[MEncoder-users] interlaced/non-interlaced

[Toerless Eckert]

On Wed, Oct 06, 2010 at 01:48:33AM -0500, Andrew Berg wrote:
> It's inefficient because takes power and CPU cycles to deinterlace. With
> hardware players, this isn't a huge issue, but it's unnecessary overhead
> for software players.

To me that is the cost of maximizing quality.
Show me any TV station that's sending out it's interlaced
material deinterlaced. Or DVDs for that matter.

> If you're not seeing any combing, then it's very likely the video is
> progressive.

No. I am quite certain it is interlaced. Heck, mplayer already tells
me the stream is 25fps. When i play it back with vlc, it shows me the
stram has 50 frames/sigh) (*sigh*). Now where is the player that correctly
tells me that is 50 FIELDS/sec interlaced and where i can single frame through
the picture ;-))

> Broadcast MPEG-TS flags can never be trusted to indicate
> whether a video stream is interlaced or progressive.

Sure. I still remember the many years when de-interlacing in DVD players
was evolving and the more often than not solely relied on that flag.
NOw this isn't an issue talked about much at all anymore because the
players deinterlacers take care of it.

So, if i single step through a movie with mplayer, would i have to press
'.' 50 times for a second, or 25 times ? It's hard to say. I see a
movement in the picture every time. It's just hard to judge whether
there are interlacing artefacts...

> I use this method
> to help determine whether video is true 30i or telecined 24p if I'm
> unsure.

Right. Haven't tried this with NTSC material for a long time but remember
from the past (not sure which program i used) to check whether i would see
the same picture for the usual 3:2 sequences.

> I tried this with pure 30i content and it's not as obvious as I
> thought to see interlacing. You'll have to look very closely for
> combing, especially if your input is the same resolution as your display.

Right. The way you described it initially i was hoping for a nice
display of alternating black and picture lines for interlaced input material ;-))

> > 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 ?
> No. If you have frames, you leave the video alone.

Well, i thought that the encoder (ffmpeg/mencoder) would ultimately
need to have real 25 frames/sec when i want it to do progressive encoding,
and if the input is 50i fields (originating from an actual 25p material),
then the final 25 frames/sec are created by combining two fields back into
a frame - without any interpolations.

> You can't, but you are free to not deinterlace. If it is such an issue,
> either don't re-encode or live with the fact that interlaced video is
> less compressible than progressive video and have either larger files or
> reduced quality. Try encoding with a constant quality (higher quality is
> better since you're testing) and compare bitrates and quality among
> 25p/50i/50p. If you still think the higher bitrate is less of a problem
> than the temporal resolution (I think that's the right term) loss, then
> by all means, encode interlaced.

Hmm. remind me, what would be the right parameter to get encoding purely
based on quality, not target bitrate ? I guess that would be possible then
to do with single pass ?

> > 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.
> If the stream truly switches like that, then you have a real problem for
> which I have no solution.

But that's just because the encoders do not support this, right ? 

> > 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.
> That depends on what you consider to be sufficient quality and on what
> kind of content you're dealing with (noise is far from the only factor).
> I've never dealt much with MPEG-4 part 2 and I don't ever plan to, so I
> can't help you here.

How about part 10 ? I have been doing 900kbps for 1280x720 at 30frames.

Cheers
    Toerless