This post shows several computer vision steps implemented on top of Spark. OpenCV is used to extract features on top of OpenStack and Spark MLLib KMeans is used to generate our KMeans dictionary. Then we use Spark and simple vector / matrix manipulation to do coding and pooling.

Workflow implemented using OpenCV and Spark:

  1. Uploading our dataset of images to Hadoop compatible storage as Sequencefile
  2. Running OpenCV feature extraction(SURF, SIFT) code using the SequenceFile of images as input
  3. Running Spark default K-means model training machine learning code on extracted features
  4. Running Feature coding and pooling using our trained K-means model and extracted feature as input
  5. (TODO): Running machine learning algortihm to do classification on the encoded features

Uploading image dataset as SequenceFile to Hadoop compatible storage (Swift)

In the first step we upload our Caltech256 dataset, 30k images totaling 1.2Gb, as SequenceFile to OpenStack Swift. For this I've created a simplistic command line tool to upload folders containing files to be stored as a SequenceFile with key=filename and value=raw_bytes. The tool has been tested with HDFS and OpenStack Swift.

The following commands show how to download Caltech-256 dataset consisting of JPG images. Then upload the downloaded images to OpenStack Swift in sequence file format:

# Download and compile the hadoop sequencefile upload tool
git clone https://github.com/samos123/hadoop-sequence-file-upload
cd hadoop-sequence-file-upload
mvn clean compile assembly:single
#  Download / extract calltech-256 dataset
axel http://www.vision.caltech.edu/Image_Datasets/Caltech256/256_ObjectCategories.tar
tar xf 256_ObjectCategories.tar
# Upload to Swift, this assumes /etc/hadoop/conf/core-site.xml is used to store Swift details
./upload-to-sequence-file.sh 256_ObjectCategories/ swift://spark.swift1/caltech-256.hseq

The dataset is also accessible through Tachyon as we configured it to use Swift as underFS.

Extract SIFT/SURF features using OpenCV on Spark

I've created a simple Spark application that uses OpenCV to extract SURF or SIFT features from an image.

from __future__ import print_function
import logging
import io
import sys
import os

import cv2
import numpy as np
from pyspark import SparkContext
from pyspark.sql import SQLContext, Row


def extract_opencv_features(feature_name):

    def extract_opencv_features_nested(imgfile_imgbytes):
        try:
            imgfilename, imgbytes = imgfile_imgbytes
            nparr = np.fromstring(buffer(imgbytes), np.uint8)
            img = cv2.imdecode(nparr, 0)
            if feature_name in ["surf", "SURF"]:
                extractor = cv2.SURF()
            elif feature_name in ["sift", "SIFT"]:
                extractor = cv2.SIFT()

            kp, descriptors = extractor.detectAndCompute(img, None)

            return [(imgfilename, descriptors)]
        except Exception, e:
            logging.exception(e)
            return []

    return extract_opencv_features_nested


if __name__ == "__main__":
    sc = SparkContext(appName="feature_extractor")
    sqlContext = SQLContext(sc)

    try:
        feature_name = sys.argv[1]
        image_seqfile_path = sys.argv[2]
        feature_parquet_path = sys.argv[3]
        partitions = int(sys.argv[4])
    except:
        print("Usage: spark-submit feature_extraction.py <feature_name(sift or surf)> "
              "<image_sequencefile_input_path> <feature_sequencefile_output_path> <partitions>")

    images = sc.sequenceFile(image_seqfile_path, minSplits=partitions)

    features = images.flatMap(extract_opencv_features(feature_name))
    features = features.filter(lambda x: x[1] != None)
    features = features.map(lambda x: (Row(fileName=x[0], features=x[1].tolist())))
    featuresSchema = sqlContext.createDataFrame(features)
    featuresSchema.registerTempTable("images")
    featuresSchema.write.parquet(feature_parquet_path)

Using the above Spark application we can start to extract features from our image dataset in Swift. As input we provide the sequencefile containing <fileName: String, image: Bytes>. The extracted features we write out as parquet file. The following command extracts the sift features from our dataset:

spark-submit --executor-memory 8g feature_extraction.py sift swift://spark.swift1/caltech-256.hseq swift://spark.swift1/caltech-256-sift1.parquet 100

K-Means Dictionary generation on SIFT features

We can now generate our dictionary of features through Spark's MLLib KMeans algorithm. The below application is used to train our KMeans model using the features generated in the previous step as input dataset.

from __future__ import print_function
import io
import sys
import os

import numpy as np
from pyspark import SparkContext
from pyspark.mllib.clustering import KMeans
from pyspark.sql import SQLContext, Row


if __name__ == "__main__":
    sc = SparkContext(appName="kmeans_dictionary_creation")
    sqlContext = SQLContext(sc)

    try:
        k = int(sys.argv[1])
        feature_parquet_path = sys.argv[2]
        kmeans_model_path = sys.argv[3]
    except:
        print("Usage: spark-submit kmeans.py <k:clusters> "
              "<feature_sequencefile_input_path> <kmeans_model_output>")

    features = sqlContext.read.parquet(feature_parquet_path)

    # Create same size vectors of the feature descriptors
    # flatMap returns every list item as a new row for the RDD
    # hence transforming x, 128 to x rows of 1, 128 in the RDD.
    # This is needed for KMeans.
    features = features.flatMap(lambda x: x['features']).cache()
    model = KMeans.train(features, k, maxIterations=10, initializationMode="random")

    model.save(sc, kmeans_model_path)
    print("Clusters have been saved as text file to %s" % kmeans_model_path)
    print("Final centers: " + str(model.clusterCenters))
    sc.stop()

Start the spark job with:

spark-submit --executor-memory 8g kmeans.py 1000 swift://spark.swift1/caltech-256-sift1.parquet swift://spark.swift1/caltech-256-dictionary

Feature coding and pooling with trained KMeans model

In this step we will use the KMeans dictionary that we trained in the previous step to encode each point of interest to a single cluster. This is done by assigning every row of our x * 128 matrix to a single cluster of the KMeans dictionary. The result is a 1 * k representation for each image utilizing coding and pooling. For pooling we've implemented a simple max and sum pooling method.

The following Spark application implements feature coding and pooling:

from __future__ import print_function
import functools
import io
import sys
import os

import numpy as np
from scipy.spatial import distance
from pyspark.mllib.clustering import KMeansModel
from pyspark import SparkContext
from pyspark.sql import SQLContext, Row

SUPPORTED_POOLING = ["max", "sum"]

def assign_pooling(row, clusterCenters, pooling):
    image_name = row['fileName']
    feature_matrix = np.array(row['features'])
    clusterCenters = clusterCenters.value
    model = KMeansModel(clusterCenters)
    bow = np.zeros(len(clusterCenters))

    for x in feature_matrix:
        k = model.predict(x)
        dist = distance.euclidean(clusterCenters[k], x)
        if pooling == "max":
            bow[k] = max(bow[k], dist)
        elif pooling == "sum":
            bow[k] = bow[k] + dist

    return Row(fileName=image_name, bow=bow.tolist())


if __name__ == "__main__":
    sc = SparkContext(appName="kmeans_assign")
    sqlContext = SQLContext(sc)

    try:
        feature_parquet_path = sys.argv[1]
        kmeans_model_path = sys.argv[2]
        bow_parquet_path = sys.argv[3]
        pooling = sys.argv[4]

    except:
        print("Usage: spark-submit feature_coding_pooling.py "
              "<feature_sequencefile_path> <kmeans_model> "
              "<bow_sequencefile_path> <pooling_method:max>")

    if pooling not in SUPPORTED_POOLING:
        raise ValueError("Pooling method %s is not supported. Supported poolings methods: %s" % (pooling, SUPPORTED_POOLING))

    features = sqlContext.read.parquet(feature_parquet_path)
    model = KMeansModel.load(sc, kmeans_model_path)
    clusterCenters = model.clusterCenters
    clusterCenters = sc.broadcast(clusterCenters)

    features_bow = features.map(functools.partial(assign_pooling,
        clusterCenters=clusterCenters, pooling=pooling))
    featuresSchema = sqlContext.createDataFrame(features_bow)
    featuresSchema.registerTempTable("images")
    featuresSchema.write.parquet(bow_parquet_path)
    sc.stop()

Execute the spark applicaiton with:

spark-submit --executor-memory 8g feature_coding_pooling.py swift://spark.swift1/caltech-256-sift1.parquet swift://spark.swift1/caltech-256-dictionary swift://spark.swift1/caltech-256-bow.parquet sum

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