An Investigation of Perceived Sharpness and Sharpness Metrics*

Zygmunt PizloDepartment of Psychological Sciences,

Purdue University

* This research is sponsored by the Hewlett-Packet Company

Buyue Zhang, Jan P. AllebachSchool of Electrical and Computer Engineering,

Purdue University

Outline

Introduction

Sharpness Metrics

Psychophysical Experiments

Conclusions

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

Outline

Introduction

Sharpness Metrics

Psychophysical Experiments

Conclusions

Sharpening and Blurring Methods

[ , ] [ ]** [ ] (2)by m n g m,n x m,n

[ , ] (1 ) [ ] [ ] (3)USM by m n x m,n - y m,n

Gaussian Filter and Unsharp Mask Filter

(1) 2

1 ][

2

22

2

)(

nm

em,ng

Sharpening/Blurring methods Vary

Vary λ

Digital Sharpness Scale (DSS)

DSS: the average of the maximal pixel difference

in a window centered at each edge pixel

2 2

22 2

22 2

2[ , ] (1 ) (4)

2

m nm n

LoG m n e

[ , ] [ , ]** [ , ] (5)LoGy m n x m n LoG m n

(6) |)](min[)](max[| 1

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edgeyInm

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DSS

Flower Girl House

Motorcyclists Red Barn Rafting

Sea Boat

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

DSS as a Function of Sharpening and Blurring Strength

Fig. 2 DSS as a function of

Normal Edge Profile based Sharpness Metrics

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

Fig. 3 (a) Fig.3 (b)

Fig. 4 (c) Edge Profiles Extracted from Pixel 1 and 2 in (a)

Fig. 4 (a) Lenna.tif Fig. 4 (b) Subset of Edge Normal Transects on the Edge Map

1

2

AETS and AETW vs. λ

Fig. 5 (b) AETW vs.λFig. 5 (a) AETS vs.λ

AE

TS

Outline

Introduction

Sharpness Metrics

Psychophysical Experiments

Conclusions

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

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

Scatter Plots and Spearman Rank Correlation Coefficients rs

Fig. 6

rs=0.97

rs=0.99

rs=0.98

rs=0.94

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

Fig. 7 Two Typical Psychometric Functions from the Detection Test

(Mu is the detection threshold for an image or an individual subject)

(a) (b)

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

Fig. 8 The Detection Thresholds vs. the Sharpness Preferences

(a) (b)

(c) (d)

Outline

Introduction

Sharpness Metrics

Psychophysical Experiments

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

Thank You!