Macroblock (MB) / Coding Tree Unit (CTU)
Definition
The basic spatial coding unit: 16x16 pixels in H.264 (macroblock) and up to 64x64 pixels in H.265 (CTU). Each unit is classified as intra-coded (from the current frame only) or inter-coded (predicted from a reference frame). The distribution of coding modes across the video is a forensic feature.
- H.264 unit
- Macroblock, 16x16 pixels
- H.265 unit
- Coding Tree Unit, up to 64x64 pixels
- Coding modes
- Intra-coded, inter-coded
- Forensic use
- Coding-mode distribution across the video
Common questions
Why does H.265 use a larger and more flexible coding unit than H.264?+
The Coding Tree Unit can subdivide down to smaller blocks as needed, letting the encoder match block size to image detail more efficiently than H.264's fixed 16x16 macroblock, improving compression at the same quality.
How does the coding-unit structure help estimate a video's compression history?+
Re-encoding imposes a new coding-unit grid on top of artifacts from an earlier encoding, and the mismatch or forced boundary pattern can reveal the file was compressed more than once, even when metadata claims otherwise.
Related terms
- DCT Coefficient Histogram
- A histogram of the discrete cosine transform coefficients across macroblocks or coding units in a frame or clip. After a single encode,...
- Double Compression
- The condition in which a video has been encoded at least twice through a lossy codec. The term is also used loosely...
- Group of Pictures (GOP)
- The periodic structure of I-frames, P-frames, and B-frames in a compressed video stream. Each encoding generation imposes its own GOP structure. When...
- Intra-Refresh Artefact
- The elevated count of intra-coded macroblocks or CTUs at frame positions that correspond to the I-frame interval of a previous encoding generation....
- Quantisation Parameter (QP)
- A per-block or per-slice value that controls the coarseness of the DCT coefficient rounding step in H.264 and H.265. Higher QP means...