Patent attributes
Techniques are generally described to determine whether a JPEG image has undergone two compressions. Probabilities can be computed for the first digits of quantized DCT (discrete cosine transform) coefficients from a set of AC (alternate current) modes to detect or determine whether the JPEG image has undergone two compressions. The set of AC modes may include a predetermined number of distinguishable AC modes where a distinguishable AC mode may be an AC mode in which a second quantization step (QS2) is not an integer multiple of the first quantization step (QS1). Classifiers may be created during a training process, and later may be used to assist in determining whether a suspect JPEG image has undergone two compressions. When the classifiers support a multi-classification system, described detection techniques may also be arranged to determine a primary quality factor for the double compressed JPEG image.
