[FFmpeg-devel] [PATCH] avfilter/vf_tile: add skip option

[Paul B Mahol]

On 11/10/17, Nicolas George <george at nsup.org> wrote:
> Le septidi 17 brumaire, an CCXXVI, Paul B Mahol a ecrit :
>> Signed-off-by: Paul B Mahol <onemda at gmail.com>
>> ---
>>  doc/filters.texi      |  5 +++++
>>  libavfilter/vf_tile.c | 53
>> ++++++++++++++++++++++++++++++++++++++++++---------
>>  2 files changed, 49 insertions(+), 9 deletions(-)
>>
>> diff --git a/doc/filters.texi b/doc/filters.texi
>> index 2ba6e04166..a6d537485b 100644
>> --- a/doc/filters.texi
>> +++ b/doc/filters.texi
>> @@ -14439,6 +14439,11 @@ refer to the pad video filter.
>>  Specify the color of the unused area. For the syntax of this option,
>> check the
>>  "Color" section in the ffmpeg-utils manual. The default value of
>> @var{color}
>>  is "black".
>> +
>
>> + at item skip
>> +Set the number of frames to skip from input before rendering output
>> frame.
>> +The default value is @code{0}, meaning skip @var{nb_frames} frames. If it
>> is more
>> +than @var{nb_frames} its is set to @var{nb_frames}.
>
> I had trouble understanding the logic of the code, until I realized the
> problem was here: I think the documentation does not explain well what
> the option is meant to do.
>
> If I understand correctly, using an example: if a 5x4 tile, the normal
> mapping is:
>
> out  0 <- in  0 .. in 19
> out  1 <- in 20 .. in 39
> out  2 <- in 40 .. in 59
> out  3 <- in 60 .. in 79
>
> But if "skip" is set to 15, it becomes:
>
> out  0 <- in  0 .. in 19
> out  1 <- in 15 .. in 34
> out  2 <- in 30 .. in 49
> out  3 <- in 45 .. in 64
>
> In other words, the output frames overlap by nb_frames - skip input
> frames.
>
> Is it what you intended?

Yes.

>
> If so, then the doc needs to be clarified. And I think the name of the
> option is misleading. What about "step"? Or maybe "period"?

It could be...

>
> Or possibly even better: reverse the direction of the option "overlap=5"
> instead of "step=15".

What if user sets overlap to same or greater than nb_frames, just error out?