[mythtv-users] 1920x1080 transcode adds "grey bar"
jonner at teegra.net
Mon Nov 5 21:47:23 UTC 2007
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Randall Hopper wrote:
> When Myth transcodes 1920x1080i programs to MPEG-4, it incorrectly
> adds a "grey bar" at the bottom of the transcoded video.
> How do I fix/suppress this?
> Note that when mythtranscode starts, it reports the source .mpg to be
> 1920x__1088__. That's incorrect. It's 1920x1080, per mytharchivehelper
> and mplayer.
> P.S. I realize this probably has something to do with MPEG-4's 16x16
> macroblocks and 1080 requiring 8 more lines to be divisible by 16.
> But it still shouldn't encode a grey bar.
That is the issue and it's not just a limitation of MPEG-4, but MPEG-2
also. The 1080i MPEG-2 video is actually 1920x1088, but players such as
MythTV and mplayer know that eight lines should be cropped to get
1920x1080. So, either no lines are being added or mythtranscode is
cropping eight lines and then adding eight new grey ones to make an
MPEG-4 compatible frame. I haven't seen the problem you're describing
since I haven't tried transcoding 1080i programs without scaling.
I'm not certain what the best way to handle the video would be. As
you've noted, the MPEG-4 video can't have 1080 lines any more than the
source MPEG-2 video can. I would guess that mythtranscode cannot or does
not put any metadata in the output MPEG-4 video stream to indicate that
eight lines should be cropped, so you see the eight grey lines that were
always there, but were properly cropped when playing back the original
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