A local mask based on the coefficient of variation
(CoV) of sets of pixels is proposed as an additional
step in FP detection. Also, the Uniformity metric is
introduced as a new criterion to evaluate FP detection
in areas under different lighting intensities. The ob-
tained results show better uniformity and repeatabil-
ity rate in most tested HDR images when compared
to standard FP detectors. Moreover, they indicate that
HDR images have great potential to be explored in
applications that rely on FP detection (Melo et al.,
2018).
2.3 FP Description Algorithms
There are several studies from Rana et al. (Rana et al.,
2016a; Rana et al., 2017b; Rana et al., 2019) fo-
cused on image description based on HDR images.
The first one (Rana et al., 2016a) compared image
matching using TM in HDR images and LDR images
using SIFT (Lowe, 2004), SURF (Bay et al., 2008),
FREAK (Alahi et al., 2012), and BRISK (Leuteneg-
ger et al., 2011) to describe the FPs. By using sev-
eral global and local TM algorithms, it is observed
that all combinations that used tone-mapped images
as input performed better than with LDR images. In
their second study (Rana et al., 2017b), Rana et al.
proposed an adaptive TM operator that uses SVR to
predict optimal modulation maps to improve FPs de-
scription, specifically to make image matching invari-
ant to day or night scene illumination. Finally, in the
third study (Rana et al., 2019), Rana et al. used the
adaptive TM operator to improve FP description in
image matching.
Khwildi and Zaid (Khwildi and Zaid, 2018) in-
troduce a descriptor based on LDR image expansion.
In this approach, the LDR image is converted to an
HDR image using a reverse TM operator (TM-HDR
image). Then, the resulting TM-HDR image is tone
mapped back to LDR image (TM-LDR) and SIFT de-
scriptor is used to describe FPs. As a result, it is
demonstrated that features described from the TM-
LDR image are more descriptive than those extracted
from LDR and TM-HDR images. In future works,
they consider exploring local TMs and improving the
efficiency of local descriptors directly in HDR im-
ages.
2.4 Applications
Ige et al. (Ige et al., 2016) developed a facial expres-
sion recognition algorithm using support vector ma-
chines (SVMs) and local binary patterns (LBP). First,
a TM algorithm is used to convert HDR into LDR
images. Then, the resulting tone-mapped image is
used as input to the SURF algorithm. Finally, the
tone-mapped images and the LDR images are com-
pared. As a result, the approach using tone-mapped
images showed better results than LDR images. Tone-
mapped images reach 79.8% accuracy, while the tra-
ditional methods that use LDR images range between
31.3% and 70.8% accuracy.
Ostiak et al. (Ostiak, 2006) used HDR images
to execute an image stitching process to generate a
panorama. In their study, a tone-mapped panorama is
generated to make the shadowed and bright areas of
the image visible. They mention a modification of the
SIFT algorithm to describe FPs in HDR images with-
out giving further details. The discussion is based on
a visual analysis of the generated panoramas, and the
results are subjective. The algorithm presents a better
performance in static scenes than in dynamic scenes,
when stitching shadowed and bright areas that are not
visible in LDR images.
The related works show that FP detection and de-
scription using HDR images as input is a field yet
to be explored. Most HDR-related studies use tone-
mapped images generated from HDR. Tone-mapped
images bring more details in shadowed and bright ar-
eas than LDR images and there is no need to adjust
detection and description algorithms.
On the other hand, few studies using HDR im-
ages as input were found and just Melo et al. (Melo
et al., 2018) detailed the modifications made in their
FP detection algorithm to support HDR images. Us-
ing HDR images brings more information but requires
adaptations in detection and description algorithms to
support floating-point values used to represent pixels.
All studies showed improvements when using
tone-mapped images as input to the detection and de-
scription algorithms. Specifically, Melo et al. (Melo
et al., 2018) showed that using HDR images as input
resulted in better detection performance in terms of
repeatability rate and uniformity, especially in shad-
owed areas of the image.
3 SIFT
The Scale Invariant Feature Transform (SIFT) de-
tects and describes FPs from an image. SIFT is one
of the most popular FP extractors, invariant to scale
and orientation (Rey Otero and Delbracio, 2014), and
shows good performance in detection and extraction,
as shown in various studies (P
ˇ
ribyl et al., 2016; Rana
et al., 2016a; Rana et al., 2017a; Rana et al., 2017b;
Rana et al., 2019).
To detect the FPs, SIFT creates an image represen-
tation from the input image called scale-space, which
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