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.
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...
Explained in
- JPEG Compression Artifacts and Error Level AnalysisA quality-dependent divisor applied to each DCT coefficient before rounding. A larger step size (lower quality) discards more information and introduces more r...