[FFmpeg-devel] [PATCH] avfilter/vf_subtitles: pass storage size to libass

[Oneric <oneric@oneric.de>]

Due to a quirk of the ASS format some tags depend on the exact storage
resolution of the video, so tell libass via ass_set_storage_size.

---
ass_set_storage_size exists since libass 0.10.2;
ffmpeg since 5.0 already requires 0.11.0.

This resolution dependences of ASS was already recognised when the
original_size parameter was added, but it actually goes farther than
just the aspect ratio. Conveniently this parameter still has all the
required information to retain rendering after resizing :)

Sample files to show the difference can be found eg here
https://code.videolan.org/videolan/vlc/uploads/b54e0761d0d3f4f79b2947ffb83a3b59/vlc-issue_libass-storage-size.tar.xz

./ffmpeg -i test_1080p.mkv -filter:v ass=./test_1080p.ass tmp_1080.mkv
./ffmpeg -i anamorphic_s720x576_d1024x576.mkv -filter:v ass=./anamorphic_s720x576_d1024x576.ass tmp_anam.mkv

---
 libavfilter/vf_subtitles.c | 7 ++++++-
 1 file changed, 6 insertions(+), 1 deletion(-)

diff --git a/libavfilter/vf_subtitles.c b/libavfilter/vf_subtitles.c
index 3fc4eeb63d..af6352b315 100644
--- a/libavfilter/vf_subtitles.c
+++ b/libavfilter/vf_subtitles.c
@@ -146,9 +146,14 @@ static int config_input(AVFilterLink *inlink)
     ff_draw_init(&ass->draw, inlink->format, ass->alpha ? FF_DRAW_PROCESS_ALPHA : 0);
 
     ass_set_frame_size  (ass->renderer, inlink->w, inlink->h);
-    if (ass->original_w && ass->original_h)
+    if (ass->original_w && ass->original_h) {
         ass_set_pixel_aspect(ass->renderer, (double)inlink->w / inlink->h /
                              ((double)ass->original_w / ass->original_h));
+        ass_set_storage_size(ass->renderer, ass->original_w, ass->original_h);
+    } else {
+        ass_set_storage_size(ass->renderer, inlink->w, inlink->h);
+    }
+
     if (ass->shaping != -1)
         ass_set_shaper(ass->renderer, ass->shaping);
 
-- 
2.30.2

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