Compression Floor
Definition
The stable low-residual state that a JPEG region approaches after many successive saves at the same quality. Once the DCT coefficients are near their quantisation levels, re-saving changes them very little, and ELA shows near-zero residual.
- Domain
- JPEG compression artefact analysis
- Mechanism
- DCT coefficients converge near quantisation levels
- ELA signature
- Near-zero residual
- Related term
- Compression equilibrium
Common questions
What is the practical difference between compression floor and compression equilibrium in an ELA report?+
The two terms describe the same underlying phenomenon from different angles, the floor is the low-residual value a region settles at, while equilibrium is the state of having reached that floor. Analysts sometimes use them interchangeably, but a careful report distinguishes the measured residual value from the description of the region's saturation state.
Why does an edited region stand out against a background at compression floor?+
A newly inserted or edited region has not gone through the same number of resave cycles as the surrounding image, so its DCT coefficients have not yet settled and it shows a higher ELA residual against the darker, equilibrated background. This contrast is what an examiner looks for, not the absolute brightness of any single region.
Related terms
- 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...
- ELA Residual
- The pixel-level absolute difference at a given location after re-compression. High residual means the pixel value moved substantially on re-compression, indicating the...
- Error Level Analysis (ELA)
- A technique that re-compresses a JPEG at a controlled quality setting and maps the pixel-level difference between the re-compressed and original images....
- Quantisation Step Size
- A quality-dependent divisor applied to each DCT coefficient before rounding. A larger step size (lower quality) discards more information and introduces more...
- Reference Quality
- The quality setting used to re-compress the image when computing ELA. The choice of reference quality controls the magnitude of all residuals...