Photo forensics from shadows: the case of the missing RPG shadow

The Reuters news agency is being accused of publishing an altered photo in it’s editor’s choice blog. The remarkable photo shows a man firing a rocket-propelled grenade (RPG). It is being argued that the shadows in the photo are improbable because the shadows of the RPG launcher and the subsequent blast cannot be seen alongside the shadow of the man. In response to these accusations, Reuters withdrew the photo from their blog. We believe, however, that the shadows and lighting in this photo are physically plausible.

In an earlier post I described how shadows can be useful in a forensic examination (“Photo forensics from shadows”). At the same time, I also said that a casual visual examination of photos can be unreliable. Here I describe a plausible explanation of the shadows and lighting in this photo. 

Note first that the cast shadow of the man onto the ground, the lighting on the man’s clothes, and the lighting on the large vase and poles in the median all suggest that the sun is above and to the right of the man. With the sun in this position, it may seem that the shadow of the RPG launcher and blast are missing. It may also seem that the shadow of the poles are missing. Consider, however, the following:

1. A magnified view of the man’s cast shadow shows a small portion of what appears to be the back of the RPG launcher.
2. The front of the launcher may not be visible because the man is standing on a part of the road that appears to slant down so that the resulting foreshortening and angle of the road obscure this part of the shadow. The debris inside the man’s shadow may also be contributing to the RPG’s shadow being obscured.
3. The blast may not be casting a shadow because it may be closer to the photgrapher than it appears in the photo (Reuthers confirmed that the photo was taken with a tele-photo lens which causes a compression in the apparent depth).
4. Contrast enhancing the road reveals what appears to be the cast shadow of the first pole – the shadow is not as pronounced partly because of the blurring caused by the narrow depth of field. Extending this shadow to the right shows that it coincides with the base of the pole. The shadows for the remaining poles are equally hard to see because of the distance, blur, haziness and resolution.

This, of course, does not prove that the photo is authentic, but it does suggest that there is a plausible explanation for the lighting and shadows in this photo. 

Because photo tampering is so common, the authenticity of spectacular photographs are now routinely, and sometimes inappropriately, questioned. This highlights the importance of quantitative and objective forensic techniques for verifying photo authenticity.

[Photo credit: Reuters/Anis Mili]

Comments

[matso]

I like the analysis, but if poles do, how come the crutches don't cast any shadows on the ground, the area under the left foot looks strange
[If you magnify and contrast enhance the region between the right foot and right crutch, you will see a faint shadow of the crutch. As with the shadow for the front of the RPG, the shadow for the left crutch may be obscured by the angle of the road. Also, the crutches are much smaller than the poles so their shadows will be proportionally smaller and harder to see. -Hany]

[chen]

But where's the shadow of the left crutch?

[Because the crutch is smaller than the person and because of the angle of the road, it is likely that the left crutch's shadow is simply too small to see. Because there is a physically plausible explanation for why we cannot see the shadow, its absence in the photo is not suspicious. -Hany]