Photo forensics from JPEG dimples

Abstract

Previous forensic techniques have exploited various characteristics of JPEG compression to reveal traces of manipulation in digital images. We describe a JPEG artifact that can arise depending on the choice of the mathematical operator used to convert DCT coefficients from floating-point to integer values. We show that the more commonly used floor or ceiling operators (but not the round operator) introduce a periodic artifact in the form of a single darker or brighter pixel — which we term a dimple — in 8 × 8 pixel blocks. We describe the nature of this artifact, its prevalence in commercial cameras, and how this artifact can be quantified and used to detect a wide range of digital manipulations from content-aware fill to re-sampling, airbrushing, and compositing.