Intra-Frame vs Inter-Frame Coding
Definition
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 difference from a reference frame. Double compression signatures are strongest in I-frames and weaken in inter-frames because motion compensation partially redistributes the DCT residual.
- I-frame coding
- Independent DCT compression, like a JPEG
- P/B-frame coding
- Stores only the difference from a reference frame
- Double-compression signal
- Strongest in I-frames
- Why weaker inter-frame
- Motion compensation redistributes the DCT residual
Common questions
Why do examiners focus on I-frames for compression history?+
I-frames carry clearer DCT quantisation artefacts because they are not diluted by motion-compensated prediction, which makes requantisation traces from an earlier encoding pass easier to detect.
Can inter-frame artefacts still be useful evidence?+
Yes. Patterns in motion-vector statistics and residual energy across P- and B-frames can corroborate an I-frame finding even when each is individually weaker.
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...
- 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...