When can a photo be trusted?

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Wednesday
Aug102011

Enhance – no, really

In our first blog entry we posted a funny video montage of the familiar (and somewhat silly) way in which Hollywood portrays digital image and video enhancement. Not surprisingly, much of what is shown on television and movies is complete fiction.  Advances in digital forensics, however, do allow us to do some pretty amazing things.

Shown below, for example, is an image of a license plate that is photographed from such a grazing angle that it is nearly impossible to reliably read the plate number. 

In general, a photograph is the result of projecting a 3D scene onto a 2D sensor.  This reduction in dimensionality leads to a significant and permanent loss of information, namely, the distance to objects in the scene. This in turn means that there is no way to directly infer the shape of objects in a scene or to rotate them in 3D to see them from a different vantage point. However, a flat surface such as a license plate is 2D to begin with, so there is no loss of information (except resolution) when it is photographed.  As a result, a photograph of a flat surface can effectively be rotated in 3D to see if from any vantage point.  Remarkably, this can be done with only 2D photo editing techniques, though it’s much easier in a tool that has some support for 3D concepts.

The Vanishing Point feature of Adobe Photoshop allows you to define perspective planes within a 2D photo so that you can clone and manipulate areas while maintaining realistic perspective. When combined with the 3D functionality of Photoshop Extended (CS4 or CS5), you can convert the perspective plane into a 3D object that can be rotated in 3D and viewed from any angle. The video below shows an example of this: (1) select Filter->Vanishing Point…; (2) select Zoom Tool and zoom in on the license plate; (3) select Create Plane Tool; (4) select the four corners of the distorted license plate; (5) select Return 3D Layer to Photoshop from the Settings menu; (6) select OK; (7) A new layer has been created that contains a 3D version of the license plate – double click on the texture layer and you will see an undistorted version of the license plate; and lastly (8) scale the license plate to the appropriate aspect ratio (2:1).

Shown below is a undistorted view of the license plate.
 

There are a few interesting things of note in this corrected image: (1) the top and bottom of the license plate are missing because the car bumper occludes the top of the plate and the bottom of the plate is bent slightly and therefore not visible; (2) the sides of the license plate are not perfectly straight because the plate is bent slightly at the edges, which violates the assumption that we are looking at a flat surface; and (3) the rusted bolts along the top are highly distorted in the corrected image – this is because the bolts are assumed to be flat and would have to look this distorted in order to appear as they do in the original photo (that is, this correction only works for flat surfaces).

Although this approach to removing distortion in Photoshop is limited to a rectangular region, the distortion of any content on a planar surface can be removed, including text on a sign or billboard.

Regardless of what Hollywood will have you believe, you cannot read a license plate number from an ATM video consisting of a half dozen pixels extracted from the reflection off of a pair of sunglasses. Modern day forensic tools, however, do allow us to do some pretty remarkable things.

[Photo credit: Hany Farid]

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