[gst-devel] [transcode-users] Two new filters for NTSC inverse telecine (fwd)
Thomas Vander Stichele
thomas at urgent.rug.ac.be
Fri Aug 9 13:20:03 CEST 2002
I'm forwarding this from the transcode list since I'm sure some of us
might find this useful ;)
Come on vektor, port a plugin !
Thomas
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---------- Forwarded message ----------
Date: Fri, 09 Aug 2002 20:58:40 +0300
From: Driver Developer <driver999dev at hotmail.com>
Reply-To: transcode-users at theorie.physik.uni-goettingen.de
To: transcode-users at theorie.physik.uni-goettingen.de
Subject: [transcode-users] Two new filters for NTSC inverse telecine
NTSC, telecine, and how to cope (using telecide and decimate)
=============================================================
Before we start, lets clear up the field a bit.
You DO NOT need any of the two filters (telecide and decimate)
if you are working with "pure interlaced data" as opposed to
"artificially interlaced data". In plain terms: You don't need
these two filters AND YOU SHOULD NOT USE THEM if your source
data are coming from:
- a decrypted PAL DVD (all European DVDs, for example)
- a captured .avi from a camera (be it PAL or NTSC)
For the above, you can use one of the other filters, like
"-J dilyuvmmx", "-I 1" or "-I 3" (or even smartdeinter).
Just remember to read the instructions.
Now that we've thrown away 90% of the readers, its time to
describe what is this artificial interlacing we (might)
meet in an NTSC DVD and why it requires special care.
23.976 -> 29.97
===============
NTSC television, for reasons that lurk deep within the American
psyche (i.e. nobody knows why), is broadcast at 29.97 frames
per second. Film however, (movies) are shot at 24 frames
per second. How are Americans watching movies on their TVs then?
Where do the extra 5.97 frames come from? :)
As one might guess, American engineers had to interpolate.
They also had to cope with the fact that TV isn't actually
showing 29.97 frames per sec, but 2 x 29.97 fields per second.
What are fields, you ask? Well, check this ASCII art:
scanline 0 --------------------------------- Field 0, 1st line
scanline 1 +++++++++++++++++++++++++++++++++ Field 1, 1st line
scanline 2 --------------------------------- Field 0, 2nd line
scanline 3 +++++++++++++++++++++++++++++++++ Field 1, 2nd line
scanline 4 --------------------------------- Field 0, 3rd line
etc...
As you can see, each frame is composed from two fields. Each of
the fields has 240 lines, thus we finally get 480 scanlines.
So, to display film content, which is 24 frames per second,
this is what American engineers came up with:
- First, each film frame is split into two fields. We thus get
48 fields per second (2 x 24)
- Then, a process called 3:2 pulldown (telecine) is performed.
It's better to see it with an example:
Assume we have 5 film frames, now split into 10 fields.
The even scanlines are forming the 'top' fields,
while the odd ones the 'bottom'.
For example, the two first frames are split as follows:
Frame 1 Frame 2
======= =======
Top Field of Frame 1 (T1) Top Field of Frame 2 (T2)
Bottom Field of Frame 1 (B1) Bottom Field of Frame 2 (B2)
Now, if we had a 24fps Television set, we could see any film easily,
if we sent it the following sequence:
T1 B1 T2 B2 T3 B3 T4 B4 T5 B5 T6 B6 ...
====== ====== ====== ====== ====== ======
Frame1 Frame2 Frame3 Frame4 Frame5 Frame6
Unfortunately, we have a stupid 29.97 standard, so this is what we send:
T1 B1 T2 B2 T2 B3 T3 B4 T4 B4 ...
===== ===== ===== ===== =====
N N A A N
As you can see, we have used fields from 4 film frames, but we have
sent them in a way that produces 5 NTSC 'frames': 3 Normal (N) ones,
and two Artificial (A) ones. The displaying is also slowed down,
from 30 'frames' per sec to 29.97 fps.
You see it?
These Artificial frames are horribly interlaced!
And this interlacing has nothing to do with the normal interlacing
found in PAL and NTSC cameras... This is a different monster.
Note that if you use one of transcode's deinterlacing filters with
such a sequence, you'll probably get something like this:
T1 B1 T2 B2 T3 B3 T4 B4 T4 B4 ...
===== ===== ===== ===== =====
... that is, two completely identical frames (IF the deinterlacing
algorithm is good enough). And yes, when you encode your sequence
to MPEG4 or whatever, you waste bandwidth for these extra frames.
Not to mention, that a skilled eye will notice that the film is not
natural - any motion will appear jerky, because of the duplicate
frames.
What can we do?
===============
Well, first of all, as good as transcode deinterlacing filters are,
they were not designed for this atrocity. Even "-I 3" falls back to
keeping odd (or even) scanlines and interpolates them to get the
ones missing. This is unacceptable.
The recovery process (called IVTC - inverse telecine) is done
in two steps, from two filters.
The first one, called 'ivtc', tries to recreate the original
film frames from the available fields. It doesn't remove the
duplicate frames; this is done from the next filter, the 'decimate'
one.
The algorithms used in these filters are the basic ones used in
the 'Decomb' package (made by Donald A. Graft and available -
unfortunately - only under Windows). Thanks for opening up the
source, Donald!
Example
=======
To summarize, let's see an example. The Dolby trailers are freely
available on the Web, and yes, they are purely telecined.
Let's transcode 'City' in a two pass perfect IVTC with Ogg sound:
First pass:
transcode -M 0 -f 23.976 -i dolby-city.vob -x vob -y xvid,null -w 740
-J ivtc,decimate -V -o /dev/null -R 1 -B 6,13,1
Second pass:
transcode -M 0 -f 23.976 -b 64 -i dolby-city.vob -x vob -y xvid,ogg -w 740
-J ivtc,decimate -V -o test.avi -R 2 -m test.o
Adding video and audio together in an Ogg stream (you'll need ogmtools):
ogmmerge -o dolby-city.ogm test.avi test.ogg
Some explaining:
-w 740 Target video bitrate is 740000 bits per second
-B 6,13,16 Resize from NTSC 720x480 to 512x384 (4:3 aspect)
-M 0 -f 23.976 it means that the demux must not drop frames on
its own; the filters will do that, producing a
23.976 frames per sec result.
-J ivtc,decimate Always use them in this order, and with no other
filter before them - especially no deinterlacing
and no resizing one!
Problems
========
As good as these filters are, there are streams out there that are
simply impossible to IVTC. For example, amazing as it may sound,
some NTSC DVDs are switching from 29.97 to 23.976 IN THE MIDDLE
of playback! When there is no constant frame rate, the filters
will fail. This is especially true of Anime DVDs (the Japanese
cartoon thingies).
Additionally, when the telecine pattern shifts, a couple of
interlaced frames will pass through. These should ideally be
deinterlaced with a simple filter - I'll probably add the code
one of these days.
Note: This doesn't mean that you should use a deinterlacing
filter after ivtc and telecide - it would blur out the otherwise
perfect progressive frames that get reconstructed from the filters.
Hope all this hasn't caused you a headache.
If your NTSC DVD contains a film, chances are these filters will
help (actually, they are the only 'correct' solution available
under Linux).
Since they work perfectly for my NTSC DVDs, I probably won't
mess them up anymore. Feel free to change them as you will,
as they are available under the GPL.
Thanassis Tsiodras
ttsiod at oreilly.softlab.ntua.gr
To e-mail, remove the well known publishing house from my address.
The joy of spam, you see...
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