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Research Article | Open Access
A Distributed -Means Segmentation Algorithm Applied to Lobesia botrana Recognition
Early detection of Lobesia botrana is a primary issue for a proper control of this insect considered as the major pest in grapevine. In this article, we propose a novel method for L. botrana recognition using image data mining based on clustering segmentation with descriptors which consider gray scale values and gradient in each segment. This system allows a 95 percent of L. botrana recognition in non-fully controlled lighting, zoom, and orientation environments. Our image capture application is currently implemented in a mobile application and subsequent segmentation processing is done in the cloud.
The grapevine moth or Lobesia botrana (Lepidoptera: Tortricidae) is an invasive insect species considered as one of the most damaging pests in vineyards (Vitis vinifera L.) . Vineyards damage is caused by direct larval feeding inside grape berries, producing rot and dehydration, inducing lowers productive yields and increasing the susceptible to other diseases such as gray and black mold (Botrytis cinerea and Aspergillus niger) [2, 3]. The L. botrana is widely distributed in Europe and is considered as a major grapevine pest since the twentieth century. In South America, L. botrana was first detected in Chile, in the area of Linderos, metropolitan region in April, 2008 . Today, L. botrana is spread to all grape growing regions of Chile.
L. botrana management has become particularly relevant in Chile due to the potential economical impact as the main grape exporter worldwide. The moth control is performed using traps containing pheromones which attracts the moth males . These traps are located on vineyards within an area formed of radius of 500 meters from a detected outbreak . Currently, there are about 33.000 traps distributed throughout Chile for the specific control of L. botrana. To calibrate and verify moth growth models, in order to take measures focused on the vineyards with outbreaks, it is needed to have information on the number of moths caught in each trap, in different time windows. However, the collection of this information is done manually, making the system costly in terms of time and not allowing to perform real-time actions on these vineyards. Therefore, it is needed to have a control system for automatically detection of L. botrana, to estimate the real number of the moths and to store the information associated with each trap for epidemiological studies. Given the number of traps and images captured, the system must have the ability to recognize specimens of L. botrana in large volumes of images. Additionally, because the jobs are executed in a specific period of the day, it is useful to manage the computing capacity according to demand.
One of the most satisfactory image recognition systems rely on binary classification using a sliding window approach. However, the sliding window approach strongly increases the computational cost, because the classifier function has to be evaluated over a large set of candidate subwindows . Considering the need for massive image processing for the recognition of L. botrana specimens, this article proposes a novel method through a mobile application that captures images using a cell phone camera, together with an algorithm based on image segmentation, which performs fast and automatic recognition of L. botrana specimens. The algorithm is based on using the -means clustering technique together with gray scale and gradient descriptors to perform the classification . Because the normal operation of the L. botrana recognition system may need to process over 30,000 images per day, a distributed version of the algorithm was implemented using Apache Spark as a distribution framework.
Experiments were performed where a version of the algorithm was evaluated using sliding window with the version that uses segmentation. The experiments evaluate the quality of the recognition and the execution times of both algorithms. For the algorithm that uses segmentation, we also evaluated the scalability of the distributed version. The results show that the segmentation algorithm has improvements to the execution time and the quality of the recognition with respect to the one that uses sliding window. The remainder of this paper is organized as follows. In Section 2 we present related work in agriculture and segmentation. The application that captures the images and stores them in the database is described in Section 3. A brief description of the distribution framework is done in Section 4. Section 5 describes the details of the segmentation algorithm together with its distributed version. Finally results and conclusions are described in Sections 6 and 7.
2. Related Work
2.1. Computer Vision in Agriculture
Computer vision has been used in agriculture and food processing over the last decades, with the aim to automate a variety of processes such us quality inspection, classification, and sorting of products . The idea was to replace traditional manual operations that are labor intensive, slow, and prone to human error . Despite the significant advances over the past decade, the creation of new algorithms, new paradigms, and new challenges to processing big complexity data allows applying computer vision research in real world problems, with a particular focus in agriculture [11–14].
One of these challenges is the application of image segmentation techniques, which separates the product region from background in image processing. It is one of the first steps in image analysis after the image capture to subdivide an image into meaningful regions. The segmentation result affects the subsequent image analysis. Image segmentation techniques in agriculture have been applied to the estimation of product size , shape , sorting , weed detection for site-specific treatment , and ground classification .
Another important area of research in the field of computer vision and agriculture has been the application of deep learning techniques [20–22]. This machine learning technique is based on the concept of neural networks and convolution to learn image features automatically by repeated training, error propagation, and incremental learning [20, 23–25]. Some deep learning architectures have been tested to identify moths and other insect pictures using images obtained mainly in laboratory conditions with great success [26–30].
2.2. Segmentation Techniques
In the next section we present the main image segmentation techniques currently being used.
(1) Edge Based Image Segmentation. Edge detection includes a variety of mathematical methods that is aimed at identifying points in a image at which the brightness changes sharply or has discontinuities. Using spectral methods and watershed morphological algorithms in , a segmentation framework based on edge detection was proposed. In  an edge detection based on illumination invariant feature detector phase congruency was proposed. The authors show that use of phase congruency for marking features has significant advantages over gradient-based methods. In  an edge based auto threshold select method to generate multiscale image segmentation was proposed. Band weight and Normalized Difference Vegetation Index (NDVI) are used to calculate edge weight.
(2) Threshold-Based Image Segmentation. In classical threshold image segmentation, an image is usually segmented and simply sorted to object and background by setting a threshold. But if there is complex information in the image, the threshold is not simple to obtain. An important line of research in threshold-based image segmentation has been to design algorithms that allow obtaining the optimum threshold. For example, in  a method to detect threshold based on entropy and applied to images in scale of grays was developed. Histograms were used in  to address the automatic detection of a threshold. In  a threshold-based level set approach including threshold-based segmentation and fast marching method was proposed. It was applied to medical image segmentation. In  they presented a new threshold segmentation method based on computational intelligence to improve the acquisition of images process in computer vision.
(3) Partial Differential Equation Based Image Segmentation. One way to perform segmentation is to detect the contours of the objects lying in the original image. The main idea of the PDE model is to translate the problem of segment objects into minimizing an energy function of a close curve. In literature there are numerous examples that apply PDE to segmentation. In  a variation model is presented using 4th-order PDE with 2nd-order PDE for the removal of finger vein image. In  a new segmentation model was used based on the geodesic contour model applied to color images. A new nonlinear partial differential equation (PDE) applied to gray images was proposed by .
3. Image Capture Application
A mobile and web application was developed in order to capture images and report relevant information to users. Figures 1 and 2 show screen shots for the mobile and web application. The mobile application is a native Android app. Images where taken on-field where traps hang, recording GPS position and date time of the capture. The image is then uploaded via cellular network (or Wi-Fi if available) to a cloud server to be processed. After processing, the image is displayed at the web application to an expert entomologist in order to asses the moth count and take action based on the pest situation for that specific place.
Figure 3 depicts the steps for capturing a trap image. First the trap must be opened and the sticky grid side must be placed facing forward to the capturing device. This device can be a cellular phone with a camera or a regular digital camera. Some traps have a triangular shape (delta traps) or a box shape. For some traps its sticky side has no grid, as shown in the example picture in Figure 3.
During September 2016 until February 2017, 50 traps where deployed between V, VI, VII, and metropolitan regions. A total of 26 users generated more than 400 on-field images. Figure 4 shows trap locations across V, VI, VII, and metropolitan regions. These traps were visited regularly once a week and a picture was obtained each time. Figure 5 shows example of images obtained on-field using the developed mobile application.
4. Spark Distributed Framework
This section describes the main concepts of the distributed Spark framework. This framework will be used in Section 5.4 for the distributed version implementation of the segmentation algorithm. Distributed implementation aims to address the problem of large volumes of images.
In recent years the amount of available data has increased significantly . This is mainly due to the simple and inexpensive storage process. However, this amount of data is useless without an adequate process of extraction of knowledge where we use machine learning methods. This volume, diversity, and complexity  of the data brings challenges to researchers, because traditional machine learning methods can not deal with this situation. Cloud-based technologies provide an ideal environment to handle the data challenge. A pioneer in addressing this problematic of massive and complex data was the MapReduce Framework . This framework is based on the principle of locality of the data  which is implemented in a distributed filesystem. However, MapReduce is insufficient for applications that need to share data through multiple steps, or for iterative algorithms . Many platforms for large-scale processing have recently emerged to overcome the issues presented by Hadoop MapReduce. Among them, Spark  appears as one of the most flexible and powerful engines to perform faster distributed computing in big data by using in-memory primitives.
Spark is a state of the art framework for high performance parallel computing designed to deal with iterative procedures that recursively perform operations over the same data. This has been used in machine learning algorithms , imaging processing , bioinformatics , computational intelligence , astronomy , medical information , and so on.
Spark was born as an in-memory cluster computing framework for processing and analyzing large amounts of data. It provides a simple programming interface, which enables an application developer to easily use the CPU, memory, and storage resources across a cluster of servers for processing large datasets in memory .
Spark has been positioned quickly as a general purpose platform. It provides a unified integrated platform for different types of data processing jobs. It can be used for batch processing, iterative process, interactive analysis, stream processing, machine learning, and graph computing.
Resilient Distributed Datasets (RDDs) are the core data units in Spark. These units are distributed and immutable; that is, the transformation of RDDs is RDDs and fault-tolerant memory abstraction. There are two types of operations: transformations, which take RDDs and produce RDDs, and actions, which take RDDs and produce values. Various cluster management options can be used for run Spark, from simple Spark’s standalone solutions, Apache Mesos, and Hadoop YARN .
Considering engineering applications we choose to use the Hadoop YARN management. Hadoop YARN, most recent implementation, uses cloud computing  which makes hundreds of machines provide services such as computing and storage on demand. Generally in-house implementations require large investments in hardware, software, and maintenance .
This section aims to describe the segmentation algorithm. First, a general description will be presented and then detailed by modules. The last section contains the distributed version of the algorithm.
Figure 6 shows the flowchart that allows constructing the classifier for L. botrana moths. As a dataset we consider 360 images obtained through the application described at Section 3. The 360 images correspond to three groups of 120 images each with resolutions , , and . 100 images are left to perform validation. For each one of the images a preprocessing, then a segmentation, and finally the generation of the descriptors that allow performing the training for the SVM classifier are applied. For the classifier training, a set of 5018 segments without moths and 2136 segments with moths were selected.
Additionally with the goal of evaluating the segmentation algorithm with respect to the recognition of L. Botrana we compared the performance to different elements usually glued on the sticky floor of the trap like other insects, leaf pieces, and equipment used in the trap. With this purpose, we analyzed 5018 segments without moths and a subset of 1325 segments containing elements other than the bottom of the trap were extracted.
5.1. Preprocessing Stage
As the first activity of the preprocessing stage, a Median Filter is used with the intention of removing particles of dust and brightness that appear at the photographs. An example is shown in Figure 7. The filter used was a matrix where each pixel has the same weight. Subsequently an equalization is applied using the Contrast Limited Adaptive Histogram Equalization (CLAHE) algorithm. This algorithm uses histograms computed over different tile regions of the image. Local details can therefore be enhanced even in regions that are darker or lighter than most of the image. Then we apply 3 scaling ratios to the image: 0.75, 0.5, and 0.25. These scaling steps aim to consider that photographs can be taken at different distances from the cell phone. To perform the scaling, nearest-neighbor interpolation was used. Finally a gray scale conversion was executed. This conversion uses the ratio .
5.2. -Means Segmentation Algorithm
The main objective of the project is to recognize L. botrana moths. A fundamental stage corresponds to the segmentation of moths. To perform the segmentation, each color image is scaled at 0.75, 0.50, and 0.25 and -means clustering technique is applied to each image. In the definition of metric space to be able to apply -means, the pixel distance shown in (1) and color distance shown in (2) are used. To achieve the proper segmentation a trade-off between pixel distance and color distance is realized. This trade-off is defined in (3). This type of segmentation has previously been used by  in benchmark datasets and in biomedical applications:
corresponds to the metric used to perform the segmentation and is a normalizing factor, where is the number of pixels and is the number of segments. The parameter allows handling the weighting between the spatial distance and the colors distance. In Figure 8 it is possible to observe two segmentations. Figure 8(a) uses and Figure 8(b) uses .
5.3. Descriptors and SVM Classifier
After segmentation was performed, the next step is to differentiate the segments containing moths from those that do not include moths. To achieve this classification, gray scale segments are used. For each segment two descriptors are defined. The first descriptor corresponds to the histogram of the gray values of the segment. The histogram is configured with 10 bins and each bin is normalized with the amount of pixels that the segment contains. This generates a 10-dimensional vector where each dimension has values in the interval [, ]. A second descriptor is obtained as a result of calculating the histogram on the gradient modulus. This histogram is also normalized by the total number of pixels in the segment. In this case the number of bins used is 5. The following steps are used to calculate the gradient modulus.
(1) Gradient Calculation. A Sobel  operator is used. The complete image, , is used to calculate each component of the gradient in all pixels of the image:(2) Calculation of the Gradient Modulus. For all pixels in image the gradient modulus is calculated according to (3) Histogram Calculation. The histogram of is calculated for each segment obtained from the -means segmentation process.
The last step corresponds to the training the classifier. At training, 5018 segments are used without moths and 2136 segments with moths are considered. A -fold cross validation was performed with to perform the parameter adjustment.
5.4. Distributed Algorithm
Due to the estimated volume of photographs that need to be processed and considering that the process of segmentation along with the generation of descriptors corresponds to the one who consumes the most time, in this section we present a distributed version of the algorithm for segmentation and generation of descriptors. This version was designed using the Spark distributed computing framework.
The algorithm can be divided into two main stages: a segmentation stage and a descriptor generation stage. In the segmentation stage the algorithm operates on the complete image. The result of this stage corresponds to a list of segments. The descriptor generation stage has as input the list of segments and, for each one of these segments, descriptors are calculated. The distributed algorithm will address the distribution of calculations in the segmentation process as well as the calculation of descriptors in the segment list.
Segmentation Stage. Let us assume that the list of images has been read from Hadoop distributed file system (HDFS), used to store images. To perform the reading, the binaryFiles function is used, generating the corresponding RDD files. In addition, the list of initial centroids has been obtained, considering an equidistant distribution for the different centroids. The distribution process in the segmentation stage is performed at the pixel and centroid level, where a pixel corresponds to the 5-dimensional distance-color vector defined in (3). The first stage of the segmentation phase is to perform a mapping shown in Algorithm 1. Each pixel is mapped to the pair (, ) where corresponds to the centroid closest to pixel . To perform this operation we use the Spark map transformation. The second stage is to perform a reducebykey, where the key corresponds to the centroid (see Algorithm 2). The vector sum of the pixels is done in the 5-dimensional space, together with the count of the pixels associated with the centroid. To perform this operation, consider the reducebykey action. Finally the value of the new centroid is updated.
Descriptor Generation Stage. This stage aims to perform the calculation of the descriptors that allows classification. At this stage the image is distributed at the segment level. To perform the distribution a map function is used that maps the segment to the pair (segment, descriptor). Each map involves the calculation on a segment (see Algorithm 3).
6.1. Parameters Setting
Considering that each algorithm involved requires parametrization, in order to find the best parameters, it was considered to perform a -fold cross validation for each of the combinations shown at the Range column in Table 1. In the cross validation, was considered. As a measure of the best configuration, was used. In Table 1, final column shows the selected value for the configuration that yields the best accuracy.
In order to have a better clarity of the effect of the segmentation on the classification, the parameter NumSegment was studied. In order to perform this study, an experiment was defined in which the remaining parameters are fixed considering the value of the final column and -fold cross validation is performed with considering the values 500, 1000, and 1500 in the parameter NumSegment. For each cross validation result, the True Positive Rate (TPR) and the False Positive Rate (FPR) were calculated. The result of the experiment is shown in Figure 9. It is observed that , represented in blue color, corresponds to the best result, for the case has a worse TPR performance and has a lower performance for FPR.
6.2. Descriptors Setting
As described at Section 5.3 a descriptor was used that combines a normalized histogram of gray scale with a normalized histogram using the gradient modulus. Equation (6) was used in order to determine the number of bins that best differentiates moth segments from those that do not contain moths corresponds to the normalized average histogram of the segments with L. botrana and WL to the normalized average histogram of the segments without L. botrana. corresponds to the function that returns the bin of a histogram. For each bin, the value obtained from the L. botrana segments is subtracted to the value of segments without L. botrana. Then the modulus function and the sum on bins are applied. The more alike the histograms are, the closer the Histogram Measure value is to 0; in our case we seek to maximize the Histogram Measure value. Ranges between 5 and 12 bins for the gray scale case and 4 to 10 bins for the gradient module were verified. The best results obtained after applying (6) are shown in Figures 10 and 11.
Finally, using the other elements dataset described in Section 5, we generate normalized average histograms for Gray and modulus of gradient. In Figures 12 and 13 the results are shown. Although the behavior of dataset histograms without L. botrana differs from histograms obtained by the dataset with other elements, when comparing the latter with L. botrana histograms, an adequate separation is observed.
6.3. -Means Segmentation Algorithm Evaluation
The evaluation of the segmentation algorithm was performed using the 100 images that were not included at the parameters configuration stage. To perform the comparisons an algorithm was designed using sliding window. When we apply sliding window, it is assumed that objects have regular appearance that do not deform much. A database is built with nxm fixed size windows containing the centered object and others that do not contain it. This is used to train the classifier. A nxm window is slid across the entire image and the classifier is queried if each of these windows contains or not the object. The sliding algorithm uses the same steps and configurations as the segmentation algorithm described at Section 5.2, except in the -means segmentation stage where it uses sliding window. The window size was and the step size was 8 pixels.
Comparisons were focused on the quality of the recognition as well as the execution speed of the different algorithms. In the case of the quality evaluation, the recognition for the rescaled images at 1, 0.75, 0.5, and 0.25 was analyzed. For the execution of the instances we use a PC running Windows 10, on an Intel® Core i7-4770 processor with 16 GB in RAM and programmed in Python 2.7.
Table 2 shows the resulting confusion matrix for both algorithms. It should be noted that the segmentation algorithm was superior to sliding window in all cases. Because our matrix has unbalanced class distribution, the following indicators were considered for comparison:(1)Sensitivity: TP/(TP + FN)(2)Precision: TP/(TP + FP)(3) Score: (4)Mathews correlation coefficient: .
The segmentation algorithm yielded an average specificity of 95.1%; in the case of sliding window this was 91.4%. In the case of the precision indicator, the segmentation algorithm yielded an average value of 92.2%, and the sliding window was 90.01%. The score yielded by the segmentation algorithm was 93.65% and in the case of sliding window was 90.76%. Finally for the Matthews coefficient the value was 93.36% for segmentation and 90.41% for sliding windows.
Additionally, a confusion matrix was constructed to evaluate the performance of the algorithm with respect to the other elements data (Table 3). This dataset was generated from the 100 images with 1135 segments containing other elements. The segmentation algorithm for the case of the other elements data, obtained an average specificity of 95.15%, an accuracy of 99.5%, an coefficient of 97.2, and a Matthews coefficient of 88.1%.
To perform the comparison of execution times, we considered the three groups of images with resolutions of , , and . For each of the images the processing time is obtained and represented by a box plot shown in Figure 14. In this figure it is observed that the segmentation algorithm has a better execution time than sliding window. For the case of the segmentation algorithm the average time was 31.8 (s) for the group , 51.5 (s) for the result , and 118 (s) for . When using sliding window the time was 316.5, 649.2, and 1515.5 respectively. Finally Figure 15 shows the classification result for two test images.
6.4. -Means Segmentation Algorithm Scalability
In the case of the distributed algorithm the experiments were focused on measuring its scalability. The algorithm was developed using Python 2.7 and Spark libraries. It was run on Azure platform using version 1.5.2 of Spark and Hadoop 2.4.1.
In order to evaluate the algorithm we considered the resolution, number of images, and different numbers of executors. As dataset the 360 images used to train and validate the algorithm were considered. These images were replicated 1, 10, and 100 times in order to generate groups of 360, 3600, and 36000 images. Each group was divided according to their image resolution. For the scalability test 1, 2, 4, 8, and 16 executors were considered. The relative speed-up by group and resolution was measured. For the Spark configuration, three parameters were considered: num-executors which controls the number of executor requested, executor-cores property which controls the number of concurrent tasks an executor can run, and executor-memory which corresponds to the memory per executor. For the proper use of an executor it is recommended to use between 3 and 5 cores. The considered Spark settings are shown in Table 4.
Figure 16 shows the relative speed-up curves for Spark distributed implementation of segmentation algorithm. The relative speed-up is calculated with respect to the execution time obtained when performing the calculation using 1 executor. The blue line corresponds to the perfect result. Curves show at the three experiments that they continue delivering speed-up to 16 executors. When over 16 executors the performance is clearly sublineal. For the dataset with 360 images the deviation from the perfect result on average was 20.6%, for the case of 3600 images 25.6%, and for the case of 36000 images 32.1%. Regarding resolutions, the one that had the best performance in all cases was the one with the lowest resolution, but the effect of the resolution was small.
In this work we have presented a segmentation algorithm to perform the early recognition of Lobesia botrana. A mechanism for the adequate parameter setting selection and definition of the descriptors that allow classification was developed. Image capture mobile and web application was created for Lobesia botrana recognition. The native Android mobile application takes the images and records the GPS position and date, making possible the real-time moth counting. The image recognition system was validated during September 2016 until 2017, in 50 traps deployed in vineyards between V, VI, VII, and metropolitan region in Chile.
The classifier algorithm developed for L. botrana recognition included a preprocessing step using a median filter and CLAHE equalization and then a segmentation step using -means clustering and the generation of two descriptors; one corresponds to the gray scale segment histogram and the other obtained calculating the gradient modulus histogram. A dataset of 360 images was used for classifier construction.
The -means segmentation algorithm was evaluated in comparison with the sliding window approach. The segmentation algorithm was superior to sliding window in quality of the recognition for the rescaled images at 1, 0.75, 0.5, and 0.25. The segmentation algorithm yielded an average specificity of 95.1.
A Spark distributed version of the system was developed. This system allows fast and scalable cloud-computing analysis of the images, providing an ideal environment for on-field applications. The scalability of the distributed -means segmentation algorithm was evaluated. The curves show they continue delivering speed-up to 16 executors. When over 16 executors the performance is clearly sublineal. For the dataset with 360 images the deviation from the perfect result on average was 20.6%, for the case of 3600 images 25.6%, and for the case of 36000 images 32.1%. The one that had the best performance in all cases was the one with the lowest resolution, but the effect of the resolution was small.
Using the additional information of timestamps and GPS positioning captured by the implemented application, future work is intended to generate models that allow determining areas of risk of Lobesia botrana in addition to conditions that favor the development of the plague. From the point of view of computer science it is interesting to work in a computational intelligence algorithm that allows determining the best parameters setting. This automatic determination of parameters would allow the algorithm to be easily adaptable to similar recognition problems.
Conflicts of Interest
The authors declare that they have no conflicts of interest.
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