1. Detecting Digital Forgeries using Blind Noise Estimation
Xunyu Pan, Xing Zhang and Siwei Lyu
Computer Science Department
University at Albany, SUNY
September 29, ACM MM&Sec 2011
Buffalo, New York

2. Outline Motivation Related Works Detection Method Experimental Results
Discussion and Future Work

3. Image Splicing
Photoshop
Photo
Credit: Liu Weiqiang

4. Creation of Visual Effects

5. Motivation
Image splicing and creation of visual effects are two common manipulations used in image tampering, where noise is introduced to either conceal tampering traces or changing image contents
The variance of noise in a authentic images is generally uniform distributed
We propose a new method to effectively expose image forgery by detecting the noise variance differences between original and tampered parts of an image

6. An Example of Image Splicing Forgery
Original images
Tampered image

7. Outline Motivation Related Works Detection Method Experimental Result
Discussion and future work

8. Previous Works
Determine the existence of forgery by extracting camera sensor fingerprint (e.g. PRNU) from training images and further detecting image forgery by supervised learning (SVM)
Knowledge of specific camera models [Lukas, 06], [Sutcu, 07] and [Filler, 08]
No extent and location of the forgery [Gou, 07]
Locate image forgery using noise level difference
Knowledge of the kurtosis of the adding noise and the original image [Popescu, 04]
Inefficient blocks merging algorithm[Mahdian, 09]

9. Outline Motivation Related Works Detection Method Experimental Results
Discussion and Future Work

10. Pipeline of the Proposed Method

11. Image Block Segmentation
Image segmentation for local noise estimation
Larger block are more stable and accurate
Smaller block represents local noise more precisely
We segment image into 64 × 64 pixel non-overlapping blocks in the first round detection

12. Image Block Segmentation
32×32
128×128
256×256
64×64
16×16
Means and standard deviations of estimated noise on
100 image blocks of various sizes by adding noise Std 25

13. Pipeline of the Proposed Method

14. Image Kurtosis
Image kurtosis is the measure of the "peakedness" of the probability distribution of pixels:
Suppose a white Gaussian noise of zero mean and unknown variance is added to the image to obtain an image , denote as:
The kurtosis of can be computed as:

15. Pipeline of Noise Estimation

16. The Objective Function
Assuming the kurtosis is scale invariant, we can estimate the kurtosis of and the variance of noise by minimizing the objective function:

17. Major Steps for the Noise Estimation
STEP 1: Conversion from image domain to DCT domain
STEP 2: Compute the variance and kurtosis for each response image
STEP 3: Noise estimation by the optimization of the objective function (MATLAB fminsearch function)

18. Pipeline of the Proposed Method

19. K-means Clustering
Classify all non-overlapping image blocks into k (k=2) clusters based on their noise level
(1) Starting with randomly selected k means
(2) Assign each block to the cluster with the closest mean
(3) Compute a new mean for each cluster
(4) Repeat (2) and (3) until the stopping criteria is satisfied

20. Initial Detection
Image blocks are classified using K-means clustering algorithm
Assume the tampered region is smaller than authentic region. The cluster with fewer blocks is treated as forgery
False positives - complex textures: tree, grass, … Need a second round refined detection

21. Refined Detection
Segment the detected suspicious region into 32 × 32 non-overlapping image blocks
Estimate noise level for each 32 × 32 blocks
keep all two clusters if their means are close enough

22. Outline Motivation Related Works Detection Method Experimental Results
Discussion and future Work

23. Quantitative Testing Results
We define two quantitative measure of performance based on the detection accuracy and false positives
Block detection accuracy (BDA)
Block false positive (BFP)

24. BDA and BFP

25. Quantitative Experiments
One uncompressed color image from Kodak dataset
A randomly located 192 × 192 image block is tampered with Gaussian noise of various noise levels
Generate 100 images for each noise level in the range of [1, 10] (total 1000 forged images)

26. Quantitative Experiments
Averages of BDA/BFP for 100 tampered image at each noise level σ = [1, 10 ]

27. Qualitative Experiments
Image tampered with noise σ=5
Detection result

28. Qualitative Experiments
Image tampered with noise σ=10
Detection result

29. Qualitative Experiments
Image tampered with noise σ=15
Detection result

30. Qualitative Experiments (cont.)
Detection result of our method on image splicing forgery

31. Qualitative Experiments (cont.)
Detection result of our method on rain/snow appearance created using image noise

32. Qualitative Testing Results (cont.)
Detection result of our method on ground glass appearance created using image noise

33. Outline Motivation Related Works Detection Method Experimental Results
Discussion and Future work

34. Summary
In this work, we propose a novel method for image forgery detection based on the clustering of image blocks with different noise variances
Experimental results with credible forgeries show the efficacy of our method

35. Discussion Advantages Limitations
Reliably expose image splicing or forgeries created for special visual effects
No prior knowledge of the imaging device or of kurtosis of the original image
Limitations
Non-overlapping can not locate the extent of tampered region precisely
Failure estimation in one block leads to missing tampered region

36. Future Work
Detect noise inconsistency due to different JPEG compression qualities
Improve the detection accuracy and efficacy by noise estimation on local overlapping image blocks using fast estimation method

37. Thank You!