Double quantisation effect
Definition
The statistical artefact that appears in DCT coefficient histograms when a video or image is quantised twice with different step sizes. The histogram of the doubly compressed data shows periodic dips at multiples of the first quantisation step, because values near those multiples were rounded away in the first pass and are unlikely to reappear.
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...
- 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...
- Macroblock
- The fundamental coding unit in most video codecs: a 16x16 pixel block in luma (brightness) and the corresponding chroma samples. Macroblocks are...
- 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 statistical artefact that appears in DCT coefficient histograms when a video or image is quantised twice with different step sizes. The histogram of the do...