[mythtv-users] keyframes in cut list editor off by one frame

[Tony Sauri]

On 23/08/14 11:13, Mark Perkins wrote:
> Just to be clear *I think* there is a mismatch of 1 between the frame viewed in the cutlist editor and the frame number. Whether that is because the wrong frame is displayed on the screen or the wrong frame number is displayed or something else I don't know. So it's not just the keyframes affected, every frame / frame number is out by 1.

I have been following this discussion because I have been investigating
this very issue.

The other day I use ImageMagick to create 15000 images with a plain
black background bearing an ascending integer starting from 000001 to
015000.

I then used FFMPEG/AVCONV to assemble these images into a mpeg2 video
stream. Saved the resulting file into my videos library.

When I play this file and start the cut editor I can now confirm that
the reported frame numbers _ARE_ correct  (ie editor frame number 1
agrees with the number on the image).

However the time stamp shown by the editor for frame 1 is 00:00.040
which would be the correct time after the frame is shown.
If using AVCONV/FFMPEG to edit the file you would use -ss 0.000 to start
at frame 1.

Another point is that the the myth cut editor when set to jump to the
next keyframe goes 1 ...16 ... 31   46 (every 15 frames
When I load the file into avidemux it says the frame bearing the frame
number 000001 is frame 0 and advancing to keyframes shows 0 ... 15 ...
30 etc

Leaves me wondering if this is all a case of the first frame of a stream
is frame 0 or frame 1?

Earlier in the discussion  the 1 frame discrepancy between the
recordedseek table and the cutlist-editor initially troubled me but then
I read the wiki documentation for the
recordrdseek table at http://www.mythtv.org/wiki/Recordedseek_table
 which says :

	       TYPE     MARK-TYPE	
		6 	MARK_GOP_START
		7 	MARK_KEYFRAME
		8 	MARK_SCENE_CHANGE
		9 	MARK_GOP_BYFRAME

If the records in the recordedseek table were pointing to Keyframes then
they would/should be Type 7's whereas what I (and you) see are type 9's.
 Now I have never been able to find a definition of GOP_BY_FRAME but I
am coming to the belief that it marks the last frame in  a Group of
Pictures.  I would love to have some clarity around exactly what the
GOP_BYFRAME does represent.

Regards

Tony Sauri