[mythtv-users] 0.24 mythpreviewgen issues - first frame always

[Michael T. Dean]

On 04/04/2011 01:49 AM, Kevin Blackham wrote:
> I've been running a user job to convert my recordings to h264, then
> update recorded.basename.  They play back fine in the frontend, DLNA
> server, even the mythweb flash player.  However, the mythpreviewgen
> command seems to be doing something odd.
>
> Even though it logs "AFD: Opened codec 0x2394660, id(H264) type(Video)
> ... Preview: Grabbed preview '/space/mythtv/2249_20110328210000.mp4'
> 640x480 at 620s" - it has not grabbed a frame from near 620s, it is
> grabbing the first frame.  In my case, that first frame is trash due
> to cheap BTTV card, analog issues, or something of the sort.

You have an invalid seek table.

> Rebuilding the map as suggested does not help.

What container are you using?  If you're putting H.264 in Matroska, 
having a seek table will actually cause problems.  If you do that, you 
need to clear the seek table.

Also, seek table building for H.264 is still very fragile--and would 
likely only work properly with H.264 that's been encoded using the exact 
same options as seen-in-the-wild broadcast H.264.

However, my recommendations are:
   a) Use only MythTV tools to transcode recordings
   b) If you transcode a recording using some non-MythTV tool, put that 
/video/ (that's no longer a MythTV recording) into MythVideo
   c) Or, better, don't waste time/energy transcoding recordings (pretty 
much the only useful reason I can imagine to transcode is to create a 
small, low-quality video for streaming to a low-resource device (cell 
phone, tablet, UPnP device, ...) that has a specific list of formats it 
supports--and those can be put into MythVideo, instead of MythTV Watch 
Recordings, if they don't work in MythTV recordings).

>    Message repeats.
> Completely re-scanning the h264 for commercial flagging does not make
> it go away either (note: after scanning h264, then simply changing
> recorded.basename back to 2249_20110328210000.nuv, It did generate the
> RTjpeg preview linked above).  What is this position map?
>
> Slightly related thread?
> http://www.mailinglistarchive.com/html/mythtv-users@mythtv.org/2010-10/msg00513.html
> mythpreviewgen version: fixes/0.24 [v0.24-231-gc2baf1b]

Exact same problem that user had--invalid seek table breaks seeking to 
retrieve the specified preview pixmap.

Mike