On Fri, 9 Jun 2017, Devin Heitmueller wrote:
Hello Marton,
On Tue, Jun 6, 2017 at 5:45 PM, Marton Balint <c...@passwd.hu> wrote:
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?
Could you please expand on where this was discussed? Is there any
design documentation for side data infrastructure that's been
introduced into ffmpeg? Is there some list of other known design
constraints/limitations?
While I agree it would be great to simply say that you should never
have multiple side data items of the same type, I'm not sure how
realistic that is. It would be helpful if I could better understand
the rationale in that thinking.
I guess there are two reasons for prohibiting same type side data:
- existing API implicitly gave the impression that is is not supported
Av_frame_get_side_data gives you no possibility to retrieve multiple side
data of the same type. API users might assumed that the reason for that is
because it is not possible/permitted.
- consistency with packet/stream side data types
For stream side data is is already explicitly disallowed, API would be
inconsistent if multiple side data entries of the same types were possible
for one kind of side data (frame), but not for the other (stream).
I'm starting a rather large project which will likely stretch the
design for side data in order to support a variety of other ancillary
data protocols (e.g. SCTE-104, SMPTE 2038, etc), and it would be great
to better understand where the constraints are (so I can either plan
to address those issues, or if too significant then choose a different
multimedia framework to work with before making a significant
investment building out a bunch of features in ffmpeg).
Yeah, you definitely have to think about this, depending on your actual
goals libavcodec/format might not be the best choice for complex ancillary
data handling.
It seems multiple side data of the same type is not going to pick up
support from other developers, so you have to come up with something else
to move forward.
Regards,
Marton
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