DCT Coefficient Histogram
Definition
A histogram of the discrete cosine transform coefficients across macroblocks or coding units in a frame or clip. After a single encode, the histogram has a characteristic shape with peaks at quantisation multiples. After re-encoding, additional peaks or gaps appear that reflect the quantisation grid of the first generation.
- Domain
- Video, aggregated over macroblocks or coding units
- Single encode
- Peaks concentrate at multiples of the quantisation step
- Double compression
- Extra peaks or gaps reflect the first-generation quantisation grid
- Use
- Estimating whether and how many times a clip was re-encoded
- Limitation
- Heavy motion or scene change can mask the periodic signature
Common questions
What does a double-peaked DCT coefficient histogram tell an examiner?+
It indicates the footage passed through two rounds of quantisation with different step sizes, a signature of transcoding or re-encoding. The spacing between the peaks can help estimate the ratio of the two quantisation steps used across generations.
Can this method alone prove a video was edited?+
No, it only indicates a compression history inconsistent with a single, unedited encode. It needs to be combined with other markers such as motion vector anomalies or container metadata to support a conclusion about editing rather than routine re-encoding for distribution.
Related terms
- 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....
- Macroblock (MB) / Coding Tree Unit (CTU)
- The basic spatial coding unit: 16x16 pixels in H.264 (macroblock) and up to 64x64 pixels in H.265 (CTU). Each unit is classified...
- 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...