[FFmpeg-cvslog] avcodec/mpegpicture: Always reset motion val buffer

[Andreas Rheinhardt]

ffmpeg | branch: master | Andreas Rheinhardt <andreas.rheinhardt at outlook.com> | Mon Aug  8 16:51:04 2022 +0200| [71ff9217f769b03ce3b8081ad596ed777ac24d2f] | committer: Andreas Rheinhardt

avcodec/mpegpicture: Always reset motion val buffer

Codecs call ff_find_unused_picture() to get the index of
an unused picture; said picture may have buffers left
from using it previously (these buffers are intentionally
not unreferenced so that it might be possible to reuse them;
this is mpegvideo's version of a bufferpool). They should
not make any assumptions about which picture they get.
Yet somehow this is not true when decoding OBMC: Returning
random empty pictures (instead of the first one) leads
to nondeterministic results; similarly, explicitly
rezeroing the buffer before handing it over to the codec
changes the outcome of the h263-obmc tests, but it makes it
independent of the returned pictures. Therefore this commit
does so.

Signed-off-by: Andreas Rheinhardt <andreas.rheinhardt at outlook.com>

> http://git.videolan.org/gitweb.cgi/ffmpeg.git/?a=commit;h=71ff9217f769b03ce3b8081ad596ed777ac24d2f
---

 libavcodec/mpegpicture.c | 4 ++++
 1 file changed, 4 insertions(+)

diff --git a/libavcodec/mpegpicture.c b/libavcodec/mpegpicture.c
index 88b4d5dec1..06c82880a8 100644
--- a/libavcodec/mpegpicture.c
+++ b/libavcodec/mpegpicture.c
@@ -245,6 +245,10 @@ int ff_alloc_picture(AVCodecContext *avctx, Picture *pic, MotionEstContext *me,
         for (i = 0; i < 2; i++) {
             pic->motion_val[i] = (int16_t (*)[2])pic->motion_val_buf[i]->data + 4;
             pic->ref_index[i]  = pic->ref_index_buf[i]->data;
+            /* FIXME: The output of H.263 with OBMC depends upon
+             * the earlier content of the buffer; therefore we
+             * reset it here. */
+            memset(pic->motion_val_buf[i]->data, 0, pic->motion_val_buf[i]->size);
         }
     }