Rebroadcasting Detection: Luminosity Based
Title: FLAT SURFACE DETECTION IN PHOTOGRAPHS FOR TAMPER DETECTION
Patent Number: US 10,692,202 B2
Date of Patent: Jun . 23 , 2020
An “analog hole” or “rebroadcast attack,” are common terms for subverting provenance systems by capturing an image of a photograph or computer screen.
While such methods may seem rudimentary, they can nonetheless be very effective, and the fact that they can be executed by an unsophisticated user, only increased the importance of developing the means to negate such attacks.
When an image is captured, a 3-dimensional scene is transformed into a 2- dimensional image. Additionally, the visual representation of depth is emulated through the intensities of the scene’s respective colours.
Using a device’s flash sensor draws on the concept of irradiance from a given point light source. This concept is illustrated in Figure 5, and later expanded on to remove the need for an external light source.
The irradiance of a light source at a given distance is dictated by the inverse square law. This law implies that the brightness of an object decreases proportionally to the square of its distance from a light source. The flash of the light source will observe the differences in luminosity of the captured image.
Using the luminosity difference, the depth information maybe be inferred, in which a flat surface would hold similar depth information, as opposed to a 3-dimensional scene which result in varying distances.
The following outlines the proposed algorithm:
1. Images with and no flash are captured.
2. Down sample the images.
3. Edge detection is performed on the images.
4. Luminosity maps are obtained from the down sampled images (in 2).
5. Apply Pixel subtraction to 4
6. Extract statistical metrics (Standard Deviation, Kurtosis & Variance).
7. Create a z-score matrix from 5.
8. perform an edge detection on 7.
9. Compare 3 and 8.
10.Classification performed using 9 and 6
Removing the requirement for an external light source
The requirement to trigger the camera’s flash may have adverse user-experience effects. We have found that similar results can be achieved by performing the image capture multiple times with varying exposure values, replacing only the very first step in the algorithm set out above.