DCT (Discrete Cosine Transform)
Definition
The mathematical transformation applied to each 8x8 pixel block during JPEG compression, converting spatial pixel values into a set of frequency coefficients from low (smooth) to high (detail).
- Block size
- 8x8 pixels, the standard JPEG processing unit
- Output
- 64 frequency coefficients per block, low to high frequency
- First coefficient
- DC term represents the block's average intensity
- Stage in JPEG
- Applied before quantisation and entropy coding
- Forensic relevance
- Basis for double-compression and tampering detection methods
Common questions
Why does JPEG compression rely on the DCT specifically?+
The DCT concentrates most of a natural image block's energy into a small number of low-frequency coefficients, which lets the encoder discard or coarsely quantise the remaining high-frequency terms with minimal visible loss, giving strong compression for a given quality level.
How do forensic examiners exploit the DCT stage to detect tampering?+
Because quantisation of DCT coefficients leaves a statistical signature tied to the JPEG quality setting used, examiners compare coefficient distributions across regions of an image or across compression generations to spot inconsistencies that indicate localised editing or resaving.
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...
- 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....
- 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...
- Quantisation Table
- A matrix of 64 integer divisors, one per DCT coefficient position, that controls how much each frequency component is rounded during JPEG...