an overview of biometric image...
TRANSCRIPT
An Overview of Biometric Image
Processing
CHAPTER 2
AN OVERVIEW OF BIOMETRIC IMAGE PROCESSING
The recognition of persons on the basis of biometric features is an emerging
phenomenon in our society. Traditional systems to verify a person’s identity are based on
knowledge (secret code) or possession (ID card). However, codes can be forgotten or
overheard, and ID cards can be lost or stolen, giving impostors the possibility to extend
the identity test. These existing issues have received increasing identity in recent years
about the use of features inseparable from a person’s body which significantly decreases
the possibility of a hoax. The need for security in a wide scope of applications, such as
replacement of the Personal Identification Number (PIN) in banking and retail business,
security of transactions across computer networks, high-secure wireless access,
tele-voting, and admittance to restricted areas can be practiced with the help of biometric
based authentication. This chapter discusses about the overview of digital image
processing, about biometric system and the importance of biometric technology in
real-time applications.
2.1 Digital Image Processing
The process of obtaining and analyzing visual data from digital data processor is
called Digital Image Processing and Scene Analysis. Digital Image Processing [10] is a
rapidly developing subject area with growing applications in science and technology.
Image Processing has the likelihood of acquiring the ultimate machine that could execute
the optical functions of all existing organisms. Many theoretical and technological
discoveries are required before it could make such a machine that is there is an
abundance of Image Processing applications that can help humanity with the availability
and anticipated technology in the near future. Imaging began in the 19th century with
photography and continued with television, X-rays and electronic scanning during the
20th century. Image Processing arose as a field of study in the 1950s with pictures of the
solid ground from high-flying spy airplanes and then with pictures of the earth’s surface
taken from orbiting satellites.
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Processing of an image includes improvement in its appearance and efficient
representation. This discipline consists of not just feature extraction, analysis and
recognition of images, but also coding, filtering, enhancement and restoration. The entire
process of image processing and analysis starts from receiving of visual information to
the giving out of the description about the scene. It can be divided into three main stages
[16] as given below:
• Discretization and Representation: Translating visual information into a
discrete form suitable for computer processing, approximating visual information
to save storage space as well as time requirements in subsequent processing.
• Processing: Improving image quality by filtering, etc., reducing data to save
storage space and channel capacity during transmission.
• Analysis: Extracting image features; quantifying shapes, registration and recognition.
2.2 Outline of Biometric Technology
The emerging field of biometric technology addresses the automated
identification of individuals, depending on their physiological and behavioral traits.
The broad category of human authentication schemes, denoted as biometrics encompasses
many techniques from Computer Vision and Pattern Recognition. The personal attributes
that are used in a biometric identification system can be physiological or behavioral. The
physiological biometric identification system includes fingerprints, palm print, facial
features, iris, retinal scans, and hand and finger geometry. The behavioral biometric
identification system considers the character idiosyncratic of the individual, such as gait,
voice print, sign and key stroking. Depending on the complexity or the security level of
the application, one will opt to use one or more of these personal characteristics [11].
Biometric systems have been actively emerging in many applications of diverse
industries for the past few years and it is continuing to roll out to provide higher security
features to access control system. Many cases of single modal biometric systems have
been built up and deployed, for example, fingerprint, face, speaker, palm print and hand
geometry verification systems. These systems are capable of providing a low to middle
range of security features. For higher security features, the blend of two or more single-
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modal biometrics (also known as multimodal biometrics) is also proposed. Biometrics are
the most secured and convenient method to satisfy the need for identifying the
individuals in the society. For automatic identification of an individual, physiological or
behavioral characteristics of a person are used [11].
2.3 Prerequisites of a Good Biometric
Many different traits of human physiology, behavior or chemistry can be used for
biometric authentication. The choice of an individual biometric for use in a specific
application involves weighing of various components. Jain et al [12] acknowledged seven
such factors to be employed when evaluating the suitability of any trait for use in
biometric authentication.
• Universality means that each person using a system should hold the quality.
• Uniqueness means that the quality should be sufficiently unique for individuals in
the relevant population, such that they can be notable from each other.
• Permanence associates the manner in which a quality varies over time. More
specifically, a trait with 'good' permanence will be reasonably invariant over time
with respect to the specific matching algorithm.
• Measurability (collectability) relates to the ease of acquisition or measurement of
the trait. Additionally, acquired data must be in a form that permits subsequent
processing and extraction of the related feature sets.
• Performance relates to the speed, robustness and accuracy of technology used.
• Acceptability relates to how good the individuals in the relevant population
accept the technology such that they are willing to have their biometric trait
captured and measured.
• Circumvention relates to the efforts with which a trait might be imitated using an
artifact or substitute.
2.4 Need for Biometrics
In the current times, most of the transactions either finance or other secure
messages are automated and many of them networked, so security has gone forth a most
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important subject. Security is commonly in the form of belongings like ID cards, keys or
secret knowledge like password. This type of security is not guaranteed as for example,
ID cards may be lost, and passwords may be forgotten or compromised. A sturdy need
was therefore felt for more robust authentication methods and far-reaching research
ensued in this area. This led to the concept of using human body parts or human
mannerism itself as security and authentication measure, and eventually to the emergence
of biometrics as a subject by itself. Nowadays it is widely recognized that any positive
recognition of a person must take into account of his biometric identification.
2.5 Biometric System as Pattern Recognition
A biometric system is basically a pattern recognition system that works by
acquiring biometric data from an individual, drawing out a feature set of the acquired
data and then comparing this feature set against the template set in the database.
Depending on the context of application, a biometric system could function either in
identification mode or verification mode. The block diagram depicts the two fundamental
modes of a biometric system [13] as exposed in figure 2.1
Figure 2.1 Block Diagram of the General Biometric Detection System [14]
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First block, known as sensor acts as an interface between system and the real
world and acquires necessary data. Vision based picture acquisition system is convenient
choice for it, but can be changed as per application. Second block performs pre-processing,
i.e. removes artifacts from the sensor and enhances the input picture. In the next block,
essential characteristics are extracted. This is a critical stage where the right features are
required to be extracted in an optimal way. Image features or vector of the numbers with
specific properties is used for the formation of the template. A template is a
combinational set of related features extracted from the source. Elements of biometric
measurements those are not required for comparison algorithms are banished in templates
for reducing size of file and for protecting the individuality of the claimer.
2.6 Modes of Biometric System
2.6.1 Verification or Authentication Mode
The system performs a one-to-one comparison of a captured biometric with a
specific template stored in a biometric database in order to verify the individual is the
person they claim to be. Three steps are involved in the verification of a person [13].
Step 1: Reference models for all the users are generated and stored in the model
database.
Step 2: With samples that are matched with reference models to generate the
genuine and impostor scores and the threshold is calculated.
Step 3: In this testing, the process was performed which may use a username,
smart card or ID number (e.g. PIN) to indicate which template should be
used for comparison.
2.6.2 Identification Mode
The system performs a one-to-many comparison against a biometric data base in
an attempt to establish the identity of an unknown individual. The system will succeed in
identifying the individual if the comparison of the biometric sample to a template in
the database falls within the previously set threshold. The identification mode may be
used either for 'positive recognition’ or for 'negative recognition' of the person. The latter
recognition will only be achieved through biometrics since other methods of personal
recognition such as PINs, passwords or keys are ineffective [11].
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During the enrollment phase, the template is simply stored somewhere (on a card
or within a database or both). During the matching phase, the template obtained is passed
to a matcher that compares it with other available templates, approximating the distance
between them using any algorithm (e.g. Hamming distance). The matching program will
analyze the input with the template. This will subsequently be an output for any specified
use or purpose (e.g. Entrance in a restricted area). Selection of biometrics in any practical
application depends upon the characteristic measurements and user requirements [13] .
Selecting a biometric based on the user requirement takes into account of the availability of
sensor devices, computational time, reliability, sensor area, cost and power consumption.
2.7 Phases of Biometric System
A biometric system is designed using the following three main phases [15] as
shown in Figure 2.2.
Figure 2.2 Modules of a Biometric System [16]
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2.7.1 Biometric Image Capturing Phase
In this phase, the raw biometric is captured by a sensing device such as a
fingerprint scanner or video camera. An example of such a device is a fingerprint sensor
that captures images like the ridge and valley structure of a user’s finger.
2.7.2 Feature Extraction Phase
The second phase of processing is to extract the distinguishing characteristics of the
raw biometric sample and convert it into a processed biometric identifier record, in which the
acquired biometric data are processed to extract the salient or discriminatory features.
2.7.3 Matcher Phase
This phase works during the authentication process in which the features
extracted during recognition are compared against the stored templates to generate
matching scores. The matcher module also encapsulates a decision-making model, where
the identity of a claimed user is confirmed (verification) or a user’s identity is established
(identification) based on the matching score.
2.7.4 Template Database Phase
This phase is used by the biometric system to store the biometric templates of the
registered users. To store the biometric features or templates of enrolled users, the
enrollment module holds the responsibility for enrolling individuals in the biometric
system database. During the phase of enrollment, the biometric characteristic of an
individual is scanned first by a biometric reader to produce a digital representation of the
characteristic. The data that is captured during the enrollment process may or may not be
supervised by a human depending on the application. A quality check is generally
performed to ensure that the acquired sample can be reliably processed by succeeding
stages. In order to help the process of matching, the input digital representation is further
treated by a feature extractor to generate a compact but expressive representation, called a
template. Based on the application, the template can be stored in the central database of
the biometric system or be recorded on a smart card issued to the individual person.
Usually, several templates of an individual are stored to account for variations observed
in the biometric trait and the templates in the database may be updated over time.
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2.8 Uses of Biometrics
The uses of Biometric system are listed below
• Firstly, biometric systems can be used as physical access granting systems.
• Secondly, biometric systems can be used to establish entitlement to services and
rights.
• Thirdly, biometric systems can be used for the recording and association of facts.
2.9 Types of Biometrics
The Biometrics system falls into two categories, they are physical biometrics and
behavioral biometrics, which are explained in this section.
2.9.1 Physical Biometrics
Physiological traits are related to the shape of the body. Examples include, but are
not limited to fingerprints, face recognition, DNA, hand and palm geometry, iris
recognition, which has largely replaced retina, and odor/scent [17]. Physical biometrics
evaluates certain unique physical characteristics of a person’s body. Types of physical
biometric devices are:
• Fingerprint Scanners
• Hand Geometry Scanners
• Iris Scanners
• Retinal Scanners
• Facial Scanners
Fingerprint Scanners
Fingerprint scanners are devices that scan an individual’s fingerprint and compare
it to a pre-existing fingerprint template [16]. This is most commonly used biometric
device. The advantages of fingerprint scanners are inexpensive and it has the ability to
enroll multiple fingerprints. The disadvantage of fingerprint scanners is it’s vulnerable to
dirt and its’ association with criminality.
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Figure 2.3 Fingerprint Scanner
Hand Geometry Scanners
Hand Geometry scanners are devices which measure an individual’s hand based
on their hand’s size and shape [18]. The advantage of hand geometry scanners is its
simple to use and it works quickly and easily. The disadvantages are its limited accuracy
and the machine is large and bulky.
Figure 2.4 Hand Geometry Scanners
Iris Scanners
Iris scanners analyze the pattern of color surrounding one’s pupils [17].
The advantage of Iris scanners is its simple cameras and doesn’t require close contact and
results are accurate. The disadvantages are its being expensive and not easy to use.
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Figure 2.5 Iris Scanners
Retinal Scanners
Retinal scanners analyze patterns of blood vessels in the back of one’s eye [19].
The advantage of retinal scanners is its very accurate and the patterns don’t change often.
The disadvantages are its difficult to use, expensive, large in size and require close contact.
Figure 2.6 Retinal Scanners
Facial Scanners
Facial recognition starts by using a digital video camera to record a person’s face
as they enter a certain area. This type of biometrics does not require anyone to physically
touch a machine, just stand within a designated space. Facial Scanners analyze one’s
facial characteristics. The advantage of facial scanners, it uses normal cameras and does
not require cooperation of others. The disadvantages are it needs adequate lighting and
faces change over time.
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Figure 2.7 Facial Scanners
2.9.2 Behavioral Biometrics
Behavioral biometrics is related to the behavior of a person. Examples include, but
are not limited to typing rhythm, gait, and voice. Some researchers have coined the term
behaviometrics for this class of biometrics. Behavioral biometric devices analyze
particular behavioral characteristics of an individual. Types of behavioral biometric
devices [20] are,
• Signature verification scanners
• Voice authentication scanners
• Keystroke scanners
Signature Verification
Signature Verification Devices analyze a person’s signature based on shape of
signature, speed and pressure. The key advantage of this particular system of behavioral
biometrics is that it was based on an already accepted form of identification.
Incorporation of a security system based on Dynamic Signature Verification would
require a certain amount of investment in equipment and software to analyze the inputs,
but no real cost to train people on how to input signals. The disadvantages are high error
rates. The process is ideal for security purposes because it allows a frequently used
writing (the signature) that is unique to each user based upon the amount of time and
effort that they specifically put into their writing.
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Figure 2.8 Signature Verification Scanners
Voice Authentication
Voice authentication devices analyze an individual’s voice and transform their
words into text, which is often referred to as voice to print technology. The voiceprint is a
biometric voice identifier and not a recording or a sound file; so an imposter could not
record one’s words and replay them into the system and get access granted. A voiceprint
allows the user to gain access to information or give authorization without being
physically present; this way the user can give authorization by way of a simple phone call.
The advantage is it’s easy to use. Its disadvantage is background noise and voice changes.
Figure 2.9 Voice Authentication Scanners
Keystroke Scanners
Keystroke scanners are software programs that evaluate a person’s typing patterns
based on “flight time” and “dwell time”. One of the most likely possible uses for
keystroke dynamics in the business and information world today would be for user
identification purposes. Once proper calibrated, the template will be easily able to
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distinguish whether the acceptable user is typing or not by comparing the flight and dwell
times to those set in the template. The advantage is it’s easy to use between software
versus physical device. The disadvantage is its accuracy is very limited.
Figure 2.10 Keystroke Biometric Authentication Scanners
2.10 Performance Evaluation
Referable to the variations existing within any biometric signal, a biometric
authentication or recognition system cannot render an absolute answer about the
individual's identity; rather it provides the individual's identity information with a
certain confidence level. This is adverse to traditional authentication systems (for
instance, a password system) where the match has to be exact and an absolute “yes” or
“no” response is given. The biometric signal variations of an individual are usually
referred to as intra-class variations (Figure 2.11); whereas variations between different
individuals are called inter-class variations.
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Figure 2.11 : Intra-class variation.
Figure 2.11 shows the examples of intra-class variation. There are eight different
fingerprint impressions of the same finger. Note that huge differences of image
contrasts, locations, rotations, sizes, and qualities, exist among them.
A biometric matcher takes two biometric signals, and returns a similarity
measurement result. If the result becomes closer to 1, the matcher recognizes more
confidently that both biometric signals come from the same individual; when it becomes
closer to 0, the matcher recognizes that both biometric signals come from the same
individual with lesser confidence. Generally, the identity of a submitted biometric signal
is either a genuine type or an impostor type; hence, there are two statistical distributions
of similarity scores, which are called genuine distribution and impostor distribution
(Figure 2.12). Each type of input identity has one of the two possible results, “accept” or
“reject”, from a biometric matcher.
Consequently, there are four possible outcomes:
1. A genuine individual is accepted;
2. A genuine individual is rejected;
3. An impostor individual is accepted;
4. An impostor individual is rejected.
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The first and fourth outcomes are correct while the second and third outcomes
represent the error situations. The second outcome is referred to as “false reject” and the
corresponding error rate is called False Reject Rate (FRR); the third outcome is referred
to as “false accept” and the corresponding error rate is called False Accept Rate (FAR).
They are the most widely used measurements in today's commercial environment. Given
a genuine distribution, pg , and impostor distribution, pi, the FAR and FRR at threshold
‘th’ is given by
𝐹𝐹𝐹𝐹𝐹𝐹(𝑇𝑇) = ∫ 𝑝𝑝𝑖𝑖(𝑥𝑥)𝑑𝑑𝑥𝑥 1𝑡𝑡ℎ 2.1
𝐹𝐹𝐹𝐹𝐹𝐹(𝑇𝑇) = ∫ 𝑝𝑝𝑔𝑔(𝑥𝑥)𝑑𝑑𝑥𝑥 𝑡𝑡ℎ0 2.2
Strict tradeoff exists between FAR and FRR in every biometric system [21].
From equation 2.1 and 2.2, both FAR and FRR are actually functions of threshold ‘th’.
When ‘th’ decreases, the system would have more tolerance to intra-class variations and
noise, however the FAR will increase. Similarly, if the value of ‘th’ is lower, the system
would be more secure and the FRR decreases. The following figure 2.12 shows the
example of genuine and imposter distributions. The red line is the imposter and the blue
line is the genuine.
Figure 2.12: Example of Genuine and Impostor Distributions
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Depending on the nature of an application, the biometric system may be chosen to
operate at low FAR configuration (for example, the login process in ATMs), or to operate
at low FRR configuration (for example, the access control system for a library). A system
designer may have no prior knowledge about the nature of the application in which the
biometric system is to be applied, thus it is helpful to report the system performance at all
possible operating points (thresholds).
Other useful performance measurements are:
• Equal Error Rate (EER): the error rate where FAR equals to FRR.
• ZeroFNMR: the lowest FAR at which no false reject occurs.
• ZeroFMR: the lowest FRR at which no false accept occurs.
• Failure To Capture Rate: the rate at which the biometric acquisition device fails
to automatically capture the biometric signals. A high failure to capture rate
makes the biometric system hard to use.
• Failure To Enroll Rate: the rate at which users are not able to enroll in the
system. This error mainly occurs when the biometric signal is rejected due to its
poor quality.
• Failure to Match Rate: occurs when the biometric system fails to convert the
input biometric signal into a machine readable/understandable biometric template.
Unlike FRR, a failure to match the error occurs at a stage prior to the decision
making stage in a biometric system.
2.11 Merits and Demerits of Biometric System
The merits of the biometric system are as follows,
• It is significantly more difficult to copy, share, and distribute biometrics with as
much ease as passwords and tokens.
• Biometrics cannot be lost or forgotten and online biometrics-based recognition
systems require the person to be recognized to be present at the point of
recognition.
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• It is difficult to forge biometrics and extremely unlikely for a user to repudiate,
for example, having accessed a computer network.
• Further, all the users of the system have a relatively equal security level and one
account is no easier to break than any other (e.g., through social engineering methods).
• Biometrics introduce incredible convenience for the users (as users are no longer
required to remember multiple, long and complex, frequently changing passwords)
while maintaining a sufficiently high degree of security.
Biometric systems that operate using any single biometric characteristic have few
issues to be monitored are as follows:
• Noise in sensed data: The sensed data might be noisy or distorted
• Intra-class variations: The biometric data acquired from an individual during
authentication may be very different from the data that was used to generate the
template during enrollment, thereby affecting the matching process.
• Distinctiveness: While a biometric trait is expected to vary significantly across
individuals, there may be large inter-class similarities in the feature sets used to
represent these traits.
• Non-universality: While every user is expected to possess the biometric trait
being acquired, in reality it is possible for a subset of the users to not possess a
particular biometric.
• Spoof attacks: An impostor may attempt to spoof the biometric trait of a
legitimate enrolled user in order to circumvent the system. This type of attack is
especially relevant when behavioral traits such as signature and voice are used.
2.12 Summary
The successful installation of biometric systems in various civilian applications
does not imply those biometrics are a fully solved problem. It is clear that there is plenty of
scope for improvement in biometrics. Researchers are not only addressing issues related to
reducing error rates, but they are also looking at ways to enhance the usability of biometric
systems. Biometric systems that operate using any single biometric characteristic have some
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limitations which will lead to poor identification results. This chapter provides the
general idea of biometric system functionalities and its applications. The proposed
research work concentrates on fingerprint based image processing. The next chapter
discusses about an elaborate study on fingerprint recognition system.
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