Re[4]: [XviD-devel] scene change detecting in Bframes mode

[peter ross]

> > syskin, i think you've got I-frames and P-frames mixed up.
>
>gruel is right here

my appologies.

> > For each frame we assume its going to be a P-frame ad perform motion
> > estimation. After the m.e. we then decide on the frame type
> > - I-frame: code the i-frame
>
>No I don't like it. I frame should be on a scene change, not a few
>frames after. If you do it a few frames after, then B frames before
>this I frame will use only backward prediction, as they belong to the
>same scene as the I frame. That's gonna be very inefficient.

the IPB-encoder is rather complex, but i assure you it handles all
the issues of reording, backward prediction and such correctly.
when i designed it, i drew up a rough flow-chart sketch of the
encoder.  i'll scan it in and post for you someday.

xvid currently handles I-frames followed B-frames correctly. eg.
        (display order) PBBPBBI

and if you use DX50IVOP, xvid will ensure the frame prior to an I-frame
is encoded as a P-frame. eg.
        (display order) PBBPBPI

xvid also handles the last frame correctly, to prevent this error
        (display order) PBBPBB<end of stream>
note: this is not functional with vfw/vdub at the moment.

>Now how? To code a b-frame you need a future P frame. To have a future
>P frame, you need previous P frame. Once you have both P frames, this
>frame can ONLY be B, because if you would decide to make it P/B then
>you would have to trash the future P frame (not valid anymore).

internally xvid has two queues, one for frames which have not been
processed yet, and another for bframes. when you give xvid a frame to be 
encoded, the encoder essentially has a three-way if statement:

1. if this is the very first frame, OR a detected keyframe,
   then flag this frame as an I-frame and add it to the queue.
   now, if there are any bframes waiting, code them accordingly.
   otherwise, pop a frame from the queue and code it.
   return;

2. otherwise, if we have reached our max_bframes count, then flag
   this frame as a P-frame and add it to the queue.
   (from here onwwards, we perform the same operation as choice 1).
   if there are any bframes waiting, code them accordingly.
   otherwise, pop a frame from the queue and code it.
   return;

3. otherwise, flag this frame as a bframe and add it to the bframe
   queue.
   return;

in summary, XVID encodes frames in bitstream, not display order.

goodnight.
-- pete

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