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[FFmpeg-devel,15/57] avcodec/mpegpicture: Always reset motion val buffer

Message ID AS8P250MB0744C192E4CE7E933BCED8098F1B2@AS8P250MB0744.EURP250.PROD.OUTLOOK.COM
State Superseded
Headers show
Series [FFmpeg-devel,01/14] avcodec/get_buffer: Remove redundant check | expand

Commit Message

Andreas Rheinhardt April 29, 2024, 9:13 p.m. UTC
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@outlook.com>
 libavcodec/mpegpicture.c | 4 ++++
 1 file changed, 4 insertions(+)
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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);