[FFmpeg-devel] [PATCH 3/6] avcodec/mpegpicture: Always reset motion val buffer

[Andreas Rheinhardt]

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.

(No, this commit is not intended to be applied. I just hope
to arouse the interest of people familiar with H.263
to look at this issue.)

Signed-off-by: Andreas Rheinhardt <andreas.rheinhardt at outlook.com>
---
I still don't intend to apply this. If no one looks into this,
I will just add the necessary changes to the h263-obmc ref-files
in the next commit and leave this one out.

 libavcodec/mpegpicture.c               | 4 ++++
 tests/ref/vsynth/vsynth1-h263-obmc     | 4 ++--
 tests/ref/vsynth/vsynth2-h263-obmc     | 4 ++--
 tests/ref/vsynth/vsynth_lena-h263-obmc | 4 ++--
 4 files changed, 10 insertions(+), 6 deletions(-)

diff --git a/libavcodec/mpegpicture.c b/libavcodec/mpegpicture.c
index c57f149752..2192f74cea 100644
--- a/libavcodec/mpegpicture.c
+++ b/libavcodec/mpegpicture.c
@@ -280,6 +280,10 @@ 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);
         }
     }
 
diff --git a/tests/ref/vsynth/vsynth1-h263-obmc b/tests/ref/vsynth/vsynth1-h263-obmc
index b7a267a8cb..aed283ed53 100644
--- a/tests/ref/vsynth/vsynth1-h263-obmc
+++ b/tests/ref/vsynth/vsynth1-h263-obmc
@@ -1,4 +1,4 @@
 7dec64380f375e5118b66f3baaaa1e24 *tests/data/fate/vsynth1-h263-obmc.avi
 657320 tests/data/fate/vsynth1-h263-obmc.avi
-844f7ee27fa122e199fe20987b41a15c *tests/data/fate/vsynth1-h263-obmc.out.rawvideo
-stddev:    8.16 PSNR: 29.89 MAXDIFF:  113 bytes:  7603200/  7603200
+2a69f6b37378aa34418dfd04ec98c1c8 *tests/data/fate/vsynth1-h263-obmc.out.rawvideo
+stddev:    8.38 PSNR: 29.66 MAXDIFF:  116 bytes:  7603200/  7603200
diff --git a/tests/ref/vsynth/vsynth2-h263-obmc b/tests/ref/vsynth/vsynth2-h263-obmc
index 2cef7f551b..c0dcc3239e 100644
--- a/tests/ref/vsynth/vsynth2-h263-obmc
+++ b/tests/ref/vsynth/vsynth2-h263-obmc
@@ -1,4 +1,4 @@
 2d8a58b295e03f94e6a41468b2d3909e *tests/data/fate/vsynth2-h263-obmc.avi
 208522 tests/data/fate/vsynth2-h263-obmc.avi
-4a939ef99fc759293f2e609bfcacd2a4 *tests/data/fate/vsynth2-h263-obmc.out.rawvideo
-stddev:    6.10 PSNR: 32.41 MAXDIFF:   90 bytes:  7603200/  7603200
+3500b4227c1e6309ca5213414599266f *tests/data/fate/vsynth2-h263-obmc.out.rawvideo
+stddev:    6.19 PSNR: 32.29 MAXDIFF:  111 bytes:  7603200/  7603200
diff --git a/tests/ref/vsynth/vsynth_lena-h263-obmc b/tests/ref/vsynth/vsynth_lena-h263-obmc
index 5b963107f6..78d7cc7277 100644
--- a/tests/ref/vsynth/vsynth_lena-h263-obmc
+++ b/tests/ref/vsynth/vsynth_lena-h263-obmc
@@ -1,4 +1,4 @@
 3c6946f808412ac320be9e0c36051ea2 *tests/data/fate/vsynth_lena-h263-obmc.avi
 154730 tests/data/fate/vsynth_lena-h263-obmc.avi
-588d992d9d8096da8bdc5027268da914 *tests/data/fate/vsynth_lena-h263-obmc.out.rawvideo
-stddev:    5.39 PSNR: 33.49 MAXDIFF:   82 bytes:  7603200/  7603200
+737af7fb166e2260ba049ae6bc30673d *tests/data/fate/vsynth_lena-h263-obmc.out.rawvideo
+stddev:    5.42 PSNR: 33.44 MAXDIFF:   77 bytes:  7603200/  7603200
-- 
2.34.1