Block Grid Phase
Definition
The horizontal and vertical offset of the 8x8 encoding grid relative to the image coordinate origin. A phase of (0,0) means the grid starts at the top-left pixel. A phase of (3,5) means the grid is shifted 3 pixels right and 5 pixels down. When a pasted region comes from a differently-aligned source, its phase differs from the host image, and that difference is detectable.
- Domain
- JPEG/DCT image forensics
- Defines
- Offset of the 8x8 DCT grid from the image origin
- Range
- 0 to 7 pixels in each axis
- Forensic signal
- Phase mismatch between a pasted region and host reveals splicing
Common questions
Why would a pasted region have a different block grid phase than the host image?+
When a region is copied from a source image and pasted at an arbitrary pixel position in a target image, its original 8x8 encoding grid rarely lines up with the target's grid. Even a one-pixel horizontal shift changes the phase, because the DCT blocks are computed relative to the image's own top-left corner.
How reliable is block grid phase analysis on its own?+
It is strong when the pasted region retains visible residual blocking from its source compression, but a splice that is heavily post-processed, resized, or re-saved at a matching alignment can obscure or eliminate the phase mismatch, so examiners typically corroborate it with other double-compression or noise-based cues.
Related terms
- Blocking Artifact
- A visual discontinuity at the boundary between two adjacent 8x8 blocks in a JPEG image, caused by independent quantisation of each block....
- 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...