Quantisation Step Size
Definition
A quality-dependent divisor applied to each DCT coefficient before rounding. A larger step size (lower quality) discards more information and introduces more rounding error on the first save. The JPEG quality setting controls the step sizes through a quantisation table.
- Function
- Divisor applied to each DCT coefficient before rounding
- Controlled by
- JPEG quality setting via quantisation table
- Effect of larger step
- More information loss, more rounding error
- Field
- JPEG artefact and Error Level Analysis
Common questions
How does quantisation step size relate to Error Level Analysis results?+
Error Level Analysis re-compresses an image at a known quality and compares it to the original; regions with a different original quantisation step size than the rest of the image respond differently to this re-compression, producing a visible brightness difference that can flag a spliced or edited area.
Why can two JPEG images look identical but have different quantisation step sizes?+
Different encoders or the same encoder set to different quality levels use different quantisation tables, so two visually similar images can carry different step sizes, which is why the raw quantisation table itself, not just visual appearance, is examined during authentication analysis.
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....
- 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...