[FFmpeg-devel] Confusion over temporal filters.

[Paul B Mahol]

On 9/10/17, Richard Ling <divetec at rling.com> wrote:
> I'm thinking of adding a temporal filter (one that relies on context from
> previous frames), and I've realised I'm a bit confused about how they
> should work.
>
> Say I open a file with ffplay and let it play up to frame 100.  Then I open
> the same file with another instance of ffplay and seek directly to frame
> 100.  It seems to me that frame 100 should look the same in both cases.
> Put another way, a specific set of filters, applied to a specific file,
> should completely define what every frame of that file looks like.  Seeking
> around in the file should not change that.  Is that right (in principle)?
>
> So, looking at some of the existing temporal filters (eg. deflicker), I
> don't think that is what happens.  They filter based on the frames
> previously passed to the filter, and if the user seeks over a bunch of
> frames, the filter will see frames as consecutive that are not actually
> consecutive in the file, so it will give a different result.  Also, looking
> at the API, I can't see a way to get the behaviour I expect.  I can't see a
> way for a filter to ask its inputs for a frame from a different (specific)
> time.  Is that right?
>
> If my understanding is wrong, please let me know!
>
> If my undersanding is correct, then I guess my questions are:
> (1) is this behaviour a known issue (or a deliberate design choice)?

Both. There is no seeking in lavfi (yet).

> (2) is it OK for new temporal filters to keep the same behaviour as
> existing ones -- that is, they will give different results when seeking
> happens, compared to sequential processing?  If so I'll just do the same
> thing for my filter.

Yes, its OK.