[mythtv-users] a little OT: deinterlacing question

[ryan patterson]

If the original source was recorded with actual film (most prerecorded TV in
USA) then the two interlaced fields are in fact split from the same source
frame and can be recombined later to form a 30p fps picture.  If the
original source was recorded with a video camera (live TV: sports/news &
much European prerecorded TV) then the two interlaced fields are unique.
And combining them together will result in visual artifacts.

If the original source was recorded with movie film (24fps) it gets more
complicated.  Movie film to PAL/DVB 50fps is done by  speeding up the
framerate 24fps => 25fps.  If it is saved interlaced then each two fields
comprise one full frame.  Movie file to NTSC/ATSC 60fps is done with a 3/2
telecine process that makes six interlaced fields (three frames) out of two
progressive frames.  But with a inversetelecine process you can get the
correct 24fps progressive frames back.

On 2/15/07, jason maxwell <decepticon at gmail.com> wrote:
>
> If one single frame (30 FPS) is interlaced into 2 separate fields for
> broadcast at 60 FPS, then each pair of fields would be from the same
> point in time, and therefore should be able to be reconstructed w/o
> artifacts.
>
> However, If I understand Stephen's post correctly, the real issue is
> that interlaced material is not always generated in such a simple
> manner, and that many times each field does represent a new moment in
> time. This is sometimes further complicated by the need for pulldown
> methods to compensate for different framerate standards. Correct? This
> is stupid. Oh well, what am I gonna do about it?
>
> Thanks for the info.
> -J
>
> On 2/14/07, Mark Kendall <mark.kendall at gmail.com> wrote:
> > On 2/14/07, jason maxwell <decepticon at gmail.com> wrote:
> > > I am just curious. Why is deinterlacing so difficult? From what I've
> > > read on the topic, an interlaced image is created by breaking a single
> > > frame into 2 fields of alternating lines. It seems to me that
> > > reassembling the full frames should be as simple as combining the odd
> > > and even fields back together. Yet, from what I have read, this will
> > > always result in some artifacts, most commonly, tearing (mouse teeth).
> > > Why is this?
> >
> > You're forgetting that each interlaced field is from a different point
> > in time - in PAL terms 1/50th of a second apart (1/60th NTSC). Hence
> > you can't just put the two fields back together to make the original
> > frame - you no longer have half the original frame.
> >
> > The reason it's so difficult is because you have to come up with a
> > best guess for that missing half - easy in a static scene, almost
> > impossible in a fast moving one.
> >
> > Regards
> >
> > Mark
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-- 
_____________
Ryan Patterson
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