Frame droping when behind (catch-up)

[tempost]

VirualDub mailed me !!! :D :D :D

This is the full email:
SOF
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On Tuesday, May 6, 2003, 2:25:53 PM, you wrote:

> I used your code notes to explaine frame droping in the fourm of
> BSPlayer Suggestins to explaine the need and usefullnes of
incorporating this feature to the player (like the Divx Bundle
> Player).
> I havn't used any of your code, just your notes and the idea;

> I can't see any problems with it but i thought i should ask if
this
> is O.K with you ? ;)

Of course, although I should add a few points.

VirtualDub's tri-modal concept of independent, dependant, and
droppable frames is only useful for MPEG, which has I/P/B frames.
B-frames do not exist in AVI and as such the drop routine is much less
useful, as decoding can only be skipped for frames right before a key
frame. For a format with a commonly short interval, such as 5-15
frames, this can still provide a smooth drop rate up to around 20%,
which is likely enough. Obviously, it works just fine for a codec
that has no delta frames (Huffyuv).

Given a longer keyframe interval of 200 frames common in
high-compression codecs, however, and a pipelining window of 32
frames, the decoding engine must decode 168 frames before it can
perform an "known-delay drop." (DivX Pro can encode
B-frames if told
to, but these are hidden within delay frames in the AVI and as such
players can't tell they exist or take advantage of them unless some
serious hacking is done to the data before passing it to the decoder.)
Since MPEG-4 is a relatively low bandwidth but computationally
intensive codec, the bottleneck is almost always the CPU instead of
I/O or video output and thus the decoder not being able to drop frames
over a long interval is a big problem.

There are a couple of stages in MPEG decoding that can be ommitted if
a frame needs not be displayed, namely color conversion and
post-processing (adaptive smoothing). However, color conversion is
not necessary if hardware color conversion hardware is available --
which it is on all modern video cards -- and with post-processing,
you're better off just turning it off for all frames so fewer frames
need be dropped in the first place.

--

http://www.virtualdub.org
The latest version of VirtualDub is V1.5.1 (build 15654).

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EOF

Notes:

1.The email was from <phaeron at virtualdub dot org>. (The link has been scrambled to foil email harvesters.)

2.As you can see in the email their are a few notes/corrections about the
independent frames i mentioned i really don't fully understand.

3.Actully i forgot to point out that BSPlayer is
desinged primary for Divx so Divx being MPEG-4 was
a too avious detail for me to include.
so aviously all the conversation here is in context to MPEG/Divx.