Quality Factor
Definition
The 1-100 scale that most JPEG encoders expose to the user, controlling how aggressively the quantisation table rounds coefficients. The factor is usually stored in the file header alongside the table itself.
- Range
- 1 to 100
- Controls
- Aggressiveness of quantisation table rounding
- Storage location
- Usually in the JPEG file header
- Relationship
- Lower factor, more compression, more loss
Common questions
Why does the quality factor matter in detecting image tampering?+
A manipulated region spliced from a different JPEG often carries a different original quality factor than the surrounding image, and analysing the quantisation table can reveal this mismatch even after both regions are re-saved together at a single final quality setting.
Can the quality factor alone tell an examiner whether an image has been re-saved multiple times?+
Not by itself. The stored quality factor only reflects the most recent save; detecting prior re-saves requires examining quantisation artefacts and coefficient statistics for signs of double compression, since intermediate quality factors are not retained in the file.
Related terms
- Blocking Artefacts
- Visible rectangular discontinuities at 8x8 block boundaries, caused by independent quantisation of adjacent blocks. At splice boundaries, misaligned block grids produce a...
- 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...
- 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....
- Quantisation Table
- A matrix of 64 integer divisors, one per DCT coefficient position, that controls how much each frequency component is rounded during JPEG...