You can't watch this! Privacy-respectful photo browsing on smartphones

We present an approach to protect photos on smartphones from unwanted observations by distorting them in a way that makes it hard or impossible to recognize their content for an onlooker who does not know the photographs. On the other hand, due to the chosen way of distortion, the device owners who know the original images have no problems recognizing photos. We report the results of a user study (n=18) that showed very high usability properties for all tested graphical filters (only 11 out of 216 distorted photos were not correctly identified by their owners). At the same time, two of the filters significantly reduced the observability of the image contents.