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.
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...
Explained in
- Double Compression Analysis in VideoThe fundamental coding unit in most video codecs: a 16x16 pixel block in luma (brightness) and the corresponding chroma samples. Macroblocks are classified as...