RECOGNITION OF MULTIPLE-FOOD IMAGES BY DETECTING CANDIDATE REGIONS
Yuji Matsuda, Hajime Hoashi and Keiji Yanai
Department of Informatics, The University of Electro-Communications, Tokyo
1-5-1 Chofugaoka, Chofu-shi, Tokyo, 182-8585 Japan{matsuda-y,hoashi-h,yanai}@mm.cs.uec.ac.jp

ABSTRACTIn this paper, we propose a two-step method to recognize
multiple-food images by detecting candidate regions withseveral methods and classifying them with various kinds offeatures. In the ﬁrst step, we detect several candidate regions by fusing outputs of several region detectors including
Felzenszwalb’s deformable part model (DPM) [1], a circle detector and the JSEG region segmentation. In the second step,
we apply a feature-fusion-based food recognition method forbounding boxes of the candidate regions with various kinds ofvisual features including bag-of-features of SIFT and CSIFT
with spatial pyramid (SP-BoF), histogram of oriented gradient (HoG), and Gabor texture features.
In the experiments, we estimated ten food candidates for
multiple-food images in the descending order of the conﬁdence scores. As results, we have achieved the 55.8% classiﬁcation rate, which improved the baseline result in case of using only DPM by 14.3 points, for a multiple-food image data
set. This demonstrates that the proposed two-step method iseffective for recognition of multiple-food images.
Index Terms— multiple-food image, region detection,
window search, multiple kernel learning
1. INTRODUCTION
Recently, personal services to recode people’s food habits using mobile phones have become popular. Users can becomeaware of own diet, and evaluate nutrition by recording theirtaken meals. When recording meals, inputing the names offood items by texts or selecting food items by hierarchicallinks is the common way. To record several items of foods inevery meal in such way is a quite troublesome task. Therefore, it is desired to make recording of food items more easierand quickly. To this end, several methods to recognize foodimages have been proposed so far [2, 3, 4, 5].
However, all of these works assumed that one food image contained only one food item. They cannot handle animage which contains two or more food items such as ahamburger-and-french-fries image. In this paper, we propose
a new method to recognize food images which contain two
or more food items. In this paper, we call such images as“multiple-food images”. The proposed method detects candidate regions with several methods including Felzenszwalb’s
deformable part model (DPM) [1], a circle detector and the
JSEG region segmentation [6]. Then, we extract various kindsof image features from each candidate region. After applying the classiﬁcation models trained by multiple kernel learning [7], we obtain the names of the top N food item candidates over the given image. The experimental results demonstrate that the proposed method is very effective for recognition of multiple-food images.
Note that the objective of our system is not associating
extracted regions with names of food items directly, but listing all the names of the food items which are estimated to beshown in the given image, since our ﬁnal objective is recording the lists of eaten items and calculating total amount ofcalorie of each meal automatically.
The rest of this paper is organized as follows: Section 2
describes related work on object recognition including foodimage recognition. Section 3 explains the proposed method.
Section 4 describes the experimental results, and in Section 5
we conclude this paper.
2. RELATED WORK
2.1. Food Image Recognition
As food image recognition, S. Yang et al.[4] proposed a foodrecognition system which was specialized for American fastfood such as hamburger, pizza and tacos. They deﬁned eightbasic food materials such as bread, beef and cheese, and recognized them and their relative position in a food image. Finally, they classiﬁed images into one of 61 categories using
detected materials and their relations. They achieve the 28.2%
classiﬁcation rate for Pittsburgh fast-food image dataset[8].
Zong et al.[5] also proposed a food recognition system employing SIFT detector and Local Binary Pattern (LBP). They
achieved the better classiﬁcation rate than S. Yang et al.’s results on the same fast food dataset. These two works focus
on American fast foods, while we handle various 100 kinds
of foods which are mainly common in Japan in this paper.
Joutou et al. proposed a food recognition system the target of which are 50 kinds of common food items in Japan[2]. They have proposed a method to recognize food images
by fusing various kinds of image features include SIFT-based
bag-of-features, Gabor, and color histograms using multiple
kernel learning (MKL) [7], and have achieved the 61.34%
classiﬁcation rate. Hoashi et al. extended this system so as
to recognize 85 kinds of food items [3].
However, all of above-mentioned works assumed that one
food image contained only one food item, and the food item
was shown as large as possible in an image. They cannot
handle an image which contains two or more food items such

Given an input image, ﬁrst, the system detects candidate regions of dishes. In this paper, we use four types of detectors
including the deformable part model (DPM) [1], a circle detector, the JSEG region segmentation [6], and whole image.
Next, we integrate bounding boxes of the candidate regions
detected by the four methods. Then, we check the aspect ratio of width and height of the bounding boxes, and exclude irrelevant bounding boxes regarding their shapes from the candidate set. The system extracts various kinds of image features from the selected regions, and calculate SVM scores bymultiple-kernel learning (MKL) [7] with non-linear kernels.
Finally, we obtain the names of the top N food items over the
given image regarding the descending order of the evaluation
values.
Fig. 1. Recognition Flowas an image that represents a meal set including a hamburger
and french fries. In this paper, we propose a new method
to recognize food images which contain two or more fooditems. To our best knowledge, this is the ﬁrst work to recognize multiple-food items in one food image at the same time.
2.2. Object Detection
To detect objects in an image, a method based on sliding
window is a standard method. Viola-Jones face detector isthe most representative method among sliding-window-based
methods [9]. Recently, as a method to detect generic objects
the deformable part model (DPM) proposed by Felzenszwalb
et al. [1] is quite popular because of its discriminative power
and availability of source code. It is based on Histogram ofOriented Gradients (HoG) proposed by N. Dalal et al. [10].
In the DPM, several HoGs are used as part models, and their
spatial relations are also modeled. As a training method,
the latent SVM is used which is an extension of a standard
SVM. The Felzenszwalb’s DPM is widely-used as a standard
method of generic object detection. Since the DPM is basedon only HoG features and linear SVM, it is not sometimes effective to discriminate object categories which looks similar
to each other.
To cope with this problem, A. Vedaldi et al. proposed an
object detection method which combined linear-SVM-based
window search and non-linear-SVM classiﬁers [11]. In themethod, they detected several candidate regions using a jumping window search method roughly ﬁrst which are slightly
different from DPM, and next veriﬁed detected candidate regions with multiple kernel learning with non-linear kernelsuch as a chi-square-RBF kernel with high accuracy. Basically, we follow this idea to detect candidate regions of fooditems. In this paper, we use not only window search by DPM
but circle detector and region segmentation algorithm to detect candidate regions over the given image.
3. PROPOSED METHOD
In this paper, we propose a food image recognition system
which outputs the names of food items which are expected
to be shown in a given food image. We show the overview
of the processing ﬂow of the proposed system in Figure 1.

3.1. Candidate Region Detection
The existing works on food image recognition assumed that
one food image contained only one food item, and the food
item was shown as large as possible in an image. On the other
hand, we handle an image which contains two or more fooditems. To this end, at ﬁrst, we estimate several candidate regions where food items are expected to exist before extracting
image features. We use four kinds of candidate region detection methods including whole image, the deformable partmodel method (DPM) [1], a circle detector, and the JSEG region segmentation [6] as shown in Figure 1.
3.1.1. Whole Image
The simplest candidate region is a whole image. This candidate region is equivalent to the existing food recognition
systems [2, 3, 4, 5] which assume that one image contains
only one food item. This candidate is expected to work for
an image containing one large dish, but does not suit for animage containing multiple small dishes.
3.1.2. Deformable Part Model (DPM)
As a main detector, we use the deformable part model (DPM)
proposed by Felzenszwalb et al.[1]. The DPM is a twolayered hierarchical model, which consists of a global “root”
ﬁlter and several part models. Each part model speciﬁes aspatial model and a part ﬁlter. The spatial model deﬁnes a setof allowed placements for a part relative to a detection window, and a deformation cost for each placement. The scoreof a detection window is the score of the root ﬁlter on thewindow plus the sum over parts, of the maximum over placements of that part, of the part ﬁlter score on the resulting subwindow minus the deformation cost. Both root and part ﬁlters are scored by computing the dot product between a set ofweights and HoG features [10] within a window.
To detect object regions, sliding window approach isadopted in the DPM. In addition, the DPM is deﬁned at a ﬁxed
scale, and we detect objects by searching over an image pyramid. Therefore, to reduce computational cost, linear SVM
are used in the DPM method. To compensate less discriminative power of linear SVM than non-linear kernel SVM, inthe DPM, the latent SVM which is latent mixture of a standard SVM is used instead of a standard SVM. We trained theTable 1. Candidate region detection method

Method# of candidates
AdvantageWhole
1
is suitable forlarger food items

Disadvantage

is unsuitable forsmall food items

DPM
100 (1 for each item)
detect regions by
HoG-based partmodel
is based on only
gradient-based
features.

Circle4 (avg.)detectdishesby contours ofdishes.
dishes are not always circular.

JSEG
14 (avg.)
detect dishes bysegmentation.
segmentation
sometimes fails.

DPMs with the latent SVM for each of 100 food categories inthe experiments 1 .
3.1.3. Circle Detector
A circle detector detects regions of dishes by extracting circular contours from an image. First, it converts a given image toa gray-scale image. Then, it extracts contours by the Canny
Edge Detector. Finally, it detects circles by the Hough transform from extracted contours.
Note that we can detect not only circles but ellipses. However, in the preliminary experiments, ellipse detectors tend to
detect too many ellipses. That is why we use a circle detector
instead of more general ellipse detector.
3.1.4. Region Segmentation
Region segmentation is to divide an image into several pieces
of regions. In this paper, we use the JSEG Algorithm proposed by Deng et al. [6] 2 as a region segmentation algorithm.
JSEG divides an image by color space quantization and color
class map. In JSEG, the number of segmented regions can be
set as a parameter. In the experiment, we set the number ofregions as 10.
We calculate circularity C of each region, and add only
the regions where the circularity value exceeds the given
threshold value into a region candidate set. The circularity
C is calculated as follows:
C=

4πS
L2

(1)

where S is the size of a region, L is the perimeter of a region.
When C is closer to 1, the shape of the region is close to acircle. If C is much smaller than 1, the shape of the region ismuch far from a circle.
In addition, if circularity of the combined region of two
adjacent regions is larger than original ones and the threshold,
we add the combined region to a candidate set as shown in
Figure 2．
3.2. Integration of Candidate Region
We simply aggregate all the candidate regions detected byfour kinds of methods in a candidate set, and convert them
1 http://people.cs.uchicago.edu/˜pff/latent-release4/
2 http://vision.ece.ucsb.edu/segmentation/jseg/software/(a) input image(b) result of region seg- (c) integration of region
mentation

Fig. 2. Candidate region detection by region segmentation
into bounding boxes each of which circumscribes each detected region. In the second step, we extract various kinds ofimage features from the bounding boxes of all the detected
regions, and apply non-linear kernel SVMs which are trainedwith MKL.
Moreover, we exclude apparently irrelevant regions froma candidate set to reduce classiﬁcation cost and noisy candidates. In this paper, irrelevant regions are deﬁned regarding
their size and aspect ratio. We discard the bounding boxes theshorter side of which are less than 60 pixels and the boundingboxes the aspect ratios of which are more than twice standard deviations apart from the average aspect ratio of all thebounding boxes of food items in the training data.
3.3. Image Features
To recognize food images with visual features, just simply
using the SIFT and color does not give good results. Therefore, in this paper, we integrate various kinds of image features in the same way as Joutou et al.’s work [2]. They used
the multiple kernel learning (MKL) [7] to recognize whole
images, while we use MKL to recognize each food items ineach bounding box which is detected as a food candidate region. In this subsection, we describe the image features usedin this paper including bag-of-features with spatial pyramid(SP-BoF), histogram of oriented gradient (HoG), Gabor texture features, and color histograms.
3.3.1. Bag-of-features of SIFT and CSIFTThe bag-of-features (BoF) representation [12] attracts attention recently in the research community of object recognition,
since it has been proved that it has excellent ability to represent image concepts in the context of visual object categorization / recognition in spite of its simplicity. In the schemeof BoF, ﬁrst, a set of local image points is sampled and visual descriptors are extracted by the Scale Invariant Feature

(a) level 1(b) level 2(c) level 3

Fig. 3. Three-level pyramids for BoF
Transform (SIFT) descriptor [13] on each point. In addition to SIFT, we also extract CSIFT [14] which is extracted
SIFT from a RGB color space. CSIFT is proved to be robust against illumination changes [14]. In this paper, we use
regular grid sampling with every 10 pixels for sampling local
image points. Next, the resulting distribution of description
vectors is then quantiﬁed by vector quantization against prespeciﬁed codewords, and the quantiﬁed distribution vector isused as a characterization of the image. The codewords are
generated by the k-means clustering method based on the distribution of SIFT vectors extracted from all the training images in advance. That is, an image is represented by a set of“visual words”, which is the same way that a text document
consists of words. In the experiment, about several thousandsof points depending on images are sampled by the grid sampling. We set the number of codewords as 1000.

3.3.2. Spatial Pyramid Representation
Since an image feature vector in the bag-of-features representation represented by a histogram of distribution of SIFT vectors, spatial information of SIFT vectors is discarded. Then,
we use spatial pyramid representation [15] to take spatial information into account roughly. In spatial pyramid representation, object regions are divided by hierarchical grids. We
extract a BoF vector from each of the grids and concatenate
them into one long vector. In this paper, we use the threelevel pyramid which consists of 1 × 1, 2 × 2 and 3 × 3 grids
as shown in Figure 3. The dimension of the feature vector of
spatial pyramid BoF (SP-BOF) is 1000 in the pyramid level
1, 4000 in the pyramid level 2, 9000 in the pyramid level 3,
and 14000 totally. This SP-BoF feature is used not only for
MKL-based region classiﬁcation.

3.3.3. Histogram of Oriented Gradients
Histogram of Oriented Gradients (HoG) was proposed by N.
Dalal et al. [10]. It is similar to SIFT in terms of how to
describe local patterns which is based on gradient histogram.
The difference between HoG and BoF is that BoF completely
ignores location information of keypoints, while HoG keeps
rough location information by building histograms for eachdense grid and concatenating them as one long feature vector.
In short, HoG and BoF have different characteristics whileboth are composed of many local gradient histograms. In thispaper, we divides a given region into 8 × 8 cells. Assumingthat one block consists of 3 × 3 cells, one region corresponds
to 6 × 6 blocks. Finally, we obtain a 2916-dim vector fromeach region.

3.3.4. Gabor texture feature
A Gabor texture feature represents texture patterns of local
regions with several scales and orientations. In this paper, we
use 24 Gabor ﬁlters with four kinds of scales and six kindsof orientations. Before applying the Gabor ﬁlters, we divide agiven region into 8×8 blocks. We apply the 24 Gabor ﬁlters toeach block, then average ﬁlter responses within the block, andobtain a 24-dim Gabor feature vector for each block. Finally
we simply concatenate all the extracted 24-dim vectors into
one 1536-dim vector for each region.
3.4. Classiﬁcation for Candidate Region
After extraction of feature vectors from each candidate region, we calculate evaluation values of the candidate region
regarding each of all the given categories using support vector
machines (SVM) which are trained by multiple kernel learning (MKL).
As a kernel function of the SVM, in this paper, we use theχ2 RBF kernel which were commonly used in object recognition tasks. χ2 RBF kernel is deﬁned as follows:(∑ kxi − yi k2 )K(x, y) = exp −γ
xi + yii(2)where γ is a kernel parameter. Zhang et al.[16] reported thatthe best results were obtained in case that they set the averageof χ2 RBF distance between all the training data to the parameter γ of the χ2 RBF kernel. We followed this method to
set γ.
In this paper, we use the multiple kernel learning(MKL) [7] to integrate various kinds of image features. With
MKL, we can train a SVM with an adaptively-weighted combined kernel which fuses different kinds of image features.
The combined kernel is represented as follows:
0

Kcombined (x, x ) =K∑

βj Kj (x, x0 )

(3)

j=1

∑K
where βj ≥ 0 and j=1 βj = 1. βj is weights to combine
sub-kernels βj (x, x0 ). MKL can estimate optimal weightsfrom training data. In this paper, we train the support vector machine with one-vs-rest strategy using MKL in the same
way as [2]. Since the number of the given categories was 100
in the experiment, we trained 100 models independently.
By applying trained models for each candidate regions
regarding all the categories, we obtain evaluation values foreach candidate region. We sort the evaluation values over all
the candidate regions and all the categories in the descendingorder, and output the top N categories in terms of the evaluation values so that one food category is included in the output
food name list only once.
Note that the objective of our system is not associating
extracted regions with names of food items directly, but listing all the names of the food items which are estimated to
be shown in the given image. Therefore, we do not output
locations of bounding boxes of candidate regions.

Fig. 4. 100 kinds of food used in the experiments. Please see
this ﬁgure on a PDF viewer with magniﬁcation.

(a) Classiﬁcation rate for multiple food-item images(a) Examples of multiple food-item images.

(b) Examples of single food-item images.

Fig. 5. Examples of multiple and single food-item images.
4. EXPERIMENTS
For experiments, we build a new food image dataset as shownin Figure 4 which includes 100 categories with boundingboxes on each food item. It contains about one hundred images for each category and 9060 images totally. It includes
both of multiple and single food-item images as shown in Figure 5. In the experiments, we selected 500 multiple food-item
images which contain 1200 food items, and 1200 single fooditem images for testing, and we used the rest of all the images
for training.
In the experiments, we trained two kinds of classiﬁers
which are non-linear SVM to evaluate candidate regions,and linear-SVM to detect regions by the deformable partmodel (DPM). To train non-linear SVMs with MKL, we used
all the image features extracted from the regions within thegiven bounding boxes as positive training samples, and image features extracted from background regions and imagesof other kinds of food items as negative training samples.
To evaluate the performance, we use a classiﬁcation rate
CR regarding food items, which is deﬁned in the following
equation:(b) Classiﬁcation rate for single food-item images

Fig. 6. Classiﬁcation rates within the top N candidates
handled only single food-item images. Therefore, we regard“Whole” as a baseline method in this experiments. In addition, we compare the results with the result in case of using ground truth bounding boxes of the test images (GTBB),
which means the case that region candidate detection is perfectly correct. This results can be regarded as being ideal results and the upper performance by introducing region detection.

4.1. Experimental Results
Figure 6 (a) and (b) show the classiﬁcation rates for multiple food-item images and for single food-item images, respectively, when varying the number of the top candidates N
for evaluation. These ﬁgures contain the results by “Whole”,“DPM”, “Circle”, “JSEG” and “GTBB” as well as the resultsby the proposed methods. Note that our main objective is tonum. of correctly-detected food items in top N candidates recognize food items in multiple food-item images, but weCR =mention the results for single food-item images as well fornum. of all the food items in all the test image
reference.
When allowing top ten candidates, we achieved the 55.8%
If the top N candidates include the names of the food items
classiﬁcation rate which were improved by 40.4 points comappearing in the given food image, we count them as thepared to the baseline method for multiple food-item images,
correctly-detected food items.
and the 68.9% classiﬁcation rate which were improved by
In the experiments, we compare the result by the pro4.7 points compared to the baseline method for single foodposed method with the results in case of using only single
item images. The improvement for multiple food-item images
region candidate methods including four methods: whole imwas so much, which proves the effectiveness of the proposedage (Whole), deformable part model (DPM), circle detector
method as a method to detect multiple food items.
(Circle) and region segmentation by JSEG (JSEG). “Whole”
Since a single food-item image contains only one foodis equivalent to the existing methods as [2, 3], since they(a) Comparison for multiple item-food images

5. CONCLUSIONSIn this paper, we proposed a two-step method to detect multiple food items from one food image. In the experiments, regarding the results of the top ten candidates, we have achieved
55.8% and 68.9% on the classiﬁcation rate for multiple fooditem images and single food-item images, respectively, which
are improved by 40.4 points and 4.7 points compared to theresults without region detection, and improved by 11.3 points
and 26.7 points compared to the results by DPM alone. These
results show the effectiveness of the proposed method.
For future work, we need to improve recognition accuracy
by introducing co-occurrence probability between food items.
In addition, to estimate calories of each food items shown in afood image accurately, we plan to study a method on estimate
the amount of foods.
6. REFERENCES(b) Comparison for single item-food images

Fig. 7. Comparison between DPM with MKL and only DPM
item, single food-item images tend to be covered with theforeground regions of food items and have relatively smaller
background regions. Therefore, the difference between the
proposed method and the baseline regarding classiﬁcation
rate is not so much. On the other hand, multiple food-item
images tend to contain relatively larger background regions
and several smaller foreground regions. Thus, the proposed
method which recognizes not whole images but parts of images was required.
The results of “GTBB (ground-truth bounding boxes)”
can be regarded as being ideal results and the upper performance by introducing region detection. The difference between the results by the proposed method and the “GTBB”
results were about 10 points for both single and multiple fooditem images, which is room to improve for future work interms of candidate detection methods.
In the experiments, among single region detection methods, “DPM” was the best, since the DPM is one of the stateof-the-art object detectors. Then, we compare the resultsof “DPM MKL” which are obtained by applying both DPM
and MKL, with the results of “only DPM” by using only
DPM as shown in Figure 7. For multiple-food images, theclassiﬁcation rate of “DPM MKL” within top ten candidates
was 52.8%, while “only DPM” was 41.5%. For single-foodimages, “DPM MKL” was 65.4%, while “only DPM” was
38.7%. These results shows that the classiﬁcation rates were
improved much by the second step where we apply non-linear
kernel SVM by fusing various kinds of visual features.

[1] P.F. Felzenszwalb, R.B. Girshick, D. McAllester, and D. Ramanan,“Object detection with discriminatively trained part-based models,”IEEE Transactions on Pattern Analysis and Machine Intelligence, vol.
32, no. 9, pp. 1627–1645, 2010.
[2] T. Joutou and K. Yanai, “A food image recognition system with multiple kernel learning,” in Proc. of IEEE International Conference on
Image Processing, pp. 285–288, 2009.
[3] H. Hoashi, T. Joutou, and K. Yanai, “Image recognition of 85 foodcategories by feature fusion,” in Proc. of International Symposium on
Multimedia, pp. 296–301, 2010.
[4] S. Yang, M. Chen, D. Pomerleau, and R. Sukthankar, “Food recognition
using statistics of pairwise local features,” in Proc. of IEEE Computer
Vision and Pattern Recognition, 2010.
[5] Z. Zong, D.T. Nguyen, P. Ogunbona, and W. Li, “On the combinationof local texture and global structure for food classiﬁcation,” in Proc. ofInternational Symposium on Multimedia, pp. 204–211, 2010.
[6] Y. Deng and B. S. Manjunath, “Unsupervised segmentation of colortexture regions in images and video,” IEEE Transactions on PatternAnalysis and Machine Intelligence, vol. 23, no. 8, pp. 800–810, 2001.
[7] S. Sonnenburg, G. Rätsch, C. Schäfer, and B. Schölkopf, “Large Scale
Multiple Kernel Learning,” The Journal of Machine Learning Research, vol. 7, pp. 1531–1565, 2006.
[8] M. Chen, K. Dhingra, W. Wu, L. Yang, R. Sukthankar, and J. Yang,“PFID: Pittsburgh fast-food image dataset,” in Proc. of IEEE International Conference on Image Processing, pp. 289–292, 2009.
[9] P. Viola and M. Jones, “Rapid object detection using a boosted cascadeof simple features,” in Proc. of IEEE Computer Vision and Pattern
Recognition, pp. I:511–518, 2001.
[10] N. Dalal, B. Triggs, I. Rhone-Alps, and F. Montbonnot, “Histogramsof oriented gradients for human detection,” in Proc. of IEEE Computer
Vision and Pattern Recognition, pp. 886–893, 2005.
[11] A. Vedaldi, V. Gulshan, M. Varma, and A. Zisserman, “Multiple kernels
for object detection,” in Proc. of IEEE International Conference on
Computer Vision, 2009.
[12] G. Csurka, C. Bray, C. Dance, and L. Fan, “Visual categorization withbags of keypoints,” in Proc. of ECCV Workshop on Statistical Learningin Computer Vision, pp. 59–74, 2004.
[13] D. G. Lowe, “Distinctive image features from scale-invariant keypoints,” International Journal of Computer Vision, vol. 60, no. 2, pp.
91–110, 2004.
[14] A.E. Abdel-Hakim and A.A. Farag, “CSIFT: A SIFT descriptor withcolor invariant characteristics,” in Proc. of IEEE Computer Vision and
Pattern Recognition, vol. 2, pp. 1978–1983, 2006.
[15] S. Lazebnik, C. Schmid, and J. Ponce, “Beyond bags of features: Spatial pyramid matching for recognizing natural scene categories,” inProc. of IEEE Computer Vision and Pattern Recognition, pp. 2169–
2178, 2006.
[16] J. Zhang, M. Marszalek, S. Lazebnik, and C. Schmid, “Local Features and Kernels for Classiﬁcation of Texture and Object Categories:
A Comprehensive Study,” International Journal of Computer Vision,
vol. 73, no. 2, pp. 213–238, 2007.