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.rheinha...@outlook.com>
> ---
Will apply this patchset minus this patch tomorrow unless there are
objections. And I would really like anybody with knowledge about H.263
to take a look at the obmc code to see what is wrong with it.
> 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
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