> Andrea,
> Do you think we should try to add some of Igor's information to the manual?
> MKV seems to have a lot of differences that cause issues because it is an
> Open Standard and can be used anyway (as opposed to MP4 which is not for free
> commerical use).
@Phyllis
OK, I try to write a draft. Can you all check if it is correct or can
be improved?
@IgorBeg
How do you see the I-frames of a media? I have only found one ffprobe
command, but it is not convenient to use. Do you have any resource to
elaborate on the problems of seeking mkv?
Note on Matroska (mkv) container
Matroska is a universal container, modern and sophisticated and above all Open
Source from which follows development with community input and excellent
documentation. Also derived from this container is the Webm container used by
Google and Youtube, which use the VP8-9 and AV1 codecs. Use within \CGG{} is
therefore recommended, however, one may run into seeking problems during
playback. The internal structure of matroskas is sophisticated but requires
exact use of internal keyframes (I-frame; B-frame and P-frame) otherwise
playback on the timeline may be subject to freeze and drop frames. The mkv
container format can be problematic if the source encoding is not done well by
the program (i.e., OBS). For an easy but accurate introduction of codecs and
how they work see:
https://ottverse.com/i-p-b-frames-idr-keyframes-differences-usecases/
To find out the keyframe type (I, P, B) of our media you can use ffprobe:
ffprobe -v error -hide_banner-of default=noprint_wrappers=0 -print_format flat
-select_streams v:0 -show_entries frame=pict_type input.mkv
-v error -hide_banner: serve to hide a blob of information that is useless for
our purposes.
-of: is an alias for "-print_format" and is used to be able to use
"default=noprint_wrappers=0".
-default=noprint_wrappers=0: is used to be able to show the information from
the parsed stream that we need.
-print_format flat: is used to display the result of ffprobe according to a
"flat" format (you can choose CSV; Json; xml; etc).
-select_streams v:0: is used to choose the first stream (0) in case there are
multiple audio and video streams (tracks, in \CGG{}).
-show_entries: shows the type of data collected by ffprobe that we want to
display (there are also types: _streams; _formats; _packets and _frames. They
are called "specifiers").
frame=pict_type: within the chosen specifier indicates the data to be
displayed; in this case pict_type, that is, the keyframe type (I, P, B) of the
frame under consideration.
input.mkv: is the media to be analyzed (it can be any container and code).
We thus obtain a list of all frames in the analyzed media and their type. For
example:
frames.frame.0.pict_type="I"
frames.frame.1.pict_type="P"
frames.frame.2.pict_type="B"
frames.frame.3.pict_type="B"
frames.frame.4.pict_type="B"
...
We can now turn to the timeline of \CGG{} to see the frames that give problems
in playback. Using a codec of type Long GOP, it is probably the rare I-frames
that give the freezes.
To find a solution you can use MKVToolNix (https://mkvtoolnix.download/) to
correct and insert new keyframes into the mkv file (matroska talks about "cues
data"). It can be done even without new encoding. Or you can use the Transcode
tool within \CGG{} because during transcoding new keyframes are created that
should correct errors.
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