[FFmpeg-devel] [PATCH 1/2] avdevice/decklink_dec: mark the field flag if framerate > 30FPS
Marton Balint
cus at passwd.hu
Sat Jul 18 21:38:16 EEST 2020
On Sat, 18 Jul 2020, lance.lmwang at gmail.com wrote:
> On Sat, Jul 18, 2020 at 09:44:03AM +0200, Marton Balint wrote:
>>
>>
>> On Sat, 18 Jul 2020, lance.lmwang at gmail.com wrote:
>>
>> > 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
>>
>> Have you actually tested this with a real SDI signal containing timecode?
>
> Yes, have tested with SDI playback by decklink UltraStudio 4K mini with TC enabled.
I still don't understand how this works. Isn't timecode in the SDI signal
also contain the so called "field bit" flag to support 50fps timecode? And
if it does, then why can't you query it from decklink? Timecode in SDI
signal is also supposed to be an SMPTE 12M timecode, so all the bits you
need should be in it, you should not need to guess anything. Therefore it
makes little sense to me that the DeckLink API does not provide a proper
timecode as string, unless the original capture you made with your
UltraStudio and which you played back also has a faulty timecode.
What generated the signal which you captured with UltraStudio? Not ffmpeg
I hope...
Regards,
Marton
>
>>
>> Thanks,
>> Marton
>>
>> >
>> > 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
>> > _______________________________________________
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>
> --
> Thanks,
> Limin Wang
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