Forensics of high-quality JPEG images with color subsampling

Detecting prior compression is an essential task in image forensics and can be used to detect forgery in digital images. Many approaches focus on grayscale images and assume compressions with a low quality factor which often leave visible artifacts in the image. In practice, however, color images and high quality compression are much more relevant and widespread. Block convergence has been proposed to estimate the number of JPEG compressions with quality factor 100 for grayscale images and has been shown to produce accurate results [1]. This paper extends block convergence to the more relevant case of color images where chrominance subsampling and color conversion make the estimation more complex. By observing block convergence for macro-blocks over multiple recompressions we are able to produce accurate estimates for color images. Oftentimes block convergence for color images enables similar accuracy and allows to detect more recompressions compared to grayscale images, while maintaining a good distinction between never and once compressed images.