max
zw1
, max
zw2
are the highest two correlations.
avg-Corr(z, w) =
avg(max
zw1
, max
zw2
)
2
where
{max
zw1
, max
zw2
} = max{Corr(F-HS
z
, F-HS
w
),
{Corr(F-HS
z
, FP-HS
wi
); i = 1, ..., P}}
(5)
3.1.5 Complexity of F-HS & FP-HS
We compare the computation time of the traditional
HS, the F-HS and the FP-HS models to build color
histograms using the Ukbench dataset (the details of
this dataset are described in Subsection 4.1). Table 1
shows that the F-HS and FP-HS require a longer time
to generate their histograms than the crisp HS model.
However, the F-HS and FP-HS models significantly
improve the performance of image NDR (see Subsec-
tion 4.3.1) comparing to the HS histogram. Moreover,
F-HS and FP-HS still too faster than the SIFT algo-
rithm (which needs hours to complete the feature ex-
traction for the same image dataset). In addition, the
F-HS and the FP-HS models produce a lesser amount
of features than the SIFT algorithm. Hence, they ac-
celerate the matching process.
Table 1: Time computation of HS, F-HS and FP-HS his-
tograms using the Ukbench dataset.
Method HS F-HS FP-HS
Sub-images - - P = 3 P = 9
Time (Sec.) 151 273 381 530
3.2 SIFT Feature Extraction
In this work, we aim to present the effect of us-
ing the F-HS and FP-HS in improving the perfor-
mance of image NDR and ZIR. Therefore, in the
step of extracting the SIFT features we are not going
to discuss the optimized SIFT methods (Alyosef and
N
¨
urnberger, 2017a; Alyosef and N
¨
urnberger, 2017b;
Alyosef and N
¨
urnberger, 2016; Khan et al., 2011) in-
stead, we apply the original SIFT algorithm (Lowe,
2004) to extract the SIFT keypoints and build their
128 dimensions descriptors. The original SIFT algo-
rithm (Lowe, 2004) extracts features using grayscale
color space i.e. the color information play no role
in the building of descriptors. To match the key-
points, we utilize the Kd-tree and the best-bin-first
algorithm as described in (Lowe, 2004). However,
this method of matching obtains duplicate matches
i.e. a keypoint of one image may match with many
keypoints in the other one. To overcome this prob-
lem, we eliminate all duplicate matches except the
one which has the best matching score. This filtering
of matched features is important to reduce the number
of mismatched features. Further discussion to filter
the matched features have been discussed in (Alyosef
and N
¨
urnberger, 2019)
3.3 Re-rank the Top N Results
To optimize the NDR results obtained by the F-HS
and the FP-HS models, we apply the SIFT algorithm
on the top N retrieved results. Consequently, no need
to compare the SIFT features of a query image with
all SIFT features of dataset images. Instead, we com-
pare the features of a query image with only the top
N retrieved results where size(N) << size(Dataset).
In the Section 4, we discuss the suitable values for the
top N results. Figure 2 details the step of our method.
4 EVALUATION
We evaluate the performance of our hybrid model to
solve image NDR and ZIR tasks. We describe our
experiments as in the following subsections.
4.1 Datasets
In this work, since we aim to solve two tasks (i.e.
image NDR and ZIR), we decide to use two image
datasets. The first one is UKbench dataset which con-
tains 10200 images of 2550 various scenes (Nist
`
er and
Stew
`
enius, 2006). For each scene, there are four near-
duplicate images. We pick the first image as query
image and keep the rest three images in the dataset.
So we get 2550 queries. The second is the Oxford
building dataset to solve the ZIR task (Philbin et al.,
2007). This dataset contains images of the same sight
but not necessarily the same scene i.e. the images
present inside and outside parts of sights. To use this
dataset for solving the ZIR task, we generate three
sets of zoomed-in images by cropping and rescal-
ing the images of the oxford building dataset. These
datasets are Oxford-Zoomed-in-50, Oxford-Zoomed-
in-25 and Oxford-Zoomed-in-10 where the zoomed-
in images cover 50%, 25% and 10% of the original
scene respectively. We use these three constructed
datasets as queries to solve the task of the whole scene
(original images of oxford buildings) retrieval.
4.2 Evaluation Measures
To evaluate the performance of the proposed F-HS,
FP-HS models and the hybrid approach, we compute
the recall, MAP and VR (Alyosef and N
¨
urnberger,
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