[FFmpeg-devel] [PATCH] Add A53 Closed Captions to MPEG header if they are available.

[John P Poet]

On Tue, Jun 6, 2017 at 3:59 PM Hendrik Leppkes <h.leppkes at gmail.com> wrote:

> On Tue, Jun 6, 2017 at 11:45 PM, Marton Balint <cus at passwd.hu> wrote:
> >
> > On Tue, 6 Jun 2017, John Poet wrote:
> >
> >> ff_mpeg1_encode_picture_header: Add support for AV_FRAME_DATA_A53_CC
> >> frame: Add av_frame_get_side_data_at() to allow retrival of multiple
> side
> >>    data of the same type.
> >
> >
> > As far as I remember multiple side data of the same type is not
> something we
> > wanted to support. Why do you need it? Can't a single
> AV_FRAME_DATA_A53_CC
> > side data packet contain many CC entries?
> >
>
> Indeed, multiple entries of the same type are really a bad idea and we
> basically made them impossible with stream sidedata, although maybe
> not with frame side data yet. We should not add API for them or
> encourage their use.
> If there is a real need for multiple of the same type, maybe the type
> should be expanded to hold more information.
>
>
The cc_count is only 5 bits, which mean that only 31 3-byte "closed caption
constructs" will fit in a "block".    Testing this with 1080i60 material, I
found that 2 or 3 blocks was often necessary to hold all of the CC data.

I tried ignoring that limit of 31 "constructs" per block, and ended up with
corrupt captions.   By preserving the 2 or 3 separate blocks I observed
from the original source, the captions are perfect.

If you would like me to go about this a different way, please give me some
direction.

Thank you,

John