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Measurement of Ringing Artifacts in JPEG Images*

Measurement of Ringing Artifacts in JPEG Images*. Xiaojun Feng Jan P. Allebach Purdue University - West Lafayette, IN. * Research supported by the Hewlett-Packard Company. Motivation. Applications Image quality assessment through measuring JPEG artifact perceptibility

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Measurement of Ringing Artifacts in JPEG Images*

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  1. Measurement of Ringing Artifacts in JPEG Images* Xiaojun Feng Jan P. Allebach Purdue University - West Lafayette, IN * Research supported by the Hewlett-Packard Company

  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.

  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

  4. Outline • What is ringing artifact • Proxy object • System detail • Sample results • Conclusions and future work

  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

  6. Example of ringing artifact JPEG Compressed Image* Enlarged Lighthouse Top * Image source: http://www.beaconphotographics.com/AmericanLights.htm

  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

  8. Proxy object Isolated ringing region Ringing region with smooth neighborhood 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

  9. Step 2: Smooth region clustering Step 3: Ringing proxy assignment Step 4: Local ringing Feature calculation Step 1: Edge detection JPEG image Ringing map 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)

  10. Edge Ringing region Smooth region Smooth region identification JPEG image Edge / ringing / smooth segmentation • Segment an image into • Edges • Potential ringing regions – regions surrounding edges • Smooth regions – regions other than edges and ringing artifact

  11. Smooth region clustering Color clustering Overall clustering Map fusion Smooth regions Texture clustering

  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

  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

  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

  15. Example of ringing visibility feature JPEG image Activity contrast

  16. Sample Result JPEG image Ringing map

  17. Sample result Quality factor=70 Quality factor=90 Quality factor=50 Quality factor=30 Compression ratio increases

  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

  19. Thank you!

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