**1. Introduction**

Along with the automation of our modem life, security issues become more critical and important. There are questions asked in our daily life such as "is this the right person to be allowed to access the system?", "is this the authorized person to perform such action?", and "does this person belong to this country?" [1]. There were two methods for answering this questions: first one based on "what you have" and called (knowledge factors), such as ID cards, and the second one based on "what you know" and called (ownership factors), such as passwords as shown in **Figure 1**. However both methods can be borrowed or copied or stolen, so users need to carry many IDs and memorize a lot of passwords. As reported banks, telecommunication companies, and governments are losing millions of dollars annually because of the violations of their password-based and card-based security police [2]. To solve this person identification issue, biometrics is an opened field.

© 2016 The Author(s). Licensee InTech. This chapter is distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/by/3.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited. © 2018 The Author(s). Licensee IntechOpen. This chapter is distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/by/3.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.

**Figure 1.** Information sets.

Biometrics rely on *what you are* called (inherence factors) so can natively differentiate between a permitted and illegal person [3, 4]. Biometric traits offer the following advantages [5]:

> However, there is not a single biometric feature that satisfies all these characteristics identified above yet, so as a result, none of existing biometric system provides a precise foolproof recognition, so there is a gap for improving the recognition accuracy and speed of primary

A Survey on Soft Biometrics for Human Identification http://dx.doi.org/10.5772/intechopen.76021 39

This chapter is divided into five sections as follows: Section 2 shows the soft biometric benefits, unimodal biometric system limitation and how multimodal biometric system overcome this limitation, the need of biometric fusion for system performance, and the system performance measurement. A holistic survey on related works is presented in Section 3 while focusing on facial soft biometrics. In Section 4 we show the challenges and the limitations of the soft

Soft biometrics provide ancillary information but are not fully distinctive and permanent, so these features cannot provide a reliable person recognition. However, such ancillary information still can be used as a secondary information to complement the primary biometric traits (face, iris, etc.), and these features can be classified to physique (e.g., color skin, gender, ethnic

• Can be used to improve the recognition accuracy and speed of a primary biometric

origin), clothing (e.g., clothes' color), or accessories (e.g., glasses, hat) [11].

biometrics using soft biometrics.

**Figure 2.** Biometrics type.

**2. Soft biometrics**

**2.1. Benefits of soft biometrics**

system [12].

biometrics. Section 5 concludes the work.


A biometric-based security system is almost impossible to be fooled. The word biometric is a composite word bios, which refers to life, and metron, which refers to measure, coming from the Greek language. Biometric is sometimes defined as a research area focused on measuring and analyzing a person's unique characteristics [6] to identify or verify a person identity and is an essential daily task for a security system to make sure that the services are available for the permitted users only [7]. It can be divided into traditional, primary, and soft biometrics as shown: traditional biometric deals with physical, behavioral, and biological characteristics such as facial features, eye, signature, gait, voice, DNA, and fingerprints as shown in **Figure 2**. Soft biometrics are concerned with ancillary characteristics that provide some information not enough to identify a person clearly as gender, ethnicity, skin color, scars, and height [8, 9]. Behavioral or physiological human features must fulfill the following requirements to be recognized as can be used as a biometric characteristic [7, 10]:


**Figure 2.** Biometrics type.

Biometrics rely on *what you are* called (inherence factors) so can natively differentiate between a permitted and illegal person [3, 4]. Biometric traits offer the following advantages [5]:

A biometric-based security system is almost impossible to be fooled. The word biometric is a composite word bios, which refers to life, and metron, which refers to measure, coming from the Greek language. Biometric is sometimes defined as a research area focused on measuring and analyzing a person's unique characteristics [6] to identify or verify a person identity and is an essential daily task for a security system to make sure that the services are available for the permitted users only [7]. It can be divided into traditional, primary, and soft biometrics as shown: traditional biometric deals with physical, behavioral, and biological characteristics such as facial features, eye, signature, gait, voice, DNA, and fingerprints as shown in **Figure 2**. Soft biometrics are concerned with ancillary characteristics that provide some information not enough to identify a person clearly as gender, ethnicity, skin color, scars, and height [8, 9]. Behavioral or physiological human features must fulfill the following requirements to be recognized as can be

• They cannot easily be forgotten, stolen, borrowed, shared, or observed.

• They are unique for each individual.

**Figure 1.** Information sets.

38 Machine Learning and Biometrics

used as a biometric characteristic [7, 10]:

**1.** Universal: each person has the trait. **2.** Acceptable: available when needed.

**3.** Resistance to circumvention: not easy to cheat.

**4.** Distinctive: can be used to differentiate between persons.

**6.** Collectable: the characteristic can be easily collected and measured.

**5.** Permanence: they don't change over a period of time.

• They always vary and are always available.

• They cannot easily be transferred to another individual.

However, there is not a single biometric feature that satisfies all these characteristics identified above yet, so as a result, none of existing biometric system provides a precise foolproof recognition, so there is a gap for improving the recognition accuracy and speed of primary biometrics using soft biometrics.

This chapter is divided into five sections as follows: Section 2 shows the soft biometric benefits, unimodal biometric system limitation and how multimodal biometric system overcome this limitation, the need of biometric fusion for system performance, and the system performance measurement. A holistic survey on related works is presented in Section 3 while focusing on facial soft biometrics. In Section 4 we show the challenges and the limitations of the soft biometrics. Section 5 concludes the work.
