[MEncoder-users] interlaced/non-interlaced

[Toerless Eckert]

On Sat, Oct 02, 2010 at 12:53:40AM -0500, Andrew Berg wrote:
> There are two big reasons. First, progressive video is more compressible
> than interlaced video with any codec. Second, the video would only need
> to be deinterlaced once instead of every time you want to watch it.

There are billions of TV sets out there dong deinterlacing every
time they receive something so the second part of the argument isn't
all that persuasive ;-)

> I am not sure how 25p -> 50i is handled. It's possible that you are
> receiving 25p video flagged as interlaced if the original is recorded at
> 25p. You can play video back frame-by-frame with MPlayer by pausing it
> and pressing the period key to advance a frame. If you see full frames,
> it's progressive. If you see combing, it's interlaced.

Unfortunately i always see full frames. Have you tried it ? mplayer
seems to alwas be showing a full frame somehow interlaced from two field
*sigh*.

> If you use a deinterlacer on progressive content, you will likely have
> very noticeable distortions. If the 25p video was interlaced before
> broadcast, you've already lost (a very small amount of) quality.

"if the 25p video was interlaced" ? What do you mean with that ?

> > For such content the best encoding would simply be to merge both fields
> > back and encode as progressive 720p25. I think this is what would happen
> > in ffmpeg if i would not specify progressive encoding, but not specify any
> > deinterlacer. Right ?
> AFAIK, it's not possible to losslessly restore progressive frames once
> the video is interlaced while maintaining a constant frame rate and
> resolution throughout. I'm not sure what you mean by merging fields.

Why not ? If i receive an MPEG2 stream that indicates it's 50 fields
per second interlaced, but every 2 fields actually belong to a single
frame, then my deinterlacing operation is simple to combine the two fields
into a field without any interpolation.

Right ?

> You will get better quality if you deinterlace to 25p (Yadif can output
> one frame for every pair of fields) than if you stay with 50i. Yes,
> technically, quality is lost when you deinterlace, but it's not much of
> a loss.

Somehow this seems to be a mantra everybody repeats without knowing
real evidence.  How can you maintain 50 field/sec smoothness of motion
after deinterlacing into 25p ? 

> If you are so concerned about the quality loss from
> deinterlacing, don't re-encode it. The bits you'd save (if any) would be
> too few to justify the time spent to encode whether you encode 50i or
> 50p.  Seriously, take material that you know is recorded at 50i and
> compare the quality when encoded at 25p, 50p and 50i.

Why do i have to do this. Why does nobody here have pointers to
example encoded streams at the same bitrate ;-))

> Yadif is an excellent deinterlacer. I don't know about ffmpeg, but
> MEncoder has mcdeint, which will reduce the very small amount of quality
> loss that Yadif introduces. Of course, mcdeint is very slow and only
> worth the time if you are obsessed with reducing temporal artifacts
> (which you seem to be).

I am not obsessed. I don't want to deinterlace except when i know it
is 25p content in the first place. But everybody else here seems to
want me to deinterlace in all cases ;-)))

> > And because the TV recording will switch mid-stream between either
> > case, this decision would need to be made continuous resulting in a
> > media stream switching between 720i50 and 720p25 - and the encoder should
> > according encode interlaced and progressive.
> It shouldn't switch during a program. It could switch when ads come on,
> but you would have those cut out anyway.

Well, TV recordings usually willhave some pre and post roll which can
have separate encodings from the main program recorded. In addition,
if the recording is a made-4-tv news report or the like then it will
be cut together from all type of input clips, some of them 4:3 AR,
some 16:9, some interlaced, some 25p.

> Not if you have 
> > Short of that it still sounds most prudent to continue
> > encoding interlaced everything as 720i50.
> As I said, compare encodes. You'll see that the quality loss that comes
> from the encoder not being able to handle interlaced material
> efficiently is going to be much more than the loss from deinterlacing.

So far i' ve not been too unhappy with mencoder and interlaced, except
for what i think are bugs, and maybe those bugs are more common in
less often used combination of options, and if everybody here subscribes
to the church of deinterlacing, then i can see why i have more bugs in my
encodings ;-))

What rates do you think is sufficient for deinterlaced PAL (720 at 576x25p) ?
I am using 1500kbps for interlaced. I alredy know it's not ideal
for noisy (older) content, but its pretty good for newer (less noisy)
content.

Cheers
    Toerless