[FFmpeg-devel] [PATCH 1/2] avdevice/decklink_dec: mark the field flag if framerate > 30FPS

[lance.lmwang at gmail.com]

On Fri, Jul 17, 2020 at 07:13:28PM +0200, Marton Balint wrote:
> 
> 
> On Fri, 17 Jul 2020, lance.lmwang at gmail.com wrote:
> 
> > From: Limin Wang <lance.lmwang at gmail.com>
> > 
> > In SMPTE ST 12-1: 2014 Sec 12.2, we need to mark the frame flag if the frame rate > 30FPS to
> > avoid interoperability issues. It will be used by the encoder to identify even or odd frames
> > and correctly calculate the frame number of the SEI TC.
> 
> This feature looks like it belongs to av_timecode_get_smpte_from_framenum
> and not into decklink. Also checking previous timecode and guessing
> something is a hack, especially since you have the exact source timecode.
If I understand correctly, it's not hacky, for the framerate > 30, the source timecode will be
frame pair(it's limited by 2bits limit of frame number ten digital. Below is one exmaple for 50fps
if you tested with SDI with TC.
00:00:00:24
00:00:00:24
00:00:00:25
00:00:00:25
00:00:00:00
00:00:00:00

That's why I check the last TC to get the frame is even or odd.

Why not use av_timecode_get_smpte_from_framenum(), for it's calculated by the string TC by one time,
so it lacks the information whether it's odd or even frame. 

> 
> Regards,
> Marton
> 
> > 
> > ---
> > libavdevice/decklink_common.h |  1 +
> > libavdevice/decklink_dec.cpp  | 14 ++++++++++++++
> > 2 files changed, 15 insertions(+)
> > 
> > diff --git a/libavdevice/decklink_common.h b/libavdevice/decklink_common.h
> > index bd68c7b..8ddc411 100644
> > --- a/libavdevice/decklink_common.h
> > +++ b/libavdevice/decklink_common.h
> > @@ -151,6 +151,7 @@ struct decklink_ctx {
> >     int channels;
> >     int audio_depth;
> >     unsigned long tc_seen;    // used with option wait_for_tc
> > +    uint32_t last_tc;
> > };
> > 
> > typedef enum { DIRECTION_IN, DIRECTION_OUT} decklink_direction_t;
> > diff --git a/libavdevice/decklink_dec.cpp b/libavdevice/decklink_dec.cpp
> > index dde68ff..a60c01b 100644
> > --- a/libavdevice/decklink_dec.cpp
> > +++ b/libavdevice/decklink_dec.cpp
> > @@ -884,12 +884,26 @@ HRESULT decklink_input_callback::VideoInputFrameArrived(
> >                         int metadata_len;
> >                         uint8_t* packed_metadata;
> >                         AVTimecode tcr;
> > +                        AVRational rate = ctx->video_st->r_frame_rate;
> > 
> >                         if (av_timecode_init_from_string(&tcr, ctx->video_st->r_frame_rate, tc, ctx) >= 0) {
> >                             uint32_t tc_data = av_timecode_get_smpte_from_framenum(&tcr, 0);
> >                             int size = sizeof(uint32_t) * 4;
> >                             uint32_t *sd = (uint32_t *)av_packet_new_side_data(&pkt, AV_PKT_DATA_S12M_TIMECODE, size);
> > 
> > +                            /* set the field flag if frame rate > 30FPS */
> > +                            /* Refer to SMPTE ST 12-1:2014 Sec 12.2 */
> > +                            if (av_cmp_q(rate, (AVRational) {30, 1}) == 1) {
> > +                                /* Odd frame */
> > +                                if (ctx->last_tc == tc_data) {
> > +                                    if (av_cmp_q(rate, (AVRational) {50, 1}) == 0)
> > +                                        tc_data |= (1 << 7);
> > +                                    else
> > +                                        tc_data |= (1 << 23);
> > +                                }
> > +                            }
> > +                            ctx->last_tc = tc_data;
> > +
> >                             if (sd) {
> >                                 *sd       = 1;       // one TC
> >                                 *(sd + 1) = tc_data; // TC
> > -- 
> > 2.9.4
> > 
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-- 
Thanks,
Limin Wang