Skip PU decoding

[rosh]

Hi

I am a bit confused about PU type "Skip" from a decoder perspective.

for these kinds of PUs, are there no motion vectors ? no residuals ? What does the decoder do when it encounters a "skip" PU ? How does it reconstruct the PU in the frame ?

many thanks

[Sulik]

"no motion vector" and "no residual" only means "motion vector residual=0" and "residual coefficients are all zero", meaning the motion vector is exactly the predicted motion vector (derived from available neighbors), and the final reconstructed PU is the prediction itself (no residual added).

[rosh]

"no motion vector" and "no residual" only means "motion vector residual=0" and "residual coefficients are all zero", meaning the motion vector is exactly the predicted motion vector (derived from available neighbors), and the final reconstructed PU is the prediction itself (no residual added).

Okay good. So what about reference frames - do SKIP type PUs have dependencies with other CU/PUs in other (reference) frames ?

thanks

[Sulik]

do SKIP type PUs have dependencies with other CU/PUs in other (reference) frames ?

Yes. SKIP is no different than Inter CU

[rwill]

Yes. SKIP is no different than Inter CU

To be more precise.. SKIP is 2Nx2N mode with Merge MV set and zero CBF. 2Nx2N with Merge MV and zero CBF cannot be signaled otherwise.

My guess is that the mode is some sort of historical leftover from the other skip mode that was possible ( by MVP ) - but that one was dropped as the standard progressed.

*edit*
And to nitpick.. a Merge MV only has a dependency on CU/PUs in reference pictures when the temporal MVP is possible as indicated by the temporal_mvp_enabled_flag in the slice header AND the ( a ) co-located MV ( temporal MVP ) is actually selected as the merge MV. You only need the POC and the reconstructed pixels of a reference picture otherwise.

[rosh]

Thank you, I think I understand now.

A SKIP indicates the decoder not to apply MV and add residual. Just take copy a CU from another (ref) frame directly.

[foxyshadis]

No, a SKIP can still have MV, but it's a predicted MV, called Merge MV. Regular MVs can be absolute or offsets from the predicted MV, but a SKIP is just predicted.

It's only zero MV and can directly use the ref CU if the prediction is 0,0.