ANPR-TUTORIAL.COM
THE AUTOMATIC NUMBER PLATE RECOGNITION
TUTORIAL
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1. Description, ANPR (Automatic
number plate recognition) or LPR (License plate
recognition) |
It is the technical method of artificial vision
(OCR) that allows the recognition of number plates in images of
vehicles. Historically, it has been applied on security systems to control
accesses of vehicles and car parks.
Nowadays, the ANPR
technology has improved its reliability, some systems
are able to offer recognition rates between 95 and 98%. Also,
some ANPR equipment are able to recognize the
number plate of vehicles that drive up tol 200km/h.
Generally, the ANPR technology
can be bought in two modalities :
The ANPR engine can recognize
the number plate directly from the images stored in a hard disk. This type of
software allows to take good use of images that have been obtained from other
systems like CCTV or cameras.
The ANPR equipment incorporates
all the hardware necessary to capture the images of the vehicles and to
recognize the number plate. Moreover, it incorporates the ANPR
engine. The ANPR equipment are designed to offer the
maximum reliability.
Depending on each case the ANPR
equipment or ANPR engine will have different uses and
advantages:
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To optain high reliability |
To recognize number plates directly from images
stored in a hard disc |
To make a good use of CCTV and camera systems |
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It is not recommended |
It is a habitual
use |
It is a habitual
use |
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It is a habitual
use |
No |
No |
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2. Parts of the generic system, Capture Unit (CU) +
Process Unit (PU) |
Although there already exist some
All-in-One equipment, the great majority of systems are still
using "CU+PU" architecture:
- (CU) Capture unit
- Camera housing
- Camera
- Infrared focus
- (PU) Process unit
- Embeded computer
- Framegraber
- Recognition engine
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Operation of the system
The "UC+UP" system consists of two parts. The
Capture Unit that takes the image of the vehicle, and the
Process Unit that receive the image from the Capture
Unit and makes the recognition of the number plate. The Process
Units can control one or more Capture Units
simultaneously.
The quality of the solution depends on each
manufacturer, although generally the "CU+PU" architecture has
disadvantages respect the ANPR All-in-One, they are the
following ones:
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1o - Nonrobust architecture
- The Capture Units controlled from the one only
Process Unit will fall if the Process Unit
does not work
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2o - Complex installation and start-up
- It is necessary to install video and control wire for each one of the
cameras
- Furthermore, it is necessary to provide power supply cable to the cameras,
focus and Process Units
- If the distance between the lanes are too long, the signal of the cameras
will not arrive with clearness to the Process Unit. For
example:
- If we have to control 4 lanes gathered in groups of two and separated by a
distance of 2 km, It will not be possible to control the 4 lanes with the same
Process Unit. Sometimes the cost of the installation may increase because of the
limitation of distances between the Capture Unit and the Process Unit (click
to view example)
- If the Process Units are installed directly on the lane it
will be necessary to protect it with a weatherproof rack
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3o - High cost
- The wiring and the time of installation are mulplied
- The maintenance of the system is multiplied
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3. Evolution to All-in-One equipment
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By contrast to the generic
ANPR, the ANPR equipment integrates directly
in the housing the camera, the processor, the communications and the power
supply unit
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4. Benefits of the ANPR
equipment |
The ANPR equipment with an
All-in-One architecture deletes the disadvantages of the
generic ANPR equipments, these are the following ones:
- It is more simple: All the necessary elements
for the ANPR process are integrated in the same housing. Only
one device is necessary for each lane to be controlled. The equipment may be
connected by Ethernet or serial communication with the client
application
- Modular architecture: If an equipment with
All-in-One architecture does not work, its fall does not affect
to the other lanes, because the Process Unit are
deleted
- Installation and start-up is easier:
- It is only necessary to provide 220v, Ethernet network or serial
communication to each equipment
- The instalation is as easy as to screw the equipment with the support, to
identify ANPR equipment with an IP and to adjust the optics
- If one equipment falls, it is possible to replace it by another one
- It reduces the cost:
- The wiring is reduced
- The installation and start-up time is reduced
- The maintenance of the system is reduced
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The ANPR process is divided
into three steps. The detection of the vehicle, the capture of the images and
the process of recognition. Next, we will detail step by step how it works and
depending on each case what the advantages and disadvantages are
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Detection of the vehicle. The first step is to take an image
of the vehicle at the right time. Thus, the number plate of the vehicle will be
visible in the image. Nowadays, three kind of triger control exist
- Hardware triger: The ANPR equipment
controls physically a sensor directly installed in the lane. Whenever a vehicle
has been detected by the sensor, the ANPR equipment will know
its presence, and then the process of the capture begins
- Software triger: The ANPR equipment
communicates with the client application, who physically controls a sensor
directlly installed in the lane. Whenever a vehicle has been detected by the
sensor, the client application knows the presence of the vehicle and
communicates it to the ANPR equipment. At this moment the
process of the capture begins
- Free flow: The ANPR equipment does not
need to receive signal from any external sensor. The ANPR
equipment takes images continuously and it is able to detect the
vehicles automatically
- Capture of the images. Once the vehicle is
detected, the following step is the capture of the vehicle. In order to take a
right image, the following points will have to be considered
- Type of cameras:
- Interlace camera: The capture of the images is made in two
steps. First uneven lines and later the even lines (click to see
example). This type of cameras are cheaper but its use is not recommended
for ANPR because if the vehicle is in movement, the number plate appears
defocused
- Progressive cameras: The capture of imagenes is made at
once (click to see
example). The use of this type of cameras is totally recommended because if
the vehicle is in movement the number plate always appears
focused
- Type of light. Infrared light is used generally for the
ANPR equipment. Although some ANPR equipment
are able to use the daylight and the infrared light during the night.
- Infrared light: The ANPR systems uses
infrared light because the human eye cannot detect it without other devices. One
infrared filter located in the camera allows to emphasize the number plate, but,
in the other hand, the rest of elements of the image are darkened
- Daylight: It is perceived by the human eye. It allows to
take images in which the vehicle is distinguished.
- Light management: The type of light is as important as how
it is managed. It is possible to use any typical technique of photography, but
the ANPR manufacturers have chosen two great ways:
- To control the light emitted by the focus
- To control the light that enters in the
camera
- Number plate recognition process. Each
ANPR manufacturer has developed its own recognition algorithms,
although, these are the main ones and the common ones
- To locate and to isolate the number plate in the image
- To correct the brightness and the contrast of the number plate
- To separate each character of the number plate
- To recognize each character of the number plate
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In some countries like Spain or Greece, the law binds the
recording of the number plate on the ticket. In this way, the number plate and
the ticket number are linked. Thus, car parks improve their management.
These are some direct benefits:
- Number plate register. The ticket number, day and the time,
and the number plate are linked and registered in the management application
- Finding a lost ticket. Thanks to the registry of the number
plates it is possible to find a lost ticket and to receive the correct
amount
- Ticket interchange is avoided. It is possible to block the
exit of a vehicle, if the number plate of the vehicle does not match with the
number plate in the entrance ticket
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The ANPR equipment has been used for the
access control of vehicles, it was thought as one more tool that allows
increasing the security. The client application could control users through
personal cards, and the ANPR allows vehicles control.
Nowadays, the ANPR equipment are used for
automatizing the accesses of vehicles. |
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These are the main advantages to incorporate
ANPR equipment in access control:
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Security increased: Integrating the
ANPR technology in access control aplications together with the
traditional control devices, allows vehicle and people control. Thanks to this
convination the security is increased
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Dynamic access of vehicles: Automatizing
vehicle access is possible through ANPR equipment. If the data
base knows the vehicle, the client application will open the barrier
automatically. By contrast, if the data base does not know the vehicle, it will
not open the barrier
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Vehicle images: It is possible to store the
image used for the ANPR equipment during the recognition
process. It allows to have more information about the vehicle in the client
application.
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Some ANPR equipment are able to
recognize the number plate of vehicles that circulate up to 200
km/h with a reliability of 95%. Thanks to this, the
use of ANPR equipment for traffic control has increased
significantly in the last years.
These are some examples of ANPR
application on traffic control:
Detecting vehicles in a black
list
It is possible to control vehicles that are in search and capture, throught the installation of the
ANPR equipment in the main accesses to cities, such as highways
and roads. |
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Average speed
control
The majority of speed control devices, such as radars or
speed traps, control the vehicle instantaneous speed. With the ANPR
equipment it is possible to control the average speed during an
itinerary. By means of the installation of two ANPR equipment
in different points in the same lane, it is possible to make two consecutive
recognitions of the number plate and to calculate the average speed of the
vehicle. |
Traffic
optimization
It is important to improve the vehicles mobility
during rush hours and traffic jam.
The installation of ANPR equipment allows to
know how many time a vehicle spends to coss an itinerary. This way, the average
time can be informed. |
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Urban toll
payment
This application solution has been succesfully
experienced. This proves that the mobility in the central zones of the city
improves considerably. Another possibility of urban tolls is to maintain a
register of foreign vehicles using the public ways, without contributing in the
road tax.
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Toll
enforcement
Although the use of ANPR equipment in
tolls is a logical application, today, the ANPR equipment are
only used to control the vehicles that circulate in the toll, as a data base
that increases the security
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To jump a red light is a really dangerous infraction with
consequences for others drivers and pedestrians.
Nowadays, it is possible to control when a vehicle jumps a red
light. This systems combinate sensors and image capture equipment, such as
cameras or video recorders.
The ANPR systems reinforce the red light
control systems. Thanks to ANPR technology the fine process ca
be done automatically.
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Last update: 15/08/2006 |
Elaborated by www.
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