EI 2006 - San Jose, CA Slide No. 1

Measurement of Ringing Artifacts in JPEG Images*

Xiaojun FengJan P. Allebach

Purdue University - West Lafayette, IN

* Research supported by the Hewlett-Packard Company

EI 2006 - San Jose, CA Slide No. 2

Motivation

Applications Image quality assessment through measuring JPEG

artifact perceptibility

Automated workflow for variable data printing

Goal Develop objective no-reference measurement of visual

impact for ringing artifacts in JPEG compressed images.

EI 2006 - San Jose, CA Slide No. 3

Prior Art

Full-reference approach – Marziliano et al.

(2004) Usually perfect reference image is not available

No-reference approach – Oguz (1999) Texture of ringing artifacts is not compared with a

neighboring smooth region

EI 2006 - San Jose, CA Slide No. 4

Outline

What is ringing artifact

Proxy object

System detail

Sample results

Conclusions and future work

EI 2006 - San Jose, CA Slide No. 5

Overview of ringing artifacts

Characteristics of ringing artifact Ringing artifact is seen to be noise-like variations in the

vicinity of major edges.

Activity of the ringing region is higher than that of neighboring smooth region

Source of ringing artifact DCT coefficient quantization

EI 2006 - San Jose, CA Slide No. 6

Example of ringing artifactJPEG Compressed Image* Enlarged Lighthouse Top

* Image source: http://www.beaconphotographics.com/AmericanLights.htm

EI 2006 - San Jose, CA Slide No. 7

Ringing artifact measurement

Compare the activities of the ringing artifact and

the neighboring smooth region

Two masking effects are taken into account in

the model: Texture masking

Luminance masking

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Proxy smooth object Isolated ringing regions need proxy objects with which activities may be

compared

Proxy object is the smooth object to which ringing region belongs

Proxy object can be assigned to ringing region by color similarity

Isolated ringingregion

Proxy object

Ringing regionwith smooth neighborhood

EI 2006 - San Jose, CA Slide No. 9

Ringing artifact detection

Step 1: Detect edges using Sobel operator

Step 2: Cluster smooth regions into different object classes

according to their color and texture similarity

Step 3: Assign a proxy class to each ringing region

Step 4: For each edge pixel (x,y), compute a local ringing feature

by pooling over the visibility of ringing regions in a local window

centered at (x,y)

Step 1:Edge

detection

Step 2:Smooth region

clustering

Step 3:Ringing

proxyassignment

Step 4:Local ringing

Feature calculation

JPEGimage

Ringing map

EI 2006 - San Jose, CA Slide No. 10

Smooth region identification

Segment an image into Edges

Potential ringing regions – regions surrounding edges

Smooth regions – regions other than edges and ringing artifact

JPEG image Edge / ringing / smooth segmentation

Edge

Ringing region

Smooth region

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Smooth region clustering

Color clustering

Textureclustering

Smoothregions

Overallclustering

Mapfusion

EI 2006 - San Jose, CA Slide No. 12

Region activity calculation

The activity of a region is computed based on the

luminance changes of neighboring pixels.

- set of neighboring pixel pairs for region

- channel value for pixel

2/1

,,),(

2)(1

2

1

rjiCji

jir

rr

YYC

rC

iY Y i

EI 2006 - San Jose, CA Slide No. 13

Luminance masking

Noise perceptibility is affected by

background luminance. Noise

with very bright or very dark

background is not easily

detected

Chou measured just-noticeable

noise levels under various

background luminance (1995)

We adopt Chou’s model and

convert the noisy level threshold

into region activity threshold

EI 2006 - San Jose, CA Slide No. 14

Ringing visibility feature For a ringing region , the overall masking effect is involved as:

− region activity of proxy object (texture masking)

− luminance masking function

Ringing visibility feature for region

− size of region

− size of a block (64 for JPEG)

Local ringing feature of edge pixel

)}(ˆ,max{ rsth Yg r

s)(ˆ g

r

M

rrv

thr

thr

,0max)(

r

M

r

),( yx

/1

),()(),(

yxWrrvyxV

EI 2006 - San Jose, CA Slide No. 15

Example of ringing visibility feature

71.3s

JPEG image Activity contrast

17.1s17.1:29.6: sr

17.1:75.7: sr

71.3:57.3: sr

71.3:28.2: sr

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Sample ResultJPEG image Ringing map

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Sample result

Compression ratio increases

Quality factor=90 Quality factor=70 Quality factor=50 Quality factor=30

EI 2006 - San Jose, CA Slide No. 18

Conclusion and future work

Our ringing artifact measurement

Uses no reference image

Involves both texture masking and luminance masking effects to measure the visual impact of ringing artifact

Future work

Conduct psychophysical experiment

Generate a global ringing value from the ringing map

EI 2006 - San Jose, CA Slide No. 19

Thank you!