A Comparison of Quality Metrics for JPEG ImagesMotivationCandidate MetricsBMR: IBMR: IIBMR: IIIBMR: IVBMR: VEOBDExperimentsExperiments (cont.)Results: ComparisonResults: Post-processingResults: RMSE vs. SubjectiveResults: BMR vs. SubjectiveResults: EOBD vs. SubjectiveResults: MIX vs. SubjectiveConclusionEE368B 1A Comparison of Quality Metrics for JPEG ImagesFeng XiaoFall 2000EE368B 2Motivation•Compare performance of different image metrics for JPEG images with subjective measurement–Blocking is the dominant artifact in JPEG images (or other block-based coding), especially at low-bit-rate–Post-processing may incur blurring when reducing blocking–Need a good metricsEE368B 3Candidate Metrics•RMSE (root-mean-square error)•BMR (block-to-mask ratio, Liu 1997)•EOBD (effect-of-block-distortion, Eskicioglu 1995)•MIX (RMSE + BMR)–RMSE is pixel-based, and BMR is block-based, combination may be more robustEE368B 4BMR: I•Compute the block difference70|)]7,()8,([2)]8,()9,()6,()7,([|81nleftnSnSnSnSnSnSL4),(),(),(),(),(jiLjiLjiLjiLjiLbottomtoprightleft6 7 8 912Block BorderEE368B 5BMR: II•Include the perceptual effects),(),(log50),(jiLjiLjiBMRJND),( jiLJNDwhere is the just-noticeable difference50 is a weighted ratioEE368B 6BMR: III•Separate the blocking and blurring measure•OBMR(i,j): BMR in the original image•PBMR(i,j): BMR in the processed image. –a) PBMR(i,j) > OBMR(i,j). Block(i,j) in processed image is more blocking than that of the original image. –b) PBMR(i,j) <= OBMR(i,j). Block(i,j) is blurred in processed image.–blocking strength = mean(|OBMR(i,j)-PBMR(i,j)|) for set a–blurring strength = mean(|OBMR(i,j)-PBMR(i,j)|) for set bEE368B 7BMR: IV•Construct the single BMRBMR= blocking strength + blurring strengthEE368B 8BMR: VJPEG qualityStrengthStrengthSize of smoothing filterEE368B 9EOBD2)]1,(),([),(2)],1(),([),(2/1)]},(()],([{NmfNmfNmfnMfnMfnMfwithNmfEnMfEEOBDEE368B 10ExperimentsClick on the image with the worst qualityJPEGJPEG withFiltering (3x3)JPEG withde-blockEE368B 11Experiments (cont.)•Each experiment has18x3 images:–18 JPEG images at quality levels 5~40 (bits .25~.80 bpp)–18 smoothed (3x3) JPEG images–18 de-blocked JPEG images (Chou’s 1995) •Repeat 4 times•2 subjects, 2 image sets (‘lena’ & ‘einstein’)EE368B 12Results: ComparisonMean Rank ErrorRMSEBMR MIX EOBDRank Error for Image i:Ei= | Si – Ri |, where Si is the subjective rank of image I, Ri is the rank derived from metricsEE368B 13Results: Post-processingBit Rate (bpp)Improvement (rank order)EE368B 14Results: RMSE vs. SubjectiveSubjective Rank OrderRMSEEE368B 15Results: BMR vs. SubjectiveSubjective Rank OrderBMREE368B 16Results: EOBD vs. SubjectiveEOBDSubjective Rank OrderEE368B 17Results: MIX vs. SubjectiveMIXSubjective Rank OrderEE368B 18Conclusion•MIX is the best metrics as tested–It takes both pixel-based metrics (RMSE) and block-based metrics (BMR) into consideration.•Both smooth (3x3) and de-block (chou’s) show improvement for low
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