Blocking Artifact
Definition
A visual discontinuity at the boundary between two adjacent 8x8 blocks in a JPEG image, caused by independent quantisation of each block. Blocking artifacts manifest as abrupt changes in tone or colour at 8-pixel intervals. Their strength increases with higher compression (lower quality factor) and their spatial phase is fixed by the grid origin used during encoding.
- Cause
- Independent quantisation of neighbouring 8x8 JPEG blocks
- Spatial period
- Recurs at 8-pixel intervals
- Strength driver
- Increases with higher compression (lower quality factor)
- Phase origin
- Fixed by the grid alignment used at encoding
Common questions
How does quality factor affect how visible blocking artifacts are?+
A lower quality factor means coarser quantisation steps, so the DCT coefficients in each block are rounded more aggressively. That produces bigger tone jumps at block edges, making the artifact more visually and statistically detectable than at a high quality setting where quantisation is fine.
Why does the grid origin matter when comparing two regions of an image?+
The 8x8 grid starts wherever the encoder placed it, typically the image's top-left corner in an untouched file. If two regions were encoded with grids starting at different origins, for instance because one was cropped or pasted from elsewhere, their blocking phases will not line up, and that misalignment is a tamper indicator.
Related terms
- 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...
- 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...
- 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...