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Boundary-based extraction that uses Hough transform (HT)
was described in [13]. It detects straight lines in the image
to locate the license plate. The Hough transform has the
advantage of detecting straight lines with up to 30° inclination
[20]. However, the Hough transform is a time and memory
consuming process. In [21], a boundary line-based method
combining the HT and contour algorithm is presented. It
achieved extraction results of 98.8%.
The generalized symmetry transform (GST) is used to
extract the license plate in [22]. After getting edges, the image
is scanned in the selective directions to detect corners. The
GST is then used to detect similarity between these corners
and to form license plate regions.
Edge-based methods are simple and fast. However, they
require the continuity of the edges [23]. When combined
with morphological steps that eliminate unwanted edges, the
extraction rate is relatively high. In [8], a hybrid method based
on the edge statistics and morphology was proposed. The
accuracy of locating 9786 vehicle license plates is 99.6%.
B. License Plate Extraction Using Global Image Information
Connected component analysis (CCA) is an important tech-
nique in binary image processing [4], [24]–[26]. It scans a
binary image and labels its pixels into components based on
pixel connectivity. Spatial measurements, such as area and
aspect ratio, are commonly used for license plate extraction
[27], [28]. Reference [28] applied CCA on low resolution
video. The correct extraction rate and false alarms are 96.62%
and 1.77%, respectively, by using more than4hofvideo.
In [29], a contour detection algorithm is applied on the
binary image to detect connected objects. The connected
objects that have the same geometrical features as the plate are
chosen to be candidates. This algorithm can fail in the case of
bad quality images, which results in distorted contours.
In [30], 2-D cross correlation is used to find license plates.
The 2-D cross correlation with a prestored license plate
template is performed through the entire image to locate the
most likely license plate area. Extracting license plates using
correlation with a template is independent of the license plate
position in the image. However, the 2-D cross correlation is
time consuming. It is of the order of n
4
for n × n pixels [14].
C. License Plate Extraction Using Texture Features
This kind of method depends on the presence of characters
in the license plate, which results in significant change in
the grey-scale level between characters color and license
plate background color. It also results in a high edge density
area due to color transition. Different techniques are used in
[31]–[39].
In [31] and [39], scan-line techniques are used. The change
of the grey-scale level results in a number of peaks in the scan
line. This number equals the number of the characters.
In [40], the vector quantization (VQ) is used to locate the
text in the image. VQ representation can gives some hints
about the contents of image regions, as higher contrast and
more details are mapped by smaller blocks. The experimental
results showed 98% detection rate and processing time of
200 ms using images of different quality.
In [41], the sliding concentric windows (SCW) method
was proposed. In this method, license plates are viewed as
irregularities in the texture of the image. Therefore, the abrupt
changes in the local characteristics are the potential license
plate. In [42], a license plate detection method based on sliding
concentric windows and histogram was proposed.
Image transformations are also widely used in license plate
extraction. Gabor filters are one of the major tools for texture
analysis [43]. This technique has the advantage of analyzing
texture in unlimited orientations and scales. The result in [44]
is 98% when applied to images acquired in a fixed and specific
angle. However, this method is time-consuming.
In [32], spatial frequency is identified by using discrete
Fourier transform (DFT) because it produces harmonics that
are detected in the spectrum analysis. The DFT is used in
a row-wise fashion to detect the horizontal position of the
plate and in a column-wise fashion to detect the vertical
position.
In [36], the wavelet transform (WT)-based method is
used for the extraction of license plates. In WT, there are
four subbands. The subimage HL describes the vertical
edge information and LH describes the horizontal one. The
maximum change in horizontal edges is determined by
scanning the LH image and is identified by a reference line.
Vertical edges are projected horizontally below this line to
determine the position based on the maximum projection. In
[45], the HL subband is used to search the features of license
plate and then to verify the features by checking if in the LH
subband there exists a horizontal line around the feature or
not. The execution time of license plate localization is less
than 0 .2 s with an accuracy of 97.33%.
In [46]–[48], adaptive boosting (AdaBoost) is combined
with Haar-like features to obtain cascade classifiers for license
plate extraction. The Haar-like features are commonly used
for object detection. Using the Haar-like features makes the
classifier invariant to the brightness, color, size, and position
of license plates. In [46], the cascade classifiers use global
statistics, known as gradient density, in the first layer and
then Haar-like features. Detection rate in this paper reached
93.5%. AdaBoost is also used in [49]. The method presented a
detection rate of 99% using images of different formats, size,
and under various lighting conditions.
All the methods based on texture have the advantage of
detecting the license plate even if its boundary is deformed.
However, these methods are computationally complex, espe-
cially when there are many edges, as in the case of a complex
background or under different illumination conditions.
D. License Plate Extraction Using Color Features
Since some countries have specific colors for their license
plates, some reported work involves the extraction of license
plates by locating their colors in the image.
The basic idea is that the color combination of a plate and
characters is unique, and this combination occurs almost only
in a plate region [50]. According to the specific formats of
Chinese license plates, Shi et al. [50] proposed that all the
pixels in the input image are classified using the hue, lightness,
and saturation (HLS) color model into 13 categories.