[FFmpeg-devel] [PATCH] avcodec/samidec: fix misposition of subtitles caused by rounding

[Jeong-Hoon Seo]

SAMI and ASS have different precision of duration in FFMPEG
(SAMI: millisecond, ASS: centisecond)
Sometimes, start-time of current subtitle is overrun with end-time of previous one by rounding process
This led to misposition of subtitles
  <exmple>
  - input SAMI
  <sync start=1155>
    <p class=ENCC> start
  <sync start=2850>
    <p class=ENCC> end
  <sync start=3850>
    <p class=ENCC> !
  - convert ASS
  Dialogue: 0,0:00:01:16,0:00:02:86,Default,start
  Dialogue: 0,0:00:02:85,0:00:03:85,Default,end
  ...

  each values in ASS means "Format": "Layer","Start","End","Style","Text"
  "End" value = <sync start> + rounding(difference of <sync start>)
  0:00:02:86 = 0:00:01:16 + rounding(2850-1151)

in the converted ASS,
start-time of 2nd "Text" is overrun by end-time of 1st "Text",
and some frames will overlaid with multi-line text
(during 0:00:02:85~0:00:02:86, 2nd one is on top of 1st one)
After 1st text end-time(expired), 2nd one keep previous position(misposition)
---
 libavcodec/samidec.c | 4 ++--
 1 file changed, 2 insertions(+), 2 deletions(-)

diff --git a/libavcodec/samidec.c b/libavcodec/samidec.c
index 7705f93..92a9863 100644
--- a/libavcodec/samidec.c
+++ b/libavcodec/samidec.c
@@ -119,9 +119,9 @@ static int sami_decode_frame(AVCodecContext *avctx,
     SAMIContext *sami = avctx->priv_data;
 
     if (ptr && avpkt->size > 0 && !sami_paragraph_to_ass(avctx, ptr)) {
-        int ts_start     = av_rescale_q(avpkt->pts, avctx->time_base, (AVRational){1,100});
+        int ts_start     = av_rescale_q_rnd(avpkt->pts, avctx->time_base, (AVRational){1,100}, AV_ROUND_ZERO);
         int ts_duration  = avpkt->duration != -1 ?
-                           av_rescale_q(avpkt->duration, avctx->time_base, (AVRational){1,100}) : -1;
+                           av_rescale_q_rnd(avpkt->duration, avctx->time_base, (AVRational){1,100}, AV_ROUND_ZERO) : -1;
         int ret = ff_ass_add_rect_bprint(sub, &sami->full, ts_start, ts_duration);
         if (ret < 0)
             return ret;
-- 
1.9.4.msysgit.1