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

[Lars Kiesow]

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(-)

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..1cc28f6a56 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;
 
         if (scale->force_original_aspect_ratio == 1) {
              w = FFMIN(tmp_w, w);
@@ -589,6 +591,7 @@ static const AVOption scale_options[] = {
     { "out_v_chr_pos",   "output vertical chroma position in luma grid/256"  , OFFSET(out_v_chr_pos), AV_OPT_TYPE_INT, { .i64 = -513}, -513, 512, FLAGS },
     { "out_h_chr_pos",   "output horizontal chroma position in luma grid/256", OFFSET(out_h_chr_pos), AV_OPT_TYPE_INT, { .i64 = -513}, -513, 512, FLAGS },
     { "force_original_aspect_ratio", "decrease or increase w/h if necessary to keep the original AR", OFFSET(force_original_aspect_ratio), AV_OPT_TYPE_INT, { .i64 = 0}, 0, 2, FLAGS, "force_oar" },
+    { "force_divisible_by", "endorce that the output resolution is devisible by a defined integer", OFFSET(force_divisible_by), AV_OPT_TYPE_INT, { .i64 = 1}, 0, 256, FLAGS, "force_oar" },
     { "disable",  NULL, 0, AV_OPT_TYPE_CONST, {.i64 = 0 }, 0, 0, FLAGS, "force_oar" },
     { "decrease", NULL, 0, AV_OPT_TYPE_CONST, {.i64 = 1 }, 0, 0, FLAGS, "force_oar" },
     { "increase", NULL, 0, AV_OPT_TYPE_CONST, {.i64 = 2 }, 0, 0, FLAGS, "force_oar" },
-- 
2.21.0