[FFmpeg-devel] [PATCH 10/18] lavf: move AVStream.*index_entries* to AVStreamInternal

[Michael Niedermayer]

On Mon, Nov 16, 2020 at 09:46:01PM -0300, James Almer wrote:
> On 11/16/2020 12:25 PM, Hendrik Leppkes wrote:
> > On Mon, Nov 16, 2020 at 4:20 PM James Almer <jamrial at gmail.com> wrote:
> > > 
> > > On 11/16/2020 3:01 AM, Anton Khirnov wrote:
> > > > Quoting Xiang, Haihao (2020-11-16 06:16:55)
> > > > > 
> > > > > This change breaks the compiling of gst-libav (
> > > > > https://gitlab.freedesktop.org/gstreamer/gst-libav), I filed
> > > > > https://trac.ffmpeg.org/ticket/8988 to track this regression.
> > > > 
> > > > This is not a regression, it's a bug in gst-libav. These fields are
> > > > private and have always been private, applications accessing them are
> > > > broken.
> > > 
> > > Looking at that ticket, it seems gst-libav wants the timestamp for a
> > > given index entry. Unless this can already be achieved in some other
> > > way, it could maybe be implemented cleanly with a new public function
> > > (We have av_index_search_timestamp() to fetch an index from a timestamp
> > > after all).
> > 
> > I also like being able to access the index, it can be used by players
> > to efficiently offer seeking straight to keyframes (eg. index
> > entries), which basically requires iterating over the full index and
> > getting the full information (timestamp, flags at least)
> > 
> > - Hendrik
> 
> What do you suggest, implement something like what you added to you
> LAVFilters fork? I don't know if returning a pointer to a given AVIndexEntry
> is ideal. If that were the case, then it wouldn't be an internal field.
> 
> Perhaps something like
> 

> int av_index_get_entries_count(AVStream *st);
> int av_index_get_timestamp(AVStream *st, int64_t *timestamp, int *flags, int
> idx);

If the (only) goal is to export the whole list then a better option might be
to use an opaque state and some sort of iterator.
Because the suggestion requires N index based lookups and this is only fast
with some conceivable implementations.
for example consider an index stored in some sort of tree structure
a index based lookup might be O(log n) while one step iterating over all
elements could be O(1)

AVIndexEntry *av_index_iterate(AVStream *st, AVIndexIterator **i);

and

AVIndexIterator *i = NULL;
AVIndexEntry *entry;
for (entry = av_index_iterate(st, &i); i; entry = av_index_iterate(st, &i))
    ...
        

[...]
-- 
Michael     GnuPG fingerprint: 9FF2128B147EF6730BADF133611EC787040B0FAB

He who knows, does not speak. He who speaks, does not know. -- Lao Tsu
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