Re: [FFmpeg-devel] [PATCH 3/3] avfilter/yadif_common: fix timestamps with very small timebases

[Marton Balint]

On Thu, 1 Feb 2024, Michael Niedermayer wrote:

On Wed, Jan 31, 2024 at 03:42:46AM +0100, Marton Balint wrote:

On Wed, 31 Jan 2024, Michael Niedermayer wrote:

On Sun, Jan 28, 2024 at 04:01:36AM +0100, Marton Balint wrote:

Yadif filter assumed that the output timebase is always half of the input
timebase. This is not true if halving the input time base is not representable
as an AVRational causing the output timestamps to be invalidly scaled in such a
case.

So let's use av_reduce instead of av_mul_q when calculating the output time
base and if the conversion is inexact then let's fall back to the original
timebase which probably makes more parctical sense than using x/INT_MAX.

Fixes invalidly scaled pts_time values in this command line:
ffmpeg -f lavfi -i testsrc -vf settb=tb=1/2000000000,yadif,showinfo -f null none

Signed-off-by: Marton Balint <c...@passwd.hu>
---
 libavfilter/yadif.h        |  2 ++
 libavfilter/yadif_common.c | 20 ++++++++++++++------
 2 files changed, 16 insertions(+), 6 deletions(-)

diff --git a/libavfilter/yadif.h b/libavfilter/yadif.h
index 2c4fed62d2..c144568242 100644
--- a/libavfilter/yadif.h
+++ b/libavfilter/yadif.h
@@ -86,6 +86,8 @@ typedef struct YADIFContext {
      * the first field.
      */
     int current_field;  ///< YADIFCurrentField
+
+    int pts_divisor;
 } YADIFContext;

 void ff_yadif_init_x86(YADIFContext *yadif);
diff --git a/libavfilter/yadif_common.c b/libavfilter/yadif_common.c
index 933372529e..90a5cffc2d 100644
--- a/libavfilter/yadif_common.c
+++ b/libavfilter/yadif_common.c
@@ -61,7 +61,7 @@ FF_ENABLE_DEPRECATION_WARNINGS
         int64_t next_pts = yadif->next->pts;

         if (next_pts != AV_NOPTS_VALUE && cur_pts != AV_NOPTS_VALUE) {
-            yadif->out->pts = cur_pts + next_pts;
+            yadif->out->pts = (cur_pts + next_pts) / yadif->pts_divisor;
         } else {
             yadif->out->pts = AV_NOPTS_VALUE;
         }
@@ -150,8 +150,8 @@ int ff_yadif_filter_frame(AVFilterLink *link, AVFrame 
*frame)
         ff_ccfifo_inject(&yadif->cc_fifo, yadif->out);
         av_frame_free(&yadif->prev);
         if (yadif->out->pts != AV_NOPTS_VALUE)
-            yadif->out->pts *= 2;
-        yadif->out->duration *= 2;
+            yadif->out->pts *= 2 / yadif->pts_divisor;
+        yadif->out->duration *= 2 / yadif->pts_divisor;
         return ff_filter_frame(ctx->outputs[0], yadif->out);
     }

@@ -168,9 +168,9 @@ FF_ENABLE_DEPRECATION_WARNINGS
     yadif->out->flags &= ~AV_FRAME_FLAG_INTERLACED;

     if (yadif->out->pts != AV_NOPTS_VALUE)
-        yadif->out->pts *= 2;
+        yadif->out->pts *= 2 / yadif->pts_divisor;
     if (!(yadif->mode & 1))
-        yadif->out->duration *= 2;
+        yadif->out->duration *= 2 / yadif->pts_divisor;

you can use >> instead of division for all above

Even for the first case? Because the right shift would be implementation
defined for negative timestamps.

we are only supporting twos-complement systems
i thought that was somewhete in teh docs

Ok, I will change the /= 2 divisions to >>= 1 shifts in v2 patch then.

Thanks,
Marton
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