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An Investigation of Perceived Sharpness and Sharpness Metrics*

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22 diapositives · 405 Views · Sep 04, 2014

This study explores image sharpness perception and presents new metrics to quantify it: Digital Sharpness Scale (DSS), Average Edge Transition Slope (AETS), and Average Edge Transition Width (AETW). Through psychophysical experiments including rank ordering, detection, and preference tests, we evaluate how these metrics correlate with perceived sharpness among various image types. Our findings indicate that these metrics effectively reflect user perceptions of sharpness, revealing that preferred sharpness levels surpass detection thresholds. This research contributes to improving image processing techniques.

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An Investigation of Perceived Sharpness and Sharpness Metrics*
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  1. An Investigation of Perceived Sharpness and Sharpness Metrics* Buyue Zhang, Jan P. Allebach School of Electrical and Computer Engineering, Purdue University Zygmunt Pizlo Department of Psychological Sciences, Purdue University * This research is sponsored by the Hewlett-Packet Company

  2. Outline • Introduction • Sharpness Metrics • Psychophysical Experiments • Conclusions

  3. Introduction • What is image sharpness? • Resolution • Acutance • Why sharpness and sharpness metrics matter? • Existing sharpness metrics • Depend on the MTF of the system • Require a reference image • Our approach to sharpness metric • Device-independent and no-reference

  4. Outline • Introduction • Sharpness Metrics • Psychophysical Experiments • Conclusions

  5. Sharpening and Blurring Methods • Gaussian Filter and Unsharp Mask Filter • Sharpening/Blurring methods • Vary • Vary λ

  6. Digital Sharpness Scale (DSS) • DSS: the average of the maximal pixel difference in a window centered at each edge pixel

  7. Flower Girl House Motorcyclists Red Barn Rafting Fig. 1 Seven Selected Images. Resolution: 3072 x 2048 Sea Boat

  8. DSS as a Function of Sharpening and Blurring Strength Fig. 2 DSS as a function of

  9. Normal Edge Profile based Sharpness Metrics Fig. 3 (a) Fig.3 (b) • Average Edge Transition Width (AETW) • Average ETW over all edge pixels • Average Edge Transition Slope (AETS) • ETS=AC/ETW • Average ETS over all edge pixels

  10. 1 2 Fig. 4 (b) Subset of Edge Normal Transects on the Edge Map Fig. 4 (a) Lenna.tif Fig. 4 (c) Edge Profiles Extracted from Pixel 1 and 2 in (a)

  11. AETS and AETW vs. λ AETS Fig. 5 (a) AETS vs.λ Fig. 5 (b) AETW vs.λ

  12. Outline • Introduction • Sharpness Metrics • Psychophysical Experiments • Conclusions

  13. Psychophysical Experiments • Psychophysics • Study perception by analyzing the response of subjects to visual stimuli • Four Psychophysical Experiments • Rank order test • Sharpness equality test • Detection test • Preference test

  14. Rank Order Test • Objective • How well do DSS, AETS and AETW agree with perceived sharpness? • Experimental design • Four image contents: Girl, House, Rafting, Sea Boat • Sharpen images with to 8~10 different levels • Images are printed with inkjet printer • “Arrange images according to your ranking of sharpness” • Six subjects

  15. Scatter Plots and Spearman Rank Correlation Coefficients rs rs=0.98 rs=0.97 rs=0.99 rs=0.94 Fig. 6

  16. Detection Test • Objective • What is the perceptual threshold for detecting sharpness increase? • Experimental Design • Test image set: eight different levels in sharpness • Reference image: the original image • “Is the test image sharper or the same compared with the reference image?” • Seven image contents in Fig. 1 • Twenty-one subjects

  17. Psychometric Functions (a) (b) Fig. 7 Two Typical Psychometric Functions from the Detection Test (Mu is the detection threshold for an image or an individual subject)

  18. Preference Test • Objective • What is the observers' preferred level of sharpness? • Experimental Design • Seven image contents, Fig. 1 • For each content: • nine different levels of sharpness, • including the original image • “Select the one image with your most preferred level of sharpness.” • Twenty-one subjects

  19. (a) (b) (c) (d) Fig. 8 The Detection Thresholds vs. the Sharpness Preferences

  20. Outline • Introduction • Sharpness Metrics • Psychophysical Experiments • Conclusions

  21. Conclusions • We proposed three new sharpness metrics: DSS, AETS, and AETW • The sharpness metrics are well correlated with perceived sharpness • The average preferred level of sharpness is consistently higher than the detection threshold across image contents as well as across observers • The average preference has much more variation than the detection threshold across subjects and images. Compared with the detection threshold

  22. Thank You!