Forensics of visual privacy protection in digital images

Visual privacy protection (VPP) is to protect individual’s privacy information in digital images or videos against being seen, such as portrait etc. Once being protected, the privacy may be imperceptible. Forensics of visual privacy protection is becoming a new challenge. As a main visual privacy protection technology, blur operation is always used in VPP. When using the existing approaches such as blur segmentation to detect the images, natural blur or other solid color regions will be falsely alarmed, resulting in low precision. In this paper, we present a novel metric invisibility degree (IvD) to measure the privacy protected blur degree of each pixel. The proposed IvD is defined by calculating the similarity between the test image and the re-blurred image in joint transformation and spatial domain, which could significantly enhance the difference between the privacy protected blur region and the other regions. Then, an effective method based on IvD is developed to automatically forensic and localize the visual privacy protection regions. Firstly, the IvD in block DCT domain of each pixel is calculated, and a IvD map is obtained. Secondly, using the IvD map, the test image is segmented and followed by morphological operation to decrease the mis-alarm regions. Finally, a spatial texture feature descriptor, including gray statistics, smoothness and information capacity, is developed, based on which the detection result is further refined. Experimental results show that the proposed method can detect the privacy regions accurately.