[mythtv-users] Keyframe identification in cutlist editor
mikep at randomtraveller.org.uk
Wed Feb 22 11:12:12 UTC 2012
On 21/02/12 20:16, John Pilkington wrote:
> On 21/02/12 18:34, Thomas Boehm wrote:
>> John Veness wrote:
>>> Personally I've never skipped forward or back by keyframe either, and
>>> for me it's just an annoying step to get past (with the up/down arrows)
>>> between Frame and 0.5 Seconds, both of which are useful. It would be
>>> interesting to see if anyone really used Keyframe when editing, and if
>>> not, just remove it from the list.
>> I use it and it looks like MPEG-2 can only be cut on a keyframe, correct
>> me if I'm wrong. I tried to cut on non-keyframes in the past and AFAIR
>> there where always more frames in the show after cutting or less.
>> I just tried it and cut 1500 frames from a show. The result after
>> transcoding (ProjectX + HandBrakeCLI) had only 1462 frames.
>> It looks like MythTV automatically sets the cut point to the preceding
>> keyframe. I double checked my next trial before transcoding and the cut
>> point was moved by MythTV when I set it to a non-keyframe...
> It must surely depend on the tools being used. ProjectX as used in
> mythcutprojectx often reports that it is 'dropping useless B-frames' -
> and as I said before, I have rarely used keyframe positions within the
> editor - but I'm subjectively convinced that it isn't as coarse-grained
> as just cutting at keyframes. Similarly I'm sure that more accurate
> cutting is possible; nice as that would be, I'm just not sure it's
> worth pursuing in the general ad-cutting context.
> Time I looked at the rapidly evolving web-based editor, perhaps? A week
> ago I thought I needed a HowTo :-)
Having recently done some manual editing using avidemux, I can tell you that if
your cuts are /not/ on keyframes the end result will likely be noticeable on
screen as random blocks, hashes or other constructs.
Makes sense really, only keyframes are complete frames, which means that when
one partial frame (the one you cut on) is followed by another partial frame (the
next one you cut on, but following a different keyframe) the difference
information is too great to be resolved. Only when the /next/ keyframe comes
along can the GPU/TV resolve everything correctly.
More information about the mythtv-users