If user opted to present fields as they come, then the first field
picture needs to be submitted to the HW for decoding. In particular,
this fixes MPEG-2 decoding of interlaced streams.

Tested on Intel Cedar Trail, Sandy Bridge and Ivy Bridge platforms.
Someone reported on the ffmpeg-devel@ list this also works on DXVA
(Windows) and other Linux platforms (NVIDIA, through the VA wrapper).

This also means a similar patch to non-hwaccel VDPAU may be necessary.

Note: I believe the SLICE_FLAG_ALLOW_FIELD is useless since the first
field shall always be submitted to the HW anyway. Nobody uses HW accels
(dxva, vaapi, vdpau, etc.) without that flag though.

Signed-off-by: Gwenole Beauchesne <[email protected]>
---
 libavcodec/mpeg12.c |    6 ++++++
 1 files changed, 6 insertions(+), 0 deletions(-)

diff --git a/libavcodec/mpeg12.c b/libavcodec/mpeg12.c
index fde4e02..18ef6da 100644
--- a/libavcodec/mpeg12.c
+++ b/libavcodec/mpeg12.c
@@ -1627,6 +1627,12 @@ static int mpeg_field_start(MpegEncContext *s, const 
uint8_t *buf, int buf_size)
             return -1;
         }
 
+        if (s->avctx->hwaccel &&
+            (s->avctx->slice_flags & SLICE_FLAG_ALLOW_FIELD)) {
+            if (s->avctx->hwaccel->end_frame(s->avctx) < 0)
+                av_log(avctx, AV_LOG_ERROR, "hardware accelerator failed to 
decode first field\n");
+        }
+
         for (i = 0; i < 4; i++) {
             s->current_picture.f.data[i] = s->current_picture_ptr->f.data[i];
             if (s->picture_structure == PICT_BOTTOM_FIELD) {
-- 
1.7.5.4

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