Double-JPEG compression
Definition
The result of decoding a JPEG, applying any edit, and saving as JPEG again. The coefficient histograms of doubly-compressed images have a distinct periodic shape that singly-compressed images lack.
Related terms
- Blocking artefacts
- Visible rectangular discontinuities at 8x8 block boundaries, caused by independent quantisation of adjacent blocks. At splice boundaries, misaligned block grids produce a...
- 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...
- JPEG ghost
- A localisation method that re-saves the image at multiple quality levels, then maps per-pixel residuals to find regions whose minimum error quality...
- 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...
- Quantisation table
- A matrix of divisors applied to DCT coefficients during JPEG compression. Higher divisors produce lower quality. The specific table used is often...
Explained in
- JPEG Compression Artefacts and Double-Compression DetectionThe result of decoding a JPEG, applying any edit, and saving as JPEG again. The coefficient histograms of doubly-compressed images have a distinct periodic sha...