**3.1 Types**

There are different kinds of developing quality assessment. The main objective of quality assessment consists in the evaluation of any equipment of codification, transmission of its conditions of work, to assure the expectations of the users that are viewing the contents.

The chain of distribution joins different processes that degrade the image on each phase. In broadcasting the phases that affects more intensively to the quality of video are commonly the contribution and the distribution in which the video suffers degradations because of the process of encoding of the signal. This degradation will be analyzed.

In this chapter, a review of the systems that are particularly used for this important purpose will be analyzed, and spreading to the experience lived by the observer to talk in terms of

The purpose of this chapter is to provide a state of the art of vision quality assessment,

Visual perception is very important to human. We are constantly receiving information and processing it, in order to interact with the environment that surrounds us. That justifies the big interest existed in video and measurement of its quality, because there is a big necessity of receiving that information in our visual system as faithful as it appears in

The evolution of technologies in codification has made video compression more efficient with the reduction of introduced artifacts. But the accuracy in developed vision models and quality metrics has increased when in consequence of the video content transfer from analogue to digital. The vision models are based on human perception, moving closer to the

The human visual system is extremely complex, but analyzing its behavior, characterizing the operation of the eye. The eye is a human body organ which is sensitive to a wide range of wavelengths from the radio-electric spectrum, from 400 to 780nm approximately. A large

For all these reasons, optimizing the performance of digital imaging systems with respect to the capture, display, storage and transmission of visual information is one of the most

Video compression schemes should reduce the visibility of the introduced artifacts, creating more effective systems to reproduce video an image: Additionally printers should use the best half-toning patterns, and so on. In all these applications, the limitations of the human visual system (HVS) can be exploited to maximize the visual quality of the output. To do this, it is necessary to build computational models of the HVS and integrate them in tools for

There are different kinds of developing quality assessment. The main objective of quality assessment consists in the evaluation of any equipment of codification, transmission of its conditions of work, to assure the expectations of the users that are viewing the contents.

The chain of distribution joins different processes that degrade the image on each phase. In broadcasting the phases that affects more intensively to the quality of video are commonly the contribution and the distribution in which the video suffers degradations because of the

process of encoding of the signal. This degradation will be analyzed.

analyzing models and metrics used in a variety of applications.

part of our neurological resources are used in visual perception.

QoE.

nature.

final consumer.

**2. Human visual system** 

important challenges in this domain.

perceptual quality assessment.

**3. Quality assessment** 

**3.1 Types** 

Fig. 1. Phases of video broadcast

The easiest way for this purpose consists in selecting a high number of observers, with a great variety of sex, age and condition, and asking them to watch a series of contents, previously and well selected to cover the range of contents which could appear on a normal TV channel. The observers will be given a questionnaire to fill with their opinions about the quality observed. Once the questionnaires are complete, the statistics will reveal a collection of consequences.

The studies must follow a protocol which is basically described in ITU-R Recommendation BT-500 about subjective assessment with variations to adapt the study to the real situation but not far from the standards. The selection of video contents and the duration of sequences are important decisions to do a proper job and to be able to compare with similar studies.

Although the subjective studies offer real results as the response of the observers is collected, that kind of studies are expensive in time and money, and they are not always so efficient, because sometimes it depends on the place to elaborate the study and its conditions of lighting or comfort of the user, being able to change a valid result because of an external condition. That is the reason for the proliferation of objective measurements which are based on mathematical algorithm using the properties of the image, which keep a higher fidelity to the subjective obtained results.

One of the main points to describe in next sections is measurement of artifacts and impairments.

All process of video encoding generates degradation on the image, with a consequent apparition of concrete defects which are called artifacts, and will affect to the perceived quality of the observer. Researchers have made studies in order to evaluate this phenomenon, artifacts such as blockiness, blurring, ringing, color bleeding or motion compensation mismatches, have been widely analyzed and a collection of metrics with or without reference have been developed in this field, which use test signals and measurement procedures to determine the level of distortion.

Finally, the theme of evolution of technologies and its influence in quality assessment will be approached, by description of the state of the art of quality measurement in stereoscopic systems. The classic methods are utilized for this purpose, but the detection of new artifacts and types of impairments, makes the necessity of developing new metrics. Additionally, the concept is not only physical, and the evolution has lead to the term Quality of Experience (QoE).

Video Quality Assessment 133

also generated when reducing the bitrate of encoding. As this effect occurs in high

Next example of "Nasa" test sequence shows the consequences of reducing the bitrate, from

The ringing artifact occurs when the quantization of individual DCT coefficients due to irregularities in high frequencies in the reconstruction of macroblocks. The effect is related to Gibb's phenomenon. It is more visible in high contrast edges, in areas with smooth textures. It is easily visible on next example, around the edges of the objects, apparing like

Noise is defined as an uncontrolled or unpredicted pattern of intensity fluctuations that

There are multiple types of noise impairments commonly produced by compression

 Mosquito noise. Mosquito noise is a temporal artifact seen mainly in smoothly textured regions as luminance/chrominance fluctuations around high-contrast edges or moving objects. It is a consequence of the coding differences for the same area of a scene in

 Quantization noise. This is a type of noise produced after a severe truncation of the values of the coefficients while performing a transform such as DCT or Hadamard. Other artifacts related are flickering, in images with high texture content, or aliasing, when

algorithms. Two common impairments are mosquito noise and quantization noise.

the content of the scene is above the Nyquist rate, either spatially or temporally

frequencies, it is more visible in sequences with higher spatial complexity.

the original one on the left to a high reduction seen on right image.

Fig. 3. Evolution of blurriness when reducing bitrate

Ringing

Noise

duplications of their contours.

Fig. 4. Example of high ringing artifact

affects to the perceived quality of an image.

consecutive frames of a sequence.
