Quantisation Table
Definition
A matrix of 64 integer divisors, one per DCT coefficient position, that controls how much each frequency component is rounded during JPEG compression. Dividing by a larger number loses more information but achieves higher compression. The quantisation table is embedded in the JPEG file header and is directly readable, giving analysts access to the compression parameters used.
- Size
- 64 integer divisors, one per DCT coefficient
- Applies to
- JPEG image compression
- Location
- Embedded in the JPEG file header
- Readability
- Directly extractable, not derived indirectly
- Sub-field
- Digital image forensics, compression analysis
Common questions
Why is the quantisation table useful for source or authenticity analysis?+
Different cameras, phone models, and editing software embed characteristic quantisation tables, so comparing a suspect image's table against known device signatures can support or contradict a claimed source or processing history.
How does a quantisation table relate to detecting double JPEG compression?+
If an image was saved once, edited, and saved again, the second quantisation step interacts with rounding from the first, leaving periodic artefacts in the coefficient histograms that differ from a single-compression image, and the table itself may also show mismatched patterns.
Can two different cameras produce the same quantisation table?+
Yes, manufacturers often reuse standard or near-standard tables across models and firmware versions, so a matching table supports but does not by itself prove a specific device was the source.
Related terms
- Double JPEG Compression
- The situation where an image has been JPEG-encoded, decoded to a bitmap, and then JPEG-encoded again. The second quantisation step leaves a...
- JPEG Ghost
- A visualisation technique introduced by Farid (2009) that reveals regions compressed at a quality factor different from the rest of the image....
- Block Grid Phase
- The horizontal and vertical offset of the 8x8 encoding grid relative to the image coordinate origin. A phase of (0,0) means the...
- Blocking Artefacts
- Visible rectangular discontinuities at 8x8 block boundaries, caused by independent quantisation of adjacent blocks. At splice boundaries, misaligned block grids produce a...
- Blocking Artifact
- A visual discontinuity at the boundary between two adjacent 8x8 blocks in a JPEG image, caused by independent quantisation of each block....
- DCT (Discrete Cosine Transform)
- The mathematical transformation applied to each 8x8 pixel block during JPEG compression, converting spatial pixel values into a set of frequency coefficients...
- 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...
- Quality Factor
- The 1-100 scale that most JPEG encoders expose to the user, controlling how aggressively the quantisation table rounds coefficients. The factor is...
Explained in these topics
- DCT Block Boundaries and Quantisation Grid Analysis
- JPEG Compression Artefacts and Double-Compression DetectionA matrix of divisors applied to DCT coefficients during JPEG compression. Higher divisors produce lower quality. The specific table used is often a manufacture...