Video Quality

Information about Video Quality

Video quality is a characteristic of a video passed through a video transmission/processing system, a formal or informal measure of perceived video degradation (typically, compared to the original video). Video processing systems may introduce some amounts of distortion or artifacts in the video signal, so video quality evaluation is an important problem.

From analog to digital video

Since the time when the world's first video sequence was recorded, many video processing systems have been designed. In the ages of analog video systems, it was possible to evaluate quality of a video processing system by calculating the system's frequency response using some traditional test signal (for example, a collection of color bars and circles).

Nowadays digital video systems are replacing analog ones, and evaluation methods have changed. Performance of a digital video processing system can vary significantly and depends on dynamic characteristics of input video signal (e.g. amount of motion or spatial details). That's why digital video quality should be evaluated on diverse video sequences, often - from user's database.

Objective video quality

Objective video evaluation techniques are mathematical models that approximate results of subjective quality assessment, but are based on criteria and metrics that can be measured objectively and automatically evaluated by a computer program. Objective methods are classified based on the availability of the original video signal, which is considered to be of high quality (generally not compressed). Therefore, they can be classified as Full Reference Methods (FR), Reduced Reference Methods (RR) and No-Reference Methods (NR). FR metrics compute the quality difference by comparing every pixel in each image of the distorted video to its corresponding pixel in the original video. RR metrics extract some features of both videos and compare them to give a quality score. They are used when all the original video is not available, e.g. in a transmission with a limited bandwidth. NR metrics try to assess the quality of a distorted video without any reference to the original video. These metrics are usually used when the video coding method is known.

The most traditional ways of evaluating quality of digital video processing system (e.g. video codec like DivX, XviD) are calculation of the Signal-to-noise ratio (SNR) and peak signal-to-noise ratio (PSNR) between the original video signal and signal passed through this system. PSNR is the most widely used objective video quality metric. However, PSNR values do not perfectly correlate with a perceived visual quality due to non-linear behavior of human visual system. Recently a number of more complicated and precise metrics were developed, for example UQI, VQM, PEVQ and SSIM.

The performances of an objective video quality metric are evaluated by computing the correlation between the objective scores and the subjective tests results. The latters are called Mean Opinion Score (MOS). The most frequently used correlation coefficients are : linear correlation coefficient, Spearman's rank correlation coefficient, Kurtosis, Kappa coefficient and Outliers Ratio.

When estimating quality of a video codec, all the mentioned objective methods may require repeating post-encoding tests in order to determine the encoding parameters that satisfy a required level of visual quality, making them time consuming, complex and impractical for implementation in real commercial applications. For this reason, a lot of research has been focused on developing novel objective evaluation methods which enable prediction of the perceived quality level of the encoded video before the actual encoding is performed [1].

Subjective video quality

Main article: Subjective video quality

The main goal of many objective video quality metrics is to automatically estimate average user (viewer) opinion on a quality of video processed by the system. But the best way to find out users' opinion is just to ask them! Sometimes however, measurement of subjective video quality can also be challenging because it may require a trained expert to judge it. Many “subjective video quality measurements” are described in ITU-T recommendation BT.500. Their main idea is the same as in Mean Opinion Score for audio: video sequences are shown to the group of viewers and then their opinion is recorded and averaged to evaluate the quality of each video sequence. However details of testing may vary greatly.

References

Digital Video Quality, Stefan Winkler, Wiley, March 2005, ISBN 0-470-02404-6

Video (Latin for "I see", first person singular present, indicative of videre, "to see") is the technology of electronically capturing, recording, processing, storing, transmitting, and reconstructing a sequence of still images representing scenes in motion.
..... Click the link for more information.

Analogue electronics (or analog in American English) are those electronic systems with a continuously variable signal. In contrast, in digital electronics signals usually take only two different levels.
..... Click the link for more information.

Frequency response is the measure of any system's response at the output to a signal of varying frequency (but constant amplitude) at its input. In the audible range it usually referred to in connection with Electronic amplifiers, microphones and loudspeakers.
..... Click the link for more information.

Theory — boolean logic | digital signal processing | computer architecture
Applications — digital photography | digital audio | digital video
..... Click the link for more information.

A video codec is a device or software that enables video compression and or decompression for digital video. The compression usually employs lossy data compression. Historically, video was stored as an analog signal on magnetic tape.
..... Click the link for more information.

DivX are a brand name of products created by DivX, Inc. (formerly DivXNetworks, Inc.), including the DivX Codec which has become popular due to its ability to compress lengthy video segments into small sizes while maintaining relatively high visual quality.
..... Click the link for more information.

Xvid (formerly "XviD") is a video codec library following the MPEG-4 standard. Xvid features MPEG-4 Advanced Simple Profile features such as b-frames, global and quarter pixel motion compensation, lumi masking, trellis quantization, and H.
..... Click the link for more information.

Signal-to-noise ratio (often abbreviated SNR or S/N) is an electrical engineering concept defined as the ratio of a signal power to the noise power corrupting the signal.
..... Click the link for more information.

The phrase peak signal-to-noise ratio, often abbreviated PSNR, is an engineering term for the ratio between the maximum possible power of a signal and the power of corrupting noise that affects the fidelity of its representation.
..... Click the link for more information.

The Structural SIMilarity (SSIM) index is a method for measuring the similarity between two images. The SSIM index is a full reference metric, in other words, the measuring of image quality based on an initial uncompressed or distortion-free image as reference.
..... Click the link for more information.

Subjective video quality is a subjective characteristic of video quality. It is concerned with how video is perceived by a viewer and designates his or her opinion on a particular video sequence.
..... Click the link for more information.

Mean Opinion Score (MOS) provides a numerical indication of the perceived quality of received media after compression and/or transmission. The MOS is expressed as a single number in the range 1 to 5, where 1 is lowest perceived quality, and 5 is the highest perceived quality.
..... Click the link for more information.

correlation, also called correlation coefficient, indicates the strength and direction of a linear relationship between two random variables. In general statistical usage, correlation or co-relation refers to the departure of two variables from independence.
..... Click the link for more information.

In statistics, Spearman's rank correlation coefficient, named after Charles Spearman and often denoted by the Greek letter ρ (rho), is a non-parametric measure of correlation – that is, it assesses how well an arbitrary monotonic function could describe the relationship
..... Click the link for more information.

kurtosis (from the Greek word kurtos, meaning bulging) is a measure of the "peakedness" of the probability distribution of a real-valued random variable. Higher kurtosis means more of the variance is due to infrequent extreme deviations, as opposed to frequent
..... Click the link for more information.

Cohen's kappa coefficient is a statistical measure of inter-rater reliability. It is generally thought to be a more robust measure than simple percent agreement calculation since κ takes into account the agreement occurring by chance.
..... Click the link for more information.

In objective video quality assessment, the Outliers Ratio (OR) is a measure of the performance of an objective video quality metric. It is the ratio of "false" scores given by the ojective metric to the total number of scores.
..... Click the link for more information.

The ITU Telecommunication Standardization Sector (ITU-T) coordinates standards for telecommunications on behalf of the International Telecommunication Union (ITU) and is based in Geneva, Switzerland.
..... Click the link for more information.

Quality in everyday life and business, engineering and manufacturing has a pragmatic interpretation as the non-inferiority, superiority or usefulness of something. This is the most common interpretation of the term.
..... Click the link for more information.

The media delivery index (MDI) is a measurement for IPTV networks which gives an indication of video transport performance based on network level (IP) measurements. It is independent of the video encoding scheme and is a lightweight, scalable alternative to measurements that decode
..... Click the link for more information.

This article is copied from an article on Wikipedia.org - the free encyclopedia created and edited by online user community. The text was not checked or edited by anyone on our staff. Although the vast majority of the wikipedia encyclopedia articles provide accurate and timely information please do not assume the accuracy of any particular article. This article is distributed under the terms of GNU Free Documentation License.

Herod_Archelaus

iPedia Free Information Center