Subjective video quality
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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.
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[edit] How it can be measured
Main idea of measuring subjective video quality is the same as in Mean Opinion Score for audio. Assume that you want to evaluate subjective video quality of a video processing system. To do this you should go through following steps:
- Choose video sequences for testing (they are often named SRC).
- Choose settings of system that you want to evaluate (often named HRC).
- Choose test method (how sequences are presented to experts and how their opinion is collected).
- Invite sufficient number of experts (not less than 15 are recommended).
- Carry out testing.
- Calculate average marks for each HRC based on experts' opinion.
Of course, to make professional comparison you should take into account lots of parameters of viewing conditions: room illumination, display type, brightness, contrast, resolution, viewing distance, age and education level of experts - all this can influence the results.
[edit] Testing methods
There are an enormous number of ways of showing video sequences to experts and to record their opinions. A few of them have been standardized. They are thoroughly described in ITU-T recommendation BT.500. One of the standardized methods is DSIS - Double Stimulus Impairment Scale: the expert is presented with an unimpaired reference video, then with the same video impaired, and after that he is asked to vote on the second video using an impairment scale (from "impairments are imperceptible" to "impairments are very annoying").
[edit] Analysis of results
Opinions of experts can be averaged; average mark is usually given with confidence interval. Additional procedures can be used for averaging, for example experts who give unstable results can be rejected (for instance, if their correlation with average opinion is small).
