On Wed, Mar 13, 2024 at 01:24:25PM +0100, Niklas Haas wrote:
> From: Niklas Haas <git@haasn.dev>
> 
> This filter's existing design has a number of issues:
> 
> - There is no guarantee whatsoever about the order in which frames are
>   pushed onto the main and ref link, due to this being entirely
>   dependent on the order in which downstream filters decide to request
>   frames from their various inputs. As such, there is absolutely no
>   synchronization for ref streams with dynamically changing resolutions
>   (see e.g. fate/h264/reinit-*).
> 
> - For some (likely historical) reason, this filter outputs its ref
>   stream as a second ref output, which is in principle completely
>   unnecessary (complex filter graph users can just duplicate the input
>   pin), but in practice only required to allow this filter to
>   "eventually" see changes to the ref stream (see first point). In
>   particular, this means that if the user uses the "wrong" pin, this
>   filter may break completely.
> 
> - The default filter activation function is fundamentally incapable of
>   handling filters with multiple inputs cleanly, because doing so
>   requires both knowledge of how these inputs should be internally
>   ordered, but also how to handle EOF conditions on either input (or
>   downstream). Both of these are best left to the filter specific
>   options. (See #10795 for the consequences of using the default
>   activate with multiple inputs).
> 
> Switching this filter to framesync fixes all three points:
> 
> - ff_framesync_activate() correctly handles multiple inputs and EOF
>   conditions (and is configurable with the framesync-specific options)
> - framesync only supports a single output, so we can (indeed must) drop
>   the redundant ref output stream
> 
> Update documentation, changelog and tests to correspond to the new usage
> pattern.
> 
> Fixes: https://trac.ffmpeg.org/ticket/10795
> ---
>  Changelog                                |   2 +
>  doc/filters.texi                         |  10 +-
>  libavfilter/vf_scale.c                   | 130 ++++++++++++-----------
>  tests/filtergraphs/scale2ref_keep_aspect |   3 +-
>  4 files changed, 76 insertions(+), 69 deletions(-)

this causes
./ffplay --help
to segfault

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

It is dangerous to be right in matters on which the established authorities
are wrong. -- Voltaire