JPEG Ghost Scanner

Every JPEG image carries invisible traces of its compression level. When you re-save a JPEG at the same quality it was originally encoded, the error introduced is minimal — the image is already "adapted" to that quality. But if a region was pasted from a different JPEG saved at a different quality, that spliced region responds differently to recompression. The JPEG Ghost Scanner exploits this by re-encoding the image at every quality level from 60 to 99 and measuring pixel-by-pixel differences. At the estimated original quality, spliced regions can "glow" because they show higher error than the background — though so does any sharp detail, which is why the map is read for coherent patches rather than for brightness alone. Two limits are worth knowing before you read a result. The re-encoding uses your browser's own Canvas JPEG encoder and its quantization tables, not the tables stored in your file, so the estimated quality is a proxy for the original setting rather than a reading of it. And images larger than 1500 px on the long side are scaled down first: resampling rewrites the 8×8 blocks the whole technique depends on, so an estimate from a scaled image is not usable. The technique complements the ELA Scanner, which detects compression inconsistencies from a single re-save rather than a full quality sweep.

JPEG ghost analysis is most effective when you suspect that parts of an image were copied from another JPEG and pasted into the current one — a technique known as splicing or compositing. Journalists use it to verify whether press photos have been manipulated. Insurance investigators check submitted damage photos for pasted elements. Social media researchers use it to detect fabricated screenshots and doctored evidence. The quality sweep chart also serves as a standalone tool for estimating the JPEG save quality of any image, which is useful for archival and quality assurance. For metadata-based tampering detection, combine this with the Authenticity Checker.

The quality sweep chart plots mean squared error at each re-encode quality from 60 to 99. A characteristic "dip" in the curve marks the estimated original save quality — at that level, re-encoding barely changes the image because it's already optimized for those quantization tables. A sharp, clear dip indicates a single-compression JPEG. A flat or noisy curve may indicate the image has been through multiple compression rounds or was converted from a non-JPEG format. If the lowest error lands on 60 or on 99 — the ends of the swept range — the curve never turned back up, so no dip was bracketed at all: the original quality may lie outside the range, and the scanner reports no estimate rather than printing the boundary value as one. Click any bar to view the ghost heatmap at that quality level. The Quality Analyzer provides additional quality metrics including sharpness, noise, and exposure analysis.

JPEG ghost analysis is strongest when combined with other forensic tools. The ELA Scanner reveals compression inconsistencies through error level amplification — a complementary approach that sometimes catches edits JPEG ghosts miss, and vice versa. The Thumbnail Scanner checks if the embedded EXIF thumbnail still matches the main image. The Stego Scanner detects hidden data encoded in pixel values. Together, these tools cover pixel-level, compression-level, metadata-level, and steganographic analysis — the four pillars of image forensics. Our photo authenticity guide walks through the complete verification workflow.