**1. Introduction**

26 will be set by intech

128 Video Compression

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One of the main aspects which affects the video compression and needs to be deeply analyzed is the quality assessment. The chain of transmission of video over a determined channel of distribution, such as broadcast or a digital way of storage, is limited, and requires a process of compression, with a consequent degradation and the apparition of artifacts which are necessary to evaluate, in order to offer a suitable and appropriate quality to the final user.

The quick evolution of technology, especially referred to television and multimedia services, which has evolved from analog to digital. The constant increasement of resolution from standard television to high definition and ultrahigh-definition or the creation of advanced production of contents systems such as 3-dimensional video, make necessary new quality studies to evaluate the video characteristics to provide the observer the best viewing that could expect.

Once the change from analog to digital television has been completely developed, the next step was encoding the video in order to obtain high compression without damaging the quality contemplated by the observer. In analog television the quality systems were wellestablished and controlled, but in digital television it is required new metrics and procedures of measurement of the quality of video.

The quality assessment must be adapted to the human visual system, which is why researchers have performed subjective viewing experiments in order to obtain the conditions of encoding of video systems to provide the best quality to the user.

There has been a process of standardization in video encoding, the group of experts of MPEG developed techniques that assure the quality which would be improved with the evolution of the standards. MPEG-2 offered a reasonably good quality, but the evolution of the standards developed another one which was twice efficient as MPEG-2, which is called AVC/H.264, i.e. to obtain a similar quality than the first standard it was only necessary half the bitrate used in the new standard.

The quality assessment has also been force to evolve Parallel to technologies. The concept is not any more limited to the perceived quality of the video, but now there are other additives carried to this concept, making appear a new term called Quality of Experience (QoE) which is becoming more popular because it is a more complete definition, just because the user is not only observing the video, is living a real experience which depends on the content and expectatives placed on it.

Video Quality Assessment 131

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

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

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

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

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

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

Fig. 1. Phases of video broadcast

higher fidelity to the subjective obtained results.

of consequences.

studies.

impairments.

level of distortion.

(QoE).

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 QoE.

The purpose of this chapter is to provide a state of the art of vision quality assessment, analyzing models and metrics used in a variety of applications.
