I recently read a few stories about this set of photo composites created by Everett Hiller, who inserts a variety of celebrities into his holiday party photos each year. What impressed me about these photos was that the creator did an unusually good job of making them look — at least at first glance — plausible. I thought these photos would be a good test of some of our image forensic techniques.
In an earlier post (“Photo forensics from shadows”) I described a forensic technique for determining if cast shadows in a scene are consistent with a single light source. When a scene is illuminated by a distant light source (the sun, for example), then lines that connect points on a shadow to their corresponding points on the object should all converge to a single point in the image. Although the sun is obviously not illuminating the scene below, the camera flash did fire (as can be seen by the cast shadow near each person’s body). Shadows generated by a flash or the sun obey the same basic constraint — lines connecting shadow and object must converge to a single point.
Shown below is a shadow analysis applied to the above photo. The green lines connect points on the cast shadow with the corresponding part of the body. As expected, these lines intersect at a single point. Note, however, that the cast shadow under Tom Cruise’s right arm is inconsistent with this location. Even though the location of this shadow is quite wrong, it is not immediately obvious at first glance.
Shown below is another of the celebrity photos, but this time there are no cast shadows on Laurance Fishburne (far left) which can be analyzed. However, his reflection appears in the mirror behind the door and as I described in an earlier post (“Photo forensics from reflections”) lines connecting points on an object and their reflection must intersect at a single point.
Shown below are lines connecting three points on the women’s face to her reflection (green) and two points on Fishburne’s face and his reflection (red). The failure of all of these lines to intersect at a single point means that the reflections are physically impossible. Notice also that there is a highlight in the middle of Fishburne’s head, but in his reflection in the mirror the highlight is on the side of his face.
The creator of these photos added shadows and reflections in a way that was visually compelling. However, our visual system is not always good at seeing the correct shape and location of shadows and reflections. A quick and relatively easy to execute geometric analysis can detect these inconsistencies. Knowing about this analysis may now make it easier to create consistent shadows and reflections — but we have more tricks up our sleeves… stay tuned.