of today’s demonstration from the link below and can also follow me on GitHub for future code updates.

Source Code : TL With PySpark

What’s next?

Next, We’ll do Distributed Hyperparameter Tuning with Spark, and will try custom Keras model and some new challenging examples

However, If you’re working on healthcare using Machine Learning, don’t hesitate to reach out if you think there’s an opportunity for collaboration. Would love to discuss. Get in touch, we keep active always

Say Hi On: Email | LinkedIn | Quora | GitHub | Medium | Twitter | Instagram

Related Technologies

• Distributed DL with Keras & PySpark — Elephas
• Distributed Deep Learning Library for Apache Spark — BigDL
• TensorFlow to Apache Spark clusters — TensorFlowOnSpark

References

• Databricks: Deep Learning Guide
• Apache Spark: PySpark Machine Learning