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.
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...
Explained in
- DCT Block Boundaries and Quantisation Grid AnalysisThe 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 pix...