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Video quality
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Video quality can be measured with formal metrics like PSNR or with
subjective video quality using expert observation. The subjective video quality of a video processing system may be evaluated as follows: Choose the video sequences (the SRC) to use for testing. Choose the settings of the system to evaluate (the HRC). Choose a test method for how to present video sequences to experts and to collect their ratings. Invite a sufficient number of experts, preferably not fewer than 15. |
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Carry out testing.Calculate the average marks for each HRC based on the experts' ratings.Many subjective video quality methods are described in the ITU-T recommendation BT.500. One of the standardized method is the Double Stimulus Impairment Scale (DSIS). In DSIS, each expert views an unimpaired reference video followed by an impaired version of the same video. The expert then rates the impaired video using a scale ranging from "impairments are imperceptible" to "impairments are very annoying". Video quality is a characteristic of video passed through a video processing system. Since the time when the first video sequence was recorded, lots of video processing systems have been designed. Different systems may have different influence on a video sequence, so video quality evaluation is a very important task. In the ages of analog video systems it was possible to evaluate the quality of video processing system by calculating the frequency response of some traditional test signal (for example, a collection of color bars and circles). Nowadays digital video systems are replacing analog ones, and evaluation methods must be changed. Performance of digital video processing system can vary powerfully and depends on dynamic characteristics of input video signal (e.g. motion, level of spatial details). That's why digital video quality must be evaluated on video sequences that can be actually received by users. The objective evaluation techniques are mathematical models that successfully emulate the subjective quality assessment results, based on criteria and metrics that can be measured objectively. The objective methods are classified, according to the availability of the original video signal, which is considered to be in high quality. Therefore, they can be classified as Full Reference Methods, Reduced Reference Methods and No-Reference Methods. The most traditional way of evaluating the quality of digital video processing system (e.g. video codec like DivX, XviD) is counting of PSNR between source signal and video passed through this system. PSNR is one of objective video quality metrics - metrics that can be automatically computed by a computer program. Recently a number of more complicated and precise metrics were developed, for example VQM and SSIM. All the aforementioned post-encoding objective methods may require repeating post-encoding tests in order to determine the encoding parameters that satisfy a specific level of user satisfaction, making them time consuming, complex and impractical for implementation on real commercial applications. For this reason, a lot of research has been focused on developing novel objective evaluation methods, which enable the prediction of the perceived quality level of an encoded video at a pre-encoding stage. The main goal of many objective video quality metrics is to automatically estimate user's opinion on a video processed by the system. But the best way to find out user's opinion is just to ask him! There are a lot of ways to do it; they are called “subjective video quality measurements”. Many of this methods 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 averaged to evaluate the quality of each video sequence, but details of testing may vary greatly. |
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