What's the difference between BI frame and I frame？

[Ahlucard]

in VC-1 standard
BI frame seems the same as I frame in structure.
So why they propose this kind of frame structure??
It seems useless.

[Dark Shikari]

A BI frame is an I-frame that isn't a keyframe; it is like a B-frame, except intra-only. It's useful for camera flashes and such where you might want an intra-only frame, but for which you don't want to lose the ability to reference previous frames.

This is needed because VC-1 doesn't have multiple reference frames, which allow an easy bypassing of the problem.

[Ahlucard]

thanks!
Do u mean that : in VC-1 , All the I-frame must be the reference frame for next frame?
(if current frame is an I-frame)Can't the next frame use the previous I or P-frame to be the reference frame?

[Dark Shikari]

thanks!
Do u mean that : in VC-1 , All the I-frame must be the reference frame for next frame?
(if current frame is an I-frame)Can't the next frame use the previous I or P-frame to be the reference frame?Every non-B frame references the previous non-B frame.

[Ahlucard]

Thank you very very much, i get it

[benwaggoner]

From a frame strucuture perspective, is essentially the same as MPEG-2, MPEG-4 part 2, etcetera. The BI frame is a result of the ability to use intra coding in B-frames, but is really the same as any other B-frame from a decoder perspective.

Of course, for frame-dropping heuristics, it's a lot better to drop B than BI frames, or else your flash frames could go missing entirely, thus leaving rave and gunfight scenes looking odd.