Compression Equilibrium
Definition
The state of an image region after sufficient JPEG compression cycles at a given quality: further compression at the same quality produces minimal additional change. Regions at equilibrium appear dark in an ELA map.
- Domain
- JPEG image forensics
- State
- Minimal further change on repeated compression
- ELA appearance
- Dark region
- Cause
- Repeated saves at the same quality level
Common questions
Does a dark region in an ELA map always mean that part of the image is original and untouched?+
Not necessarily. A region can reach compression equilibrium either because it was never re-edited or because it was pasted from a source that had already been compressed many times at a similar quality, so it starts near equilibrium. ELA darkness indicates stability under further compression, not authenticity on its own.
How many resave cycles does it typically take for a region to reach compression equilibrium?+
There is no fixed count, it depends on the JPEG quality setting and the original content, but most regions converge within a handful of resaves at a constant quality once the DCT coefficients settle near their quantisation steps. Lower quality settings converge faster than high-quality settings.
Related terms
- ELA Amplification Factor
- The multiplier applied to the per-pixel difference before display. A higher factor reveals subtle differences but also amplifies noise, making regions look...
- ELA (Error Level Analysis)
- A passive image forensics technique that re-saves the input image at a chosen JPEG quality and amplifies the per-pixel absolute difference to...
- False Positive (ELA)
- An ELA anomaly in a region that was not manipulated, typically caused by high-frequency texture, JPEG artefacts in the original, or camera...
- High-Frequency Detail
- Fine textures, sharp edges, and small text that JPEG compresses poorly. These regions inherently show high ELA values in authentic, unmodified images...
- Saturation
- In ELA context, the condition where a region's error level has reached near-zero, indicating it has already been compressed to near-equilibrium. A...