[mythtv-users] Lossless Cut and keyframe cuts off

Ian Evans dheianevans at gmail.com
Wed Nov 14 22:53:33 UTC 2012

On Wed, Nov 14, 2012 at 2:55 PM, HP-mini <blm-ubunet at slingshot.co.nz> wrote:
>> There could be several causes for this..I have no experience with mpeg2
>> recordings but have spent months playing with cut-lists & dd based
>> cutting.
>> 1. The cut-list editor is misleading  with H264 video recordings
>> (mpeg2-ts from dvb-t). H264 uses out-of-order frames & keyframes can be
>> 2 sec apart & can even be intra-refresh B frames.
>> Mpeg2 uses out of order frames as well.
>> For example a recording with first keyframe at frame =24.
>> The editor shows a valid picture at frames that it can not (before the
>> first keyframe).
>> The editor can not frame advance from frame 1 (not wrong) but there is
>> no possible image.
>> The editor seems to display the picture created by the first possible
>> returned decoded frame. It seems to stuff as many frames in until the
>> decoder returns anything. So the editor shows an image from 1 frame to
>> 20+ frames in advance of the current time.
>> This results in  cut-list offset error.
>> A possible explanation is that ffmpeg decoder only returns decoded
>> frames when "enough" frames are passed to it. With out-of-order
>> encoding, this can be of the order of 1 sec (25+ frames)..
>> 2. mkvmerge cut points are at the keyframe immediately preceding the cut
>> time.
>> 3. could be bug in lossless_cut keyframe adjust..moving the wrong way..
>> Possibly setting the tuner to start on a sequence header might help
>> reduce the offset error.
>> I have not tried this.
> The start on sequence header is a tick box in mythtv-setup tuner setup..
> Try a simple expt:
> - invoke cut-list editor on a obvious transition.
> - note the frame number
> - navigate around transition with cut-list editor
> - does the transition still occur at same frame number ?
> - some recordings could be worse than others due to location of
> keyframes.

I'll give that a try

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