Can P frames reference B-frames ?

[rosh]

Hi,

When using hierarchical B-frames (B-pyramids) or generalised B-frames in HEVC encoders, can P-frames refer to B-frames ?

I know b-frames can refer to I/P/B-frames when the hierarchical B-frames is enabled, but what about P-frames ?

For example in a GoP of 35 frames, can a P-frame at decoding index 15 refer to a generalised B-frame at decoding index 12 ?

when using x265, or the reference encoder, I haven't seen this happening, but is this possible ?

Also if this situation can occur, then is it valid according to the HEVC specification ?

Are there any inefficiency issues of using a B-frame as reference to a P-frame ?

Many thanks!

[Sulik]

It's legal (reference frames and slice types are independent in AVC & HEVC), but having a P reference a B is typically not good for coding efficiency.