[FFmpeg-devel] [PATCH v8 3/3] avdevice/decklink_dec: export timecode with s12m side data

Andreas Rheinhardt andreas.rheinhardt at gmail.com
Sun Jul 12 19:03:04 EEST 2020


lance.lmwang at gmail.com:
> On Sun, Jul 12, 2020 at 03:07:18AM +0200, Andreas Rheinhardt wrote:
>> lance.lmwang at gmail.com:
>>> From: Limin Wang <lance.lmwang at gmail.com>
>>>
>>> Signed-off-by: Limin Wang <lance.lmwang at gmail.com>
>>> ---
>>>  libavdevice/decklink_dec.cpp | 14 ++++++++++++++
>>>  1 file changed, 14 insertions(+)
>>>
>>> diff --git a/libavdevice/decklink_dec.cpp b/libavdevice/decklink_dec.cpp
>>> index a499972..146f56f 100644
>>> --- a/libavdevice/decklink_dec.cpp
>>> +++ b/libavdevice/decklink_dec.cpp
>>> @@ -42,6 +42,7 @@ extern "C" {
>>>  #include "libavutil/imgutils.h"
>>>  #include "libavutil/intreadwrite.h"
>>>  #include "libavutil/time.h"
>>> +#include "libavutil/timecode.h"
>>>  #include "libavutil/mathematics.h"
>>>  #include "libavutil/reverse.h"
>>>  #include "avdevice.h"
>>> @@ -882,6 +883,19 @@ HRESULT decklink_input_callback::VideoInputFrameArrived(
>>>                          AVDictionary* metadata_dict = NULL;
>>>                          int metadata_len;
>>>                          uint8_t* packed_metadata;
>>> +                        AVTimecode tcr;
>>> +
>>> +                        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;
>>> +                            uint8_t *sd = av_packet_new_side_data(&pkt, AV_PKT_DATA_S12M_TIMECODE, size);
>>> +
>>> +                            if (sd) {
>>> +                                AV_WL32(sd, 1); // one TC ;
>>
>> Nit: why is there a space after TC?
> 
> Will remove it, it's not intentional.
> 
>>
>>> +                                AV_WL32(sd + 4, tc_data); // TC;
>>
>> This contradicts the documentation of AV_PKT_DATA_S12M_TIMECODE, because
>> you are always using little endian here. But the documentation [1] does
>> not specify an endianness, it uses native endianness (in accordance with
>> what AV_FRAME_DATA_S12M_TIMECODE does). This very same patch also uses
>> native endianness in libavformat/dump.c.
> 
> good catch, at first, I think it's little endian order, and Nicolas comment
> uint32_t is native, so I remove the commens for the byte order. I'll change
> to use AV_WB32() for native write. 
> 

AV_WB uses big-endian. Instead you could simply use an uint32_t* to
write the numbers.

>>
>> I consider it a mistake to use native endianness for the frame side data
>> and an uint32_t in av_timecode_get_smpte (none of the fields cross a
>> byte boundary, so each entry could easily have been an uint8_t[4]), as
>> this will make it harder to test this side data via fate; but I also see
>> the advantage of using the same format and the same helper functions for
>> both. What do others think about this? After all, we can still change
>> the format for the packet side data.
> 
> I didn't get the SMPTE S314M:2005 specs for checking, it's not free. I think
> use native format is fine for local system, but if the side data is packed into
> or read from specific format like DV, AV_WB32() and AV_RB32() had to be used.
> For the fate testing, I think we can't test the side data binary only, if needed,
> we can test the format which use the sidedata like dv.

The framecrc muxer calculates a checksum of every side data. There is
already a special way of calculating the side-data for palettes in
big-endian and it seems that we can simply reuse this (as both are based
on uint32_t in native format). So it seems it won't be much of a problem.

- Andreas


More information about the ffmpeg-devel mailing list