Determining JPEG Image Standard Quality Factor from the Quantization Tables

Identifying the quality factor of JPEG images is very useful for applications in digital image forensics. Though several command-line tools exist and are used in widely used software such as GIMP (GNU Image Manipulation Program), the well-known image editing software, or the ImageMagick suite, we have found that those may provide inaccurate or even wrong results. This paper presents a simple method for determining the exact quality factor of a JPEG image from its quantization tables. The method is presented briefly and a sample program, written in Unix/Linux Shell bash language is provided.