Characterization of the Effects of Translations on JPEG-

Compressed Images

Characterization of the Effects of Translations on JPEG-

Compressed Images

Keith Jacoby

Advisor: Roger L. Easton, Ph. D.

Keith Jacoby

Advisor: Roger L. Easton, Ph. D.

Rochester Institute of TechnologyCenter for Imaging Science

Rochester Institute of TechnologyCenter for Imaging Science

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Introduction...

IntroductionIntroduction BackgroundBackground ExperimentationExperimentation

ResultsResults ConclusionConclusion

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image1.jpg

image2.jpg

“RECOMPRESSION OF SHIFTED IMAGE”“RECOMPRESSION OF SHIFTED IMAGE”

Intentions...

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Shifted and Recompressed

Intentions of ResearchIntentions of Research

Isolate the effect of shifts from the effects of JPEG’s quantization

Show trend of error due to increasing amount of shift (1 through 7 pixels in one direction)

Determine if there is a consistent relationship between shift and error across different images

Isolate the effect of shifts from the effects of JPEG’s quantization

Show trend of error due to increasing amount of shift (1 through 7 pixels in one direction)

Determine if there is a consistent relationship between shift and error across different images

JPEG Background...

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Brief Background of JPEGBrief Background of JPEG 8 x 8 blocking DCT DCT matrix of coef’s

that represent freq. content of the block

higher frequency info. is thrown out

reconstruction of block from lower freq. components using inv. DCT

8 x 8 blocking DCT DCT matrix of coef’s

that represent freq. content of the block

higher frequency info. is thrown out

reconstruction of block from lower freq. components using inv. DCT

BASIS FUNCTIONS

HIGHER

LOWER

Artifacts...

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Close-up of Shift ArtifactsClose-up of Shift Artifacts

Question: How does one quantify the artifacts due to shift? Question: How does one quantify the artifacts due to shift? Original JPEG imageOriginal JPEG image Shifted, recompressed JPEG imageShifted, recompressed JPEG image

Experiments...

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Several ExperimentsSeveral Experiments

Histograms of Error images* Analyze DCTs of shifted JPEGs Analyze DCTs of Error images Ensemble Averaging of DCT image Correlation of unshifted and shifted images

Histograms of Error images* Analyze DCTs of shifted JPEGs Analyze DCTs of Error images Ensemble Averaging of DCT image Correlation of unshifted and shifted images

* Results are shown in this presentation. Results from other experiments are to be covered in the Thesis.

Measure error...

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How is the error measured?How is the error measured?1st Gen.

JPEGimage

(“original”)

2nd Gen. JPEG image

(shifted)

| |

Difference(or Error)

Image

histogram of error...

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Histograms of Error ImagesHistograms of Error Images

Error image (inverted andequalized for better visi-bility of the artifacts)

Error Histogram showing error due to a shift of just 2 pixels (over 75% of the pixelshave changed their value***)

***Note: This could be an interesting value to investigate...

!!!

Mean Plot...

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This graph shows the Mean pixel value of the error image as a functionof pixel shift and quality factor for the photographic image.

Std. Dev. Plot...

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This graph shows the Std. Dev. of the pixel value of the error image asa function of pixel shift and quality factor for the photographic image.

ANOVA of mean...

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Two-way Analysis of Variance for Mean Analysis of Variance for Mean Source DF SS MSshift 6 1.2970 0.2162Qfactor 9 28.6188 3.1799Error 54 1.3273 0.0246Total 69 31.2431

Variance due to shift Individual 95% CI shift Mean --+---------+---------+---------+--------- 1 2.023 (------*-----) 2 2.197 (-----*------) 3 2.304 (------*-----) 4 2.409 (------*-----) 5 2.299 (-----*------) 6 2.159 (------*------) 7 2.020 (------*-----) --+---------+---------+---------+--------- 1.950 2.100 2.250 2.400

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Variance due to Quality Factor Individual 95% CI Qfactor Mean ----------+---------+---------+---------+- 10 3.451 (-*-) 20 2.920 (-*-) 30 2.601 (-*-) 40 2.444 (-*-) 50 2.233 (-*-) 60 2.063 (-*-) 70 1.894 (-*-) 80 1.680 (-*-) 90 1.329 (-*-) 100 1.400 (-*-) ----------+---------+---------+---------+- 1.800 2.400 3.000 3.600

Note: A similar statistical analysis will be done for the “percentage”value obtained from the histograms of error images. Results will bereported in the Web Thesis.

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What do these results suggest?

What do these results suggest?

For photographic images, the JPEG artifacts increase for increasing shifts up to a shift of 4 pixels in any axial direction. Beyond 4 pixels, the additional error due to shift decreases until 8 pixels, when the additional error disappears. This agrees with the initial hypothesis of the research project.

For photographic images, the JPEG artifacts increase for increasing shifts up to a shift of 4 pixels in any axial direction. Beyond 4 pixels, the additional error due to shift decreases until 8 pixels, when the additional error disappears. This agrees with the initial hypothesis of the research project.

Why it is important to know this...

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Concluding StatementsConcluding Statements This phenomenon is difficult to characterize

because JPEG images are all different in their spatial frequency characteristics.

This attempt to characterize the error due to shift reinforces the importance of taking care not to crop JPEG images along their 8x8 border rows or columns.

All investigation into the artifacts due to shift by analyzing frequency domain (DCT) images proved inconclusive.

This phenomenon is difficult to characterize because JPEG images are all different in their spatial frequency characteristics.

This attempt to characterize the error due to shift reinforces the importance of taking care not to crop JPEG images along their 8x8 border rows or columns.

All investigation into the artifacts due to shift by analyzing frequency domain (DCT) images proved inconclusive.

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www.cis.rit.edu/~kaj3230www.cis.rit.edu/~kaj3230

For complete details on all experiments performed, discussion of results, and conclusions, follow

link to Thesis.

Senior Research Thesis posted on:Senior Research Thesis posted on:

Thank You!!!

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