[FFmpeg-user] ffmpeg architecture question

[Paul B Mahol]

On 4/17/20, Mark Filipak <markfilipak.windows+ffmpeg at gmail.com> wrote:
> Thanks to pdr0 -at- shaw.ca, My quest for the (nearly perfect) p24-to-p60
> transcode has concluded.
>
> But remaining is an ffmpeg behavior that seems (to me) to be key to
> understanding ffmpeg
> architecture, to wit: The characteristics of frame traversal through a
> filter chain.
>
>  From a prior topic:
> -----
> Filter graph:

That is not filter graph. It is your wrong interpretation of it.
I can not dechiper it at all, because your removed crucial info like ','

>
> split[A]    select='not(eq(mod(n+1\,5)\,3))'       [C]interleave
>       [B]split[D]select='eq(mod(n+1\,5)\,2)'[F]blend[D]
>               [E]select='eq(mod(n+1\,5)\,3)'[G]
>
> What I expected/hoped:
>
> split[A]         0 1 _ 3 4         [C]interleave 0 1 B 3 4  //5 frames
>       [B]split[D] _ 1 _ _ _ [F]blend[D]               |
>               [E] _ _ 2 _ _ [G]                       blend of 1+2
>
> What appears to be happening:
>
> split[A]         0 1 _ 3 4         [C]interleave 0 1 _ 3 4  //4 frames
>       [B]split[D] _ _ _ _ _ [F]blend[D]
>               [E] _ _ 2 _ _ [G]
>
> The behavior is as though because frame 1 (see Note) can take the [A][C]
> path, it does take it &
> that leaves nothing left to also take the [B][D][F] path, so blend never
> outputs.
> -----
> (Note: I originally wrote "frame n+1==1" but that was an error.)
>
> I assume that frame numbers are assigned at the input of the filter chain as
> frames are encountered,
> and that the following actions occur.
> Frame 0: Traverses [A][C] and is enqueued at [C].
> Frame 1: Traverses [A][C] and is enqueued at [C] (see Proposition).
> Frame 2: Traverses [B][E][G] and is enqueued at [G].
> Frame 3: Traverses [A][C] and is enqueued at [C].
> Frame 4: Traverses [A][C] and is enqueued at [C].
>
> Proposition: Frame 1 could also traverse [B][D][F] and be enqueued at [F]
> but since it's already
> enqueued at [C], it does not do so.
>
> Specifically, it appears that ffmpeg does not recurse the filter chain for
> frames that are already
> enqueued, thus Frame 1 is not enqueued at [F], thus 'blend' doesn't activate
> when Frame 2 arrives at
> [G], thus Frame 2 is never enqueued at [D] and never appears in the output
> of 'interleave'.
>
> Is what I've written correct? Authoritative confirmation or correction of
> this architectural detail
> is desired.
>
> Regards,
> Mark.
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