Reference Quality
Definition
The quality setting used to re-compress the image when computing ELA. The choice of reference quality controls the magnitude of all residuals in the map. Using inconsistent reference qualities when comparing images makes the maps incomparable.
- Controls
- Magnitude of residuals in an error level analysis map
- Technique
- Error level analysis (ELA)
- Pitfall
- Inconsistent reference quality makes maps incomparable
Common questions
How does an examiner choose the reference quality setting for an ELA comparison?+
The setting is typically chosen to sit close to the image's own apparent original compression level or held constant across a batch of images being compared side by side, since the goal is a consistent basis for comparison rather than any single universally correct value.
Can a mismatched reference quality create a false impression of tampering?+
Yes, recompressing at a quality far from the image's native level can produce bright residuals across the whole image rather than just at edited regions, which an inexperienced examiner could misread as evidence of manipulation when it is really an artefact of the setting chosen.
Related terms
- Compression Floor
- The stable low-residual state that a JPEG region approaches after many successive saves at the same quality. Once the DCT coefficients are...
- 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...