Exposing Photo Manipulation with Inconsistent Reflections

James F. O’BrienUniversity of California,

Berkeley

Hany FaridDartmouth College

Presented by: Jinjie Lin

All images presented in this document is from the paper except the ones whose source link are provided.

Where can we get the lastet SIG papers?

http://kesen.realtimerendering.com/

Photographs can no longer be trusted!

Is this a real photo?

A forged image

Is this a real photo?

http://img2.zol.com.cn/product/22/406/ceIi7JPRpyOnI.jpg

Is it really so easy?

Is this a real photo?

Digital watermarking

• Embedding information into a digital signal which may be used to verify its authenticity or the identity of its owners

• Drawback– Watermark must be

inserted at the time of recording

Passive forensic techniques

• Operate in the absence of watermarks or signatures.

• Assumption:– Although photo manipulation may leave no

obvious visual clues, it may alter some geometric or statistical property in the image.

Exposing Photo Manipulation with Inconsistent Reflections

Planar reflection geometry

viewer

objectreflection

Reflecting mirror

Surface normal

Planar reflection geometry:The reflection Vanishing Point

Observation:- PR must be perpendicular to MB- Photo: liner perspective projection- Parallel lines must all converge to

the same vanishing point.

Planar reflection geometry:Reflection Line Midpoints

Observation:The reflection line midpoint must lie in the plane of the reflecting surface.

Planar reflection geometry:Reflection Line Midpoints

pr

Test:- for a line pr, check whether it converge to the same vanishing point v;- Compute the midpoint, m;- Compare m to other scene elements for consistency.

Planar reflection geometry:The Center of Projection

Observation:For a given image, the center of projection is unique.

http://www.cse.unr.edu/~bebis/CS791E/Notes/PerspectiveProjection.pdfhttp://cs.hust.edu.cn/webroot/courses/csgraphics/jiaocai.php?bookpage=7_d

How to find out the center of projection from an image

• When vanishing points for three mutually orthogonal directions are known.

Common in man-made environments

How to find out the center of projection from images

• The computation is extremely unstable.• Instead, they compute a cloud of plausible

locations rather than a single location.

Detecting Inconsistencies

• Three tests for exposing inconsistencies in an image– ill-defined reflection vanishing points– Implausible midpoints– Inconsistent centers of projection

• Not all of these tests are applicable to any given image.– Require some human understanding of the image

content.

Results

lack of a well-defined reflection vanishing point

each object has a well-defined reflection vanishing point, but they are mutually inconsistent.

Blue lines: scene features with their corresponding reflections in the building windows. Green: linear features that should be perpendicular to the building front

head has been removed

the height of her hair has been altered.

Conclusion

• Present a set of test to expose photo manipulation.

• Require the input photo should contain planar reflectors where parts of the scene can be observed both directly and indirectly through reflection.

• Is not able to prove a photo is authentic, but it can prove a photo is fake.