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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.

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an investigation of perceived sharpness and sharpness metrics

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

slide2

Outline

  • Introduction
  • Sharpness Metrics
  • Psychophysical Experiments
  • Conclusions
slide3

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
slide4

Outline

  • Introduction
  • Sharpness Metrics
  • Psychophysical Experiments
  • Conclusions
sharpening and blurring methods
Sharpening and Blurring Methods
  • Gaussian Filter and Unsharp Mask Filter
  • Sharpening/Blurring methods
    • Vary
    • Vary λ
digital sharpness scale dss
Digital Sharpness Scale (DSS)
  • DSS: the average of the maximal pixel difference in a window centered at each edge pixel
slide7

Flower Girl House

Motorcyclists Red Barn Rafting

Fig. 1 Seven Selected Images. Resolution: 3072 x 2048

Sea Boat

normal edge profile based sharpness metrics
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
slide10

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)

aets and aetw vs
AETS and AETW vs. λ

AETS

Fig. 5 (a) AETS vs.λ

Fig. 5 (b) AETW vs.λ

slide12

Outline

  • Introduction
  • Sharpness Metrics
  • Psychophysical Experiments
  • Conclusions
psychophysical experiments
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
rank order test
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
detection test
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
psychometric functions
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)

preference test
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
slide19

(a)

(b)

(c)

(d)

Fig. 8 The Detection Thresholds vs. the Sharpness Preferences

slide20

Outline

  • Introduction
  • Sharpness Metrics
  • Psychophysical Experiments
  • Conclusions
conclusions
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