When can a photo be trusted?

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Reflections on Hurricane Sandy

As Kevin described in his last post, Hurricane Sandy unleashed a slew of fake photos. Media outlets, and the public in general, have been scrambling to sort out the real from the fake. What made this situation more complex was that the real photos seemed as unlikely as the fake. The images from lower Manhattan, showing unimaginable flooding, were particularly striking. Here I will describe the application of a forensic technique for analyzing these types of flooding images.

As these hard-to-fathom images began to spread online, we were asked to verify some of the images that showed the extent of the flooding in New York City. Shown below, for example, is a remarkable image from lower Manhattan showing an entire block under water. As expected, you can see in the water a reflection of the objects in the scene. As I have previously described, there is a simple forensic analysis that can be applied to these reflections to determine if they are physically plausible.

[Photo credit: http://ontherealny.com/holy-sandy/]

In the figure below, I have connected points on four different objects with their reflection (cyan-colored lines). Because the water is not perfectly still, this correspondence is not perfect, but is accurate enough for this analysis. The small dots are the mid-points of each line. Due to the basic laws of perspective projection, the heights of these mid-points should be proportional to the distance of each object to the camera. This physical constraint is respected in this image — from closest to furthest in the scene and lowest to highest in the image, the mid-points correspond to the  (1) bicycle; (2) police car; (3) traffic light; and (4) building window.

This, of course, doesn’t prove that the image is real, but it at least shows that the reflections are physically plausible. Combined with the fact that creating such a reflection would have been very difficult, and that the extent of the flooding from Sandy has now been well documented, this photo isn’t very hard to believe. 

Natural disasters have always given rise to image hoaxes, but it does seem that Hurricane Sandy led to an unprecedented number of fake photos. The mixing of the fake with the real led to confusion and general doubt about any photographic evidence. Image authentication schemes such as those described here can, hopefully, begin to return some trust to photography.

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