Blocking Artefacts
Definition
Visible rectangular discontinuities at 8x8 block boundaries, caused by independent quantisation of adjacent blocks. At splice boundaries, misaligned block grids produce a detectable phase shift.
- Cause
- Independent quantisation of adjacent 8x8 DCT blocks
- Appearance
- Visible rectangular discontinuities at block boundaries
- Forensic use
- Phase shift at splice boundary reveals tampering
- Severity driver
- Increases with higher compression / lower quality factor
Common questions
How do blocking artefacts help detect an image splice specifically?+
A genuine, unedited JPEG has one consistent block grid across the whole frame. When a region from a different source is pasted in, its own blocking pattern rarely aligns with the host's grid, producing a visible phase discontinuity right at the splice boundary that would not appear in an untouched image.
Can blocking artefacts be removed to hide evidence of editing?+
Smoothing or deblocking filters can reduce their visibility, and resaving the final composite at a low compression level can regenerate a new, uniform blocking pattern that masks the original mismatch. This is why examiners pair blocking-artefact analysis with other cues like noise inconsistency or double-compression traces.
Related terms
- 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....
- 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...