An image that appears to be a photograph may not necessarily be a normal photograph as we know it. For example, a photograph-like image can be rendered by computer graphics instead of being taken by a camera or it can be a photograph of an image instead of a direct photograph of a natural scene. What is really different between these photographic appearances is their underlying synthesis processes. Not being able to distinguish these images poses real social risks, as it becomes harder to refute claims of child pornography as non-photograph in the court of law and easier for attackers to mount an image or video replay attack on biometric security systems. This motivates digital image forensics research on distinguishing these photograph-like images from true photographs. In this chapter, we present the challenges, technical approaches, system design, and other practical issues in tackling this multimedia forensics problem. We will also share a list of the open resources and the potential future research directions in this area of research which we hope readers will find useful.
MotivationsSince the ancient times of the Greeks and Romans, artists have been playing with special painting techniques for inducing visual illusion where objects in a painting appear to be real and immersed in the real surrounding. Trompe l'oeil, the name for such visual artistry, literally means deceiving the eye. For example, the painting entitled Escaping Criticism created by Pere Borrell del Caso in 1874 depicts a person climbing out of the painting with a make-believe quality ( Figure 1a); the mural painting on the facade of the Saint-Georges Theater created by a painter Dominique Trompe l'oeil makes believe with not only the photorealistic quality in the painting, it also exploits the visual gullibility of human observers through immersion into the real surrounding. Such adversarial nature of trompe l'oeil aptly mirrors that of digital image forensics where the intention to deceive is present. While the deceptive intent is from humans, the photorealism of an image which takes many years of practice for conventional artists to master can be easily produced by laymen with modern technology. Physics-based computer graphics is capable of rendering photorealistic images that emulate images of real three-dimensional scenes, a great advancement from the two-dimensional graphics like cartoon which was popular in the early days of computer graphics. It has been shown that a scene with diffuse reflectance can be realistically rendered to the extent that the rendered scene radiance is close to that of a real scene and perceptually indistinguishable for humans [43].A photograph is photorealistic by definition. Recapturing a photograph with a camera when displayed in good quality on a paper or screen preserves the photorealism of the photograph if perspective distortion is minimized 1 . We refer to such types of images as recaptured or rephotographed image. The modern artist Richard Prince pioneered a unique art form of rephotographing...