Macroblock
Definition
The fundamental coding unit in most video codecs: a 16x16 pixel block in luma (brightness) and the corresponding chroma samples. Macroblocks are classified as intra (coded independently) or inter (coded relative to a reference frame using motion compensation). Forensic analysis of macroblock type distributions can reveal double encoding.
- Standard size
- 16x16 pixels in luma
- Types
- Intra-coded, inter-coded
- Inter-coding basis
- Motion compensation from a reference frame
- Forensic signal
- Macroblock type distribution across a video
Common questions
How does macroblock analysis help detect video tampering?+
When a video is re-encoded after editing, the type pattern of macroblocks from the first encoding can leave statistical traces that differ from a single, unedited encoding, which double-compression detectors look for.
What is the difference between an intra-coded and an inter-coded macroblock?+
An intra-coded macroblock is compressed using only information within its own frame, while an inter-coded macroblock is predicted from a reference frame using motion vectors, which is more efficient but ties its data to neighbouring frames.
Related terms
- 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 Quantisation Effect
- The statistical artefact that appears in DCT coefficient histograms when a video or image is quantised twice with different step sizes. The...
- Intra-Frame vs Inter-Frame Coding
- Intra-frame (I-frame) coding compresses each frame independently using DCT quantisation, like a JPEG image. Inter-frame (P-frame or B-frame) coding stores only the...
- Quantisation
- The step in lossy compression where DCT coefficients are divided by a quantisation parameter and rounded to integers. Higher quantisation parameters produce...
- 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...