video signal is available Reduced reference methods (RR), where only partial information of the original video is available, and No-reference methods (NR), where the original video is not available at all. Subjective video quality This is concerned with how video is perceived by a viewer, and designates their opinion on a particular video sequence. Subjective video quality tests are quite expensive with regard to time (preparation and running) and human resources. There are many ways of showing video sequences to experts and recording their opinions. A few of them have been standardized, mainly in ITU-R Recommendation BT.500-13 and ITU-T Recommendation P.910. The reason for measuring subjective video quality is the same as for measuring the mean opinion score for audio. Opinions of experts can be averaged and the average mark stated as, or accompanied by, a given confidence interval. Additional procedures can be used for averaging. For example, experts whose opinions are considered unstable (such as if their correlation with average opinion is found to be low) may have their opinions rejected. In the case of video codecs, this is a very common situation. When codecs with similar objective results show results with different subjective results, the main reasons can be: Pre- and postfilters are widely used in codecs. Codecs often use prefilters such as video denoising, deflicking, deshaking, etc. Denoising and deflicking normally maintain PSNR value while increasing visual quality (the best slow denoising filters also increase PSNR on medium and high bitrates). Deshaking greatly decreases PSNR, but increases visual quality. Postfilters show similar characteristics deblocking and deringing maintain PSNR, but increase quality; graining (suggested in H.264) essentially increases video quality, especially on big plasma screens, but decreases PSNR. All filters increase compression/decompression time, so they enhance visual quality but decrease the speed of coding and decoding. Motion estimation (ME) search strategy can also cause different visual quality for the same PSNR. So-called true motion search commonly will not reach minimum sum of absolute differences (SAD) values in codec ME, but may result in better visual quality. Such methods also require more compression time. Rate control strategy. VBR commonly cause better visual quality marks than CBR for the same average PSNR values for sequences. It is difficult to use long sequences for subjective testing. Commonly, three or four ten-second sequences are used, while full movies are used for objective metrics. Sequence selection is important those sequences that are similar