[FFmpeg-devel] [PATCH v4 1/2] avutil/timecode: add description for SMPTE binary format

[Nicolas George]

lance.lmwang at gmail.com (12020-06-30):
> From: Limin Wang <lance.lmwang at gmail.com>
> 
> AV_FRAME_DATA_S12M_TIMECODE is public API, so user need to know its format
> by the API header instead of reading the code.
> 
> Signed-off-by: Limin Wang <lance.lmwang at gmail.com>
> ---
>  libavutil/frame.h    |  4 ++--
>  libavutil/timecode.h | 16 +++++++++++++++-
>  2 files changed, 17 insertions(+), 3 deletions(-)
> 
> diff --git a/libavutil/frame.h b/libavutil/frame.h
> index 3fb8c56..c694b12 100644
> --- a/libavutil/frame.h
> +++ b/libavutil/frame.h
> @@ -162,8 +162,8 @@ enum AVFrameSideDataType {
>      /**
>       * Timecode which conforms to SMPTE ST 12-1. The data is an array of 4 uint32_t
>       * where the first uint32_t describes how many (1-3) of the other timecodes are used.
> -     * The timecode format is described in the av_timecode_get_smpte_from_framenum()
> -     * function in libavutil/timecode.c.
> +     * The timecode format is described in the comments of av_timecode_get_smpte_from_framenum()
> +     * function in libavutil/timecode.h.
>       */
>      AV_FRAME_DATA_S12M_TIMECODE,
>  
> diff --git a/libavutil/timecode.h b/libavutil/timecode.h
> index ab38e66..b1c1887 100644
> --- a/libavutil/timecode.h
> +++ b/libavutil/timecode.h
> @@ -61,7 +61,21 @@ int av_timecode_adjust_ntsc_framenum2(int framenum, int fps);
>   * @param tc       timecode data correctly initialized
>   * @param framenum frame number
>   * @return         the SMPTE binary representation
> - *

> + * the format description as follows, note it's in system byte order:

Better avoid contractions in documentation.

The result is uint32_t, not char[4]: there is no byte order.

> + * 31b:     color frame flag (0: unsync mode, 1: sync mode)
> + * 30b:     drop  frame flag (0: non drop,    1: drop)
> + * 28b,29b: tens  of frames
> + * 24-27b:  units of frames
> + * 23b:     PC (NTSC) or BGF0 (PAL)
> + * 20b,22b: tens  of seconds
> + * 16-19b:  units of seconds
> + * 15b:     BGF0 (NTSC) or BGF2 (PAL)
> + * 12b,13b: tens  of minutes
> + * 8-11b:   units of minutes
> + * 7b:      BGF2 (NTSC) or PC (PAL)
> + * 6b:      BGF1
> + * 4b,5b:   tens  of hours
> + * 0b-3b:   units of hours

It seems more logical in the opposite order.

I do not think this b suffix is very standard. Better be explicit:

 * bits 24-27: units of frames

Also, no reason to make a special case when there are exactly two.

And since several numbers are stored the same way, I would suggest to
regroup:

 * bits 8-13: minutes, in BCD
 * ...
 * BCD numbers (6 bits): 4 lower bits for units, 2 higher bits for tens.

>   * @note Frame number adjustment is automatically done in case of drop timecode,
>   *       you do NOT have to call av_timecode_adjust_ntsc_framenum2().
>   * @note The frame number is relative to tc->start.

Regards,

-- 
  Nicolas George
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