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Digital Image Forensics

Digital image forensics or image forensics is a quite recent branch of computer forensics which aims to confirm the authenticity of images and to detect malicious manipulation in images denoted as digital tampering. It is assumed that images don’t include watermarks.

Contents [hide] 1 Introduction 2 Techniques 2.1 Camera based techniques 2.2 Tampering detection techniques 2.2.1 Segmentation techniques 2.2.2 Optical techniques 2.2.3 Projection related techniques 3 References 4 See also 5 External links

Introduction

Digital images and videos are everywhere in our lives, but how these objects are used is questionable: The modifications done on digital files can result in a computer crime. Removal of a person / object from the image (or its insertion to the image) is a typical digital tempering example. An intentional change in contrast or a change in the shape, color and location of objects can also lead to fraudulent images. In order to check the reliability of image contents, digital image forensics techniques are used.

Techniques

The principal techniques can be grouped into two, according to the nature of properties they benefit. Basically, camera based techniques exploit the unique signs of the camera in the image whereas tampering detection techniques are based on rather the semantic content of the image.

Camera based techniques

Tampering detection techniques “Image manipulation is denoted as tampering when it explicitly aims at modifying the semantic meaning of the visual image.” 1 In the following paragraphs some examples of digital tampering and techniques that can be used to expose them are discussed.

Segmentation techniques In these methods the image is divided into segments, and several properties of the segments are compared.

For example, in an image of street manifestation the number of people who participated in the action can be reduced or increased by copy-paste forgery. To search for such a repeated pattern, the image can be divided into blocks of different size. Afterwards, the DCT transform2, the Fourier-Mellin transform3 of image segments, or PCA on the image segments4 can be examined. Various other methods exist.1

Also if the image is in JPEG format, and if the copy-paste parts are coming from another JPEG image, then the quality factor and other properties of different regions will differ5, helping us to expose the forgery.

Optical techniques Lighting inconsistencies in different parts of the image can be an evidence of tampering. Such inconsistencies may be found by examining the optical tools like illuminant direction and specularity of the objects6. Consider a simple example: In a picture of a group of people, if their shadows fall both to left except for one person, then probably he is added to the original picture from another image.

Projection related techniques These methods benefit the projection geometry. For instance, in the presence of a copy-paste forgery or a removal, the principal point moves7.

References

1. J. A. Redi, W. Taktak and J. L. Dugelay,( October 2010). Digital image forensics: a booklet for beginners. Multimedia Tools and Applications, Springer, pp. 1-30 2. J. Fridrich, D. Soukal, J. Lukas (2003). Detection of copy-move forgery in digital images. In: Proceedings of Digital Forensic Research Workshop. 3. S. Bayram, T. Sencar, N. Memon (2009). An efficient and robust method for detecting copy-move forgery. In: Proc IEEE ICASSP 4. C. Popescu, H. Farid. (2004). Exposing digital forgeries by detecting duplicated image regions. Technical report, TR2004-515, Department of Computer Science, Dartmouth College. 5. X. Feng, G. Doerr (2010). Jpeg compression detection. In: SPIE Conference on Media Forensics and Security. 6. M. K. Johnson (September 21, 2007). Lighting and Optical Tools for Image Forensics. PhD Thesis in Computer Science, Darthmouth College. 7. H. Farid (2009). A Survey of Image Forgery Detection. IEEE Signal Processing Magazine, 26(2):16-25.

See also

- Digital imaging - Digital image processing - Forensic science - Digital forensics - Computer forensics

External Links

A digital image forgery example (http://thislooksshopped.com/2010/07/09/sir-wong-is-sir-wrong/)