When can a photo be trusted?

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Tuesday
Oct012013

Is That A Race Horse On The Roof?

When the owners of “Must Win” received a photo of their race horse on the roof of their home they were suspicious that it was a prank. A shadow analysis confirms that the photo is probably real.

[Source: CTV News]

We have previously employed a geometric analysis of cast shadows to detect image and video tampering (e.g., The Baby Snatching Eagle and The Russian Dash Cam). In the above image, however, the required object/shadow correspondences are in short supply. We have recently developed a new shadow analysis that is applicable to less well defined object/shadow pairings (as described in The Moon Landing Photos). This analysis is particularly helpful in analyzing the above image.

Shown below is the result of this analysis. Each red wedge-shaped region originates at a point on a cast shadow and encompasses the corresponding object that cast the shadow. The four shadows, from the horse and building, have a common intersection (the triangular region outlined in black) suggesting that these shadows are physically plausible.

[Click to enlarge]

The intersection of the shadow constraints localizes the projected location of the light source. As you can see, this places the light source to the right of the photographer. Looking back at the original image, this location is consistent with the lighting on the house: the walls facing right-ward are brightly illuminated while those facing the camera are less illuminated or in shadow. In addition to the shadows being physically plausible, the shadows are also consistent with the lighting in the scene. You can also see physical evidence of the horse’s adventure at: Bored horse climbs on top of New Brunswick Family’s Roof.

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Reader Comments (1)

I see a lot of shadow analysis on this blog to see if things are fake. Is that the only way you find out of photos are real or not? A more intriguing question for my is, Why is your software $890? That is as much as Photoshop itself.

[We have many different ways to authenticate photos, shadows and lighting being only one particular way. We offer a $20 on-line version of FourMatch that allows you to analyze up to 30 images. -Hany]

November 29, 2013 | Unregistered CommenterTim

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