[Mp4-tech] [H.264] Use of SPS/PPS in H.264

[arun menon]

Hi Erez,
Please my comments below.
Thanks,
Arun
On Wed, 21 Jun 2023 Steinberg Erez-RM96322 wrote :
>Hi,
>
>H.264 allows up to 32 sequence parameter sets (SPS) and 256 picture
>parameter sets (PPS) in a stream.
>
>An SPS includes information on the picture resolution, number of
>reference frame, profile and level. These are cardinal for decoder
>initialization and setup.
>
>My questions:
>
>(1) Is a decoder expected to handle spontaneous changes in these
>parameters? I.e., if an IDR picture is received with a different
>PPS/SPS, is it supposed to reallocate all its memory buffers? (This is
>effectively resetting the decoder)
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An example I can think of when the decoder will receive SPS and PPS information with different picture resolution could be during playing of live content. If at any time the encoder finds that the network bandwidth is reduced due to some reason, it could encode and send smaller frame size. In this case you should reallocate the memory buffers used for the frames. Alternatively you can allocate your memory buffers with the max size supported to prevent the freeing and reallocation.
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>
>(2) For practical implementations -- are there any known applications
>that use different SPSs in the same stream? Or, can a decoder assume
>only one SPS?
>
  It would be wrong if the decoder assumes that there would be only one SPS or PPS because you do get multiple SPS and PPS in some streams. But there is also the possibility that sometimes the same information in SPS and PPS is sent again, instead of some thing different (I have encountered a few such clips).
>
>Thanks for your help!
>
>Regards,
>Erez
>
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