[XviD-devel] [RFC] kthresholding in 2pass2
ed.gomez at free.fr
Mon Dec 1 19:15:27 CET 2003
Christoph Lampert (chl at math.uni-bonn.de) wrote:
> When do such IIIII or IPIPPIPP sequences occur? Is it a "missing feature",
> because XviD detects scenechanges where there isn't one? Or is the image
> content really changing that much and often?
It's always happening in flashing situations. Trailers are good at that
as Hollywood uses to do <1s shots of different movie parts to stress the
audience and give a higher rhythm at their Trailer.
However the first example i gave is completly theoretical, the second
one can happen in real cases.
> The only occurance I found was a TV series where from time to time
> there were short shots with every alternate frame _inverted_ to
> express some telekinetic action that wouldn't have been noticed by the
> viewer (100 points to who can tell me the series and who the
> character/actress with telekinetic powers was, together with what
> famous NBC series she stars in now). So there, the changes were real,
> and spending fewer bits on all but last I-frame would hardly have been
> noticed because of all the flashing.
exactly that ype of sequences, flashes, color inversions, strong cops
> The bigger problem I see if _not_ the sequence is really changing, but
> only XviD creates too many I-VOPs. Because then, the viewer might follow
> some action and concentrate on elements which for real scene changes would
> be masked.
> Could within min_keyframe_interval maybe the percentage of MBs be raised
> which are needed to classify a frame as I-VOP.
For this time, I don't plan any BIG change in twopass, it's just a small
adjustment i would like to do. I think we should simply how to apply bit
allocation reduction with some kind of smart decision (not a simple bits
-= distance*step_reduction, which IMO is non sense if we don't look at
the ivop burst duration versus frame rate, and importance of each
iframe insde the burst)
Moreover, all this is very hypothetical and doesn't happen so often.
> As far as I know, even at scenechanges, I-frames usually don't save
> bits, anyway, at least not many, but are mainly as keyframes for index
<out of topic>
I appreciate scene changes ivops because they are very good at one
thing: avoid scene mixage for the few frames surrounding the scene
change. An ivop creates a clear cut.
It's like some nasty scene mixage when using bframes w/o the closed
gov option. When we have this sequence:
In Display order
... whatever frames ... PPBBI...... whatever type of frames
scene 1 | scene 2
In this case, the last bframes are a ugly mix of old scene and new
scene. I had some reports about that on transcode ML or transcode IRC
channel, i don't remember exactly. That's why in the frontends i've
written for devapi4^W1.0.0 i use closed_gop as default to insert a
Pframe right before the scene changes so no scene mixage happens.
</out of topic>
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