Checking Interlacing Flags
With the replacement of conventional televisions by progressive scan
LCD and gas plasma displays it is surprising that today's high definition
camcorders still record interlaced video.
It seems plausible that interlaced video will be common in consumer
camcorders at least until the year 2015.
This document discusses how to verify that the flags are set
correctly in an interlaced video file.
Each frame from an interlaced source consists of two fields, a
top field and the bottom field. Exactly how these fields are stored
depends on the video encoding used. When the top field is displayed
it fills every other horizontal scan line starting with the top most
line in the image.
When the bottom field is displayed it fills every other horizontal
scan line starting with the scan line second from the top.
Before burning a file to a SVCD, DVD or Blu-ray disk it is important
to check that the interlacing order is correct because if the order
is not correct, the video will not play smoothly.
Required Software
Checking Interlace Order
If you play interlaced video on a computer monitor without additional
post processing then interlacing will appears as a combing effect on
fast moving objects. The combing results because the even and the
odd lines of the image which were recorded at different points in time
are being displayed at the same time.
When you see this combing effect you have interlaced or telecined video.
In either case, the first thing to do is determine the field order.
To do this play the video with
$ mplayer -vf tfields=1:0 video.mpg
If the motion looks smooth then the field dominance is top field
first; if the motion is not smooth it is bottom field first.
To verify the field order play the video again with
$ mplayer -vf tfields=1:1 video.mpg
If the motion did not play smoothly before then it should look smooth
now; if it was smooth before then it should no longer be smooth.
A related way to check the interlacing order of the video
is with ffmpeg and mjpegtools using the method given in
When to Deinterlace a Video.
Convert your clip to progressive video with a 59.96 frame
rate in two different ways: once as if it were interlaced
bottom field first and again as if it were interlaced top
field first.
Play back each progresive video file. The file that plays
smoothly is the one that was deinterlaced with the correct
interlacing order.
Detecting Telecined Video
Play the video frame by frame and look for a pattern of two
interlaced frames followed by three progressive frames. If
your video has this pattern it is telecined. Telecined
video comes from the 24p mode in some camcorders and from
transferring film source to video. The interlacing
flags for telecined video can be verified in the same way
as interlaced video. More information about telecined
video can be found
in PF24 Pulldown Removal with Linux.
Verifying the Interlacing Flags
Verify that the flags embedded in the video stream accurately
reflect the interlacing order with
$ mediainfo video.mpg
The output should contain lines like
Scan type : Interlaced
Scan order : Bottom Field First
Check that the interlacing order it correct.
Note that if the video format is Digital Video then the
scan order is not reported but should be assumed to be
bottom field first.
Fixing the Interlace Order
If the interlacing determined by examining the video with
mplayer does not agree with the interlacing order given by the flags
then the video will not play smoothly from an SVCD, DVD or Blu-ray disk.
Fix the encoding step in the workflow so the
flags are correct. The interlace options patch for ffmpeg
given on the page
DVD Compliant mpeg2 on Linux
may help.
The program
restream
can be used
to directly manipulate the interlacing flags in raw mpeg2 video
files on Windows.
Reencode the video only as a last resort.
Conclusions
The method outlined here should prevent making disks with interlaced
video that has the field dominance improperly flagged.
Proof that the workflow is correct is a test disk.
Find a short video clip containing a fast camera pan across a scene.
Check the workflow by encoding the clip as if it were
interlaced top field first and again as if it were interlaced
bottom field first.
Play the disk in a player connected to a standard definition
television with a interlaced cathode ray tube display.
Do not use a progressive scan player or a high definition
television.
The video file which shows smooth motion is the one that corresponds
to the workflow with
the correct interlacing order.
Note the hrd_pd_interlace patch is needed for x264 to include
interlacing flags in the encoded h264 video stream.
The Toshiba A2 will randomly choose an interlacing order if
the video stream is not properly flagged.
More information on encoding interlaced h264 video is given at
H264 HD Video Workflow.
Page written and maintained by Eric Olson
Last Updated:
Fri Aug 28 23:41:09 PDT 2009