Modeling the flicker effect in camcorded videos to improve watermark robustness

Camcording a screen is a common and easy way for a pirate to capture a protected video and circumvent any cryptography-based protection system like DRM. Therefore, antipiracy methods like watermarking should be robust to this type of attack. One effect very often encountered is the flicker effect: image luminance varies periodically along the video, and scrolling stripes may also be observed. Such an effect is due to aliasing, because the higher frequency backlight of the screen is sampled at a lower rate by the camcorder. The stripe effect appears because each line of the image is captured with a small delay with CMOS sensors. We show that the amplitude of the flicker depends on the luminance of the displayed image, as well as on the luminosity of the screen and on the exposure setting of the camcorder. We propose a method to blindly estimate the flicker parameters by studying the spectrum of the camcorded video; then we show how the flicker can be selectively removed without impairing other frequential components of the video. Experiments on camcorded videos show that removing the flicker enable significant improvement in the watermarking detection performances.