[XviD-devel] [RFC] kthresholding in 2pass2

Dirk Knop dknop at gwdg.de
Mon Dec 1 18:06:09 CET 2003

Hi all,

as far as I can see it, the images are really changing that much (i.e. 
luminance changes).
We introduced that feature mainly for low-bitrate encodings, therefore 
the default bitrate reduction of 20% for ivops then - those keyframes 
were producing much overflow which then prdouced "overquantized" scenes. 
I agree that this isn't a usual case and therefore the defaults were a 
little high - we more or less forgot to reset them to 0%.

I think it's still a useful feature as if those sequences aren't 
avoidable (and xvid got really good in that matter now) it should still 
be possible to save some bits there (i.e. 5 iframes with quant 4 in a 
row stack up 50kb of overflow where the first 4 iframes aren't really 
visible when watching the result - so they could be quant 5 as well and 
maybe we even get the bonus of 20 kb more to spend on the scene).

I don't know if yyou wanted such an explanation, but i felt i describe 
this for Edouard as well.

I'll write a summary of bugreports on doom9's forum later btw. so that 
we can see if  some were already fixed, if some are important and others 
can be dropped completely ( i expect beta 2 soon ;) ).

Best reards

Christoph Lampert schrieb:

>wow, this sounds all very scientific ;-)
>Unfortunately, I don't know anything about sequences of several I-frames
>in a row. So, as usual, I start asking stupid questions: 
>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? 
>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
>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. 
>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

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