IEEE Spectrum August, 2009 - 42

Seeing Is Not Believing
Continued from page 48

light source in an image by looking at a given object's 2-D surface contour, such as a person's jawline and chin. There, the
surface orientation to the light source is always perpendicular to the contour.
By measuring the brightness and orientation at several points
along the contour, we can estimate which direction the light is
coming from. Then we can compare the lighting direction for
that object to those of other objects in the photo. Any inconsistency in the lighting direction is evidence of tampering.
the associated Press Planned to run a photo of the
cast of the television series "American Idol." A photo editor at the wire service had doubts about the photo's authen-

ticity, however, and contacted my lab for a second opinion.
When my colleagues and I examined the image closely, we
immediately noticed that the small white specks of reflected
light in each person's eyes, known as specular highlights,
were inconsistent. To us, that was an obvious sign that the
cast members had posed at different times and that the individual photos had been melded together.
The eyes are a beautiful tool for digital forensics, because
they act as tiny windows into the world in which the photo was
taken. By looking at the shape, color, and location of the specular highlights, we can learn quite a bit about the lighting that
was used to take the photograph.
The location of the bright spot on the eye, for example, can
indicate where the light source was positioned; multiple spots
indicate more than one light source. The precise position of
the specular highlights depends on both the curve of the eyeball and the relative orientations of the eye, the camera, and

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LEADING MAN:
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Table of Contents for the Digital Edition of IEEE Spectrum August, 2009