avcodec/h264_slice: compute and export film grain seed

From SMPTE RDD 5-2006, the grain seed is to be computed from the
following definition of `pic_offset`:

> When decoding H.264 | MPEG-4 AVC bitstreams, pic_offset is defined as
> follows:
>   - pic_offset = PicOrderCnt(CurrPic) + (PicOrderCnt_offset << 5)
> where:
>   - PicOrderCnt(CurrPic) is the picture order count of the current frame,
>     which shall be derived from [the video stream].
>
>   - PicOrderCnt_offset is set to idr_pic_id on IDR frames. idr_pic_id
>     shall be read from the slice header of [the video stream]. On non-IDR I
>     frames, PicOrderCnt_offset is set to 0. A frame shall be classified as I
>     frame when all its slices are I slices, which may be optionally
>     designated by setting primary_pic_type to 0 in the access delimiter NAL
>     unit. Otherwise, PicOrderCnt_offset it not changed. PicOrderCnt_offset is
>     updated in decoding order.

Co-authored-by: James Almer <jamrial@gmail.com>
Signed-off-by: Niklas Haas <git@haasn.dev>
Signed-off-by: James Almer <jamrial@gmail.com>

1 files changed, 14 insertions, 0 deletions

diff --git a/libavcodec/h264dec.c b/libavcodec/h264dec.c
index 07ff9110d2..f46d29baa6 100644
--- a/libavcodec/h264dec.c
+++ b/libavcodec/h264dec.c
@@ -485,6 +485,8 @@ static void h264_decode_flush(AVCodecContext *avctx)
 static int get_last_needed_nal(H264Context *h)
 {
     int nals_needed = 0;
+    int slice_type = 0;
+    int picture_intra_only = 1;
     int first_slice = 0;
     int i, ret;
 
@@ -516,11 +518,23 @@ static int get_last_needed_nal(H264Context *h)
                 !first_slice ||
                 first_slice != nal->type)
                 nals_needed = i;
+            slice_type = get_ue_golomb_31(&gb);
+            if (slice_type > 9) {
+                if (h->avctx->err_recognition & AV_EF_EXPLODE)
+                    return AVERROR_INVALIDDATA;
+            }
+            if (slice_type > 4)
+                slice_type -= 5;
+
+            slice_type = ff_h264_golomb_to_pict_type[slice_type];
+            picture_intra_only &= (slice_type & 3) == AV_PICTURE_TYPE_I;
             if (!first_slice)
                 first_slice = nal->type;
         }
     }
 
+    h->picture_intra_only = picture_intra_only;
+
     return nals_needed;
 }