JPEG Ghost
Definition
A visualisation technique introduced by Farid (2009) that reveals regions compressed at a quality factor different from the rest of the image. The image is repeatedly re-compressed at different quality factors and the pixel-wise difference from the original is computed. A pasted region that was originally saved at quality Q will show minimum difference when recompressed at Q, creating a visible ghost of its extent against the surrounding area.
Related terms
- Double JPEG Compression
- The situation where an image has been JPEG-encoded, decoded to a bitmap, and then JPEG-encoded again. The second quantisation step leaves a...
- Quantisation Table
- A matrix of 64 integer divisors, one per DCT coefficient position, that controls how much each frequency component is rounded during JPEG...
- Block Grid Phase
- The horizontal and vertical offset of the 8x8 encoding grid relative to the image coordinate origin. A phase of (0,0) means the...
- Blocking Artefacts
- Visible rectangular discontinuities at 8x8 block boundaries, caused by independent quantisation of adjacent blocks. At splice boundaries, misaligned block grids produce a...
- Blocking Artifact
- A visual discontinuity at the boundary between two adjacent 8x8 blocks in a JPEG image, caused by independent quantisation of each block....
- DCT (Discrete Cosine Transform)
- The mathematical transformation applied to each 8x8 pixel block during JPEG compression, converting spatial pixel values into a set of frequency coefficients...
- Discrete Cosine Transform (DCT)
- A mathematical transform that expresses a block of pixel values as a sum of cosine functions at different frequencies. JPEG applies the...
- Quality Factor
- The 1-100 scale that most JPEG encoders expose to the user, controlling how aggressively the quantisation table rounds coefficients. The factor is...
Explained in these topics
- DCT Block Boundaries and Quantisation Grid AnalysisA visualisation technique introduced by Farid (2009) that reveals regions compressed at a quality factor different from the rest of the image. The image is rep...
- JPEG Compression Artefacts and Double-Compression DetectionA localisation method that re-saves the image at multiple quality levels, then maps per-pixel residuals to find regions whose minimum error quality differs fro...