[FFmpeg-devel] [PATCH] Ensure scaled video is divisible by n

[Michael Niedermayer]

On Wed, Jul 03, 2019 at 06:16:51PM +0200, Lars Kiesow wrote:
> This patch adds a new option to the scale filter which ensures that the
> output resolution is divisible by the given integer similar to using -n
> in the `w` and `h` options. But this works even if the
> `force_original_aspect_ratio` is used.
> 
> The use case for this is to set a fixed target resolution using `w` and
> `h`, to use the `force_original_aspect_ratio` option to make sure that
> the video always fits in the defined bounding box regardless of aspect
> ratio, but to also make sure that the calculated output resolution is
> divisible by n so in can be encoded with certain encoders/options if
> that is required.
> 
> Signed-off-by: Lars Kiesow <lkiesow at uos.de>
> ---
>  doc/filters.texi       | 5 +++++
>  libavfilter/vf_scale.c | 9 ++++++---
>  2 files changed, 11 insertions(+), 3 deletions(-)

commit message should begin with a prefix like 
avfilter/vf_scale: ...

> 
> diff --git a/doc/filters.texi b/doc/filters.texi
> index 700a76f239..1694fdda28 100644
> --- a/doc/filters.texi
> +++ b/doc/filters.texi
> @@ -15215,6 +15215,11 @@ Please note that this is a different thing than specifying -1 for @option{w}
>  or @option{h}, you still need to specify the output resolution for this option
>  to work.
>  
> + at item force_divisible_by
> +Ensures that the output resolution is divisible by the given integer similar
> +to using -n in the @option{w} and @option{h} options. But this works even if
> +the @option{force_original_aspect_ratio} is used.
> +
>  @end table
>  
>  The values of the @option{w} and @option{h} options are expressions
> diff --git a/libavfilter/vf_scale.c b/libavfilter/vf_scale.c
> index f741419e7e..d1b486f3d6 100644
> --- a/libavfilter/vf_scale.c
> +++ b/libavfilter/vf_scale.c
> @@ -86,6 +86,7 @@ typedef struct ScaleContext {
>      int in_v_chr_pos;
>  
>      int force_original_aspect_ratio;
> +    int force_divisible_by;
>  
>      int nb_slices;
>  
> @@ -237,10 +238,11 @@ static int config_props(AVFilterLink *outlink)
>          goto fail;
>  
>      /* Note that force_original_aspect_ratio may overwrite the previous set
> -     * dimensions so that it is not divisible by the set factors anymore. */
> +     * dimensions so that it is not divisible by the set factors anymore
> +     * unless force_divisible_by is defined as well */
>      if (scale->force_original_aspect_ratio) {
> -        int tmp_w = av_rescale(h, inlink->w, inlink->h);
> -        int tmp_h = av_rescale(w, inlink->h, inlink->w);
> +        int tmp_w = av_rescale(h, inlink->w, inlink->h) / scale->force_divisible_by * scale->force_divisible_by;
> +        int tmp_h = av_rescale(w, inlink->h, inlink->w) / scale->force_divisible_by * scale->force_divisible_by;

what does this feature have to do with force_original_aspect_ratio ?
i think it should not be under this if()

also the rounding is always down, it probably should be rounding to closest
(for n=2 it doesnt matter but for larger divisibility like 16 rounding down
by 15 instead of rounding up by 1 seems not ideal


[...]

thx

-- 
Michael     GnuPG fingerprint: 9FF2128B147EF6730BADF133611EC787040B0FAB

Awnsering whenever a program halts or runs forever is
On a turing machine, in general impossible (turings halting problem).
On any real computer, always possible as a real computer has a finite number
of states N, and will either halt in less than N cycles or never halt.
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