Re: [FFmpeg-devel] [PATCH 11/11] avcodec/mpegpicture: Always reset motion val buffer

[Andreas Rheinhardt <andreas.rheinhardt@outlook.com>]

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@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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