Haakon Riiser wrote:
I'm doing some experimentation with Microsoft's Smooth streaming,
and I have some questions regarding picture/sequence parameter
sets in H.264.
In Microsoft's layout, this data is stored out of band in a
so-called "manifest" file. The data is marked "CodecPrivateData",
and, as I understand it, corresponds to FFmpeg's extradata
concept.
Unfortunately, it wasn't as simple as filling in the extradata
array with the data from the CodecPrivateData string. Here's a
specific example. It shows what happens if I take a video file,
encode it to work with Smooth streaming, and compare the
CodecPrivateData from Microsoft with extradata from FFmpeg. Note
that only remuxing is done, no transcoding, so the parameters sets
are identical.
>From Microsoft's manifest-file (line-breaks at strategic places to
illustrate the differences):
Microsoft's CodecPrivateData = \
00000001 \
6764000dac2cc506077e5840000003004000000ca3c50a6580 \
00000001 \
68ebecb22c
If I use libavformat to parse the source material used to
generate the Smooth streaming stuff that produced the above
CodecPrivateData, I get the following extradata:
FFmpeg's extradata = \
0164000dffe10019 \
6764000dac2cc506077e5840000003004000000ca3c50a6580 \
010005 \
68ebecb22c
So, it looks like the two blocks
6764000dac2cc506077e5840000003004000000ca3c50a6580
and
68ebecb22c
are the same in both cases (I assume these are respectively the
sequence and picture parameter sets). The question is, why are
the lead-in bytes different, and what do they mean?
00000001 != 0164000dffe10019 (lead-in to sequence parameter set)
and
00000001 != 010005 (lead-in to picture parameter set)
In summary: If I only have the CodecPrivateData as Microsoft encodes
it:
000000016764000dac2cc506077e5840000003004000000ca3c50a65800000000168ebecb22c
then how can I use it to properly decode video with FFmpeg's
H.264 decoder? I don't know the algorithm used to convert this into
0164000dffe100196764000dac2cc506077e5840000003004000000ca3c50a658001000568ebecb22c
which FFmpeg seems to require.
FFmpeg stores H264 PPS&SPS headers in avcC format. CodecPrivateData
stores PPS&SPS headers as NAL units. Use function ff_isom_write_avcc
from file "libavformat/avc.c" to convert CodecPrivateData to normal
extradata form.
Anatoly.
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