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<p>We know to be wary of the photo hoaxes that litter the online landscape, the impossibly perfect women in fashion magazines, and the scandalous images in tabloids. But can we trust photographs in reputable news outlets, prestigious scientific journals, and government publications? In this series of posts, I will examine how the ubiquity of photographic tampering has eroded our faith in images. I will also discuss recent technological advances in the field of photo forensics that have the potential partially to restore this faith.</p> <p></p>

<p>When asked to judge the location of the sun relative to this scene, most people would respond quickly that it is on the right. When asked to localize the sun more precisely, most people would need to think before responding and they would still be inaccurate. (For evidence of this perceptual failure see <span class="frzfn fn"><span class="marker">1</span><span class="text">Hany Farid and Mary J. Bravo. Image Forensic Analyses that Elude the Human Visual System. <em>SPIE Symposium on Electronic Imaging</em>, San Jose, CA, 2010.</span></span>, but also test yourself: the answer is below.) The difficulty we have localizing light sources is surprising because the scene contains abundant information for making this judgment. The fact that we cannot easily perceive information that is clearly contained within the image provides an opening for forensic analysis because forgers may be unaware that they have introduced telltale clues that we can easily detect. In this post, we will see how to determine whether the cast shadows in an image are consistent with a single light source. If the shadows are inconsistent in a scene that is clearly illuminated by a single source (e.g., the sun), then we have strong evidence of image tampering.</p>

<p>Bright lights may appear to emanate streaks of light, and this could explain why the sun is often depicted as a circle surrounded by rays. The streaks of light are created by imperfections in the lens of the eye and are referred to as lens flare. Lenses refract (bend) the light rays that enter the pupil of the eye so that the rays focus on the retina. Because of slight imperfections in the lens, some light rays may be scattered or reflected rather than refracted. It is this wayward light that gives rise to lens flare aberrations.</p>

<p>A digital camera contains a vast array of sensor cells, each with a photo detector and an amplifier. The photo detectors measure incoming light and transform it into an electrical signal. The electrical signals are then converted into pixel values. In an ideal camera, there would be a perfect correlation between the amount of light striking the sensor cells and the pixel values of the digital image. Real devices have imperfections, however, and these imperfections introduce noise in the image.</p>

<p>When you view a sign from a highly oblique angle, the perspective distortion may cause the text and graphics to be unrecognizable. To make the sign interpretable, you could change your vantage point and view the sign head-on. But what if the sign appears at an oblique angle in a photograph? Is it possible to remove the perspective distortion and view the sign head-on? This question arises in forensic settings when the evidence contained within an image is obscured by perspective distortion. In this post, I describe how perspective distortion can be removed to reveal, for example, the identity of a barely visible license plate.</p>