[MEncoder-users] interlaced/non-interlaced

[James Hastings-Trew]

  On 10-10-08 11:24 PM, Toerless Eckert wrote:
> No, i totally do not understand that. Assume i have 25p material and
> want to broadcat it as 50i. What i do then is that i just split
> up every frame into even and odd lines and broadcast first the odd
> lines as one field and then the even lines as the second field. Of course
> i shuold be able to recombine these two fields to a full frame without any
> loss.
Correct. Except that there are no flags at all in the stream that will 
let you differentiate between 25p and 50i programmatically. You would 
have to do this part yourself by visual inspection of the material. If 
you want to always ensure that the output is going to be 25p, then you 
will always deinterlace, using something like yadiff. Yadiff will not 
deinterlace parts of the frame that do not need it - it is only applied 
where material differs line by line by some set threshold value.

>> You will have to deinterlace with a
>> filter like yadif to get 25p again, and it won't be 100% lossless
>> (though it will be extremely close since it was never 50Hz to begin with).
> Well, let's just assume original 25p ->  50i ->  25p and 30p ->  60i ->  30p.
> Those cases should be lossless. Even 24p ->  60i ->  24p should be lossless.
> Not sure which case you are thinking of. 24p ->  50i should be possible
> to convert lossless for vide back to 24p, just the audio needs to
> be speed up/down by 4%.
Generally, 24fps material is simply sped up a little to give you 25p 
material for PAL. In those cases, yes, it is possible to losslessly 
reconvert 60i or 50i streams back down to the original 24fps. In the 
case of NTSC material encoded with MPEG2, there are flags in the stream 
that indicate frame cadence, and these can be used to reconstruct the 
original progressive frames.

However, this discussion only really applies to productions shot on film 
or HD video for display in movie theaters. Stuff shot for television is 
going to always be a mixed bag of progressive material shot on film or 
HD, older material shot in 60i for 50i, programming where progressive 
and interlaced material has been freely edited together with no regard 
for "recoverability" of the frames, etc. All bets are off when talking 
about cable HD programming - especially sport broadcasts. Your best bet 
is simply treat it all as 60i material, deinterlace with YADIF where 
necessary. Then you are not recovering frames - you are simply making 
the material more suitable to display on a computer and more 
compressible in the stream.