Hi On Fri, Nov 24, 2006 at 03:49:25PM -0800, Ralph Giles wrote:
On Fri, Nov 24, 2006 at 11:35:24PM +0100, Michael Niedermayer wrote:
hmm, i see 3 possibilities for xiph codecs 1. store the metadata packet as is 2. dont store the metadata packet 3. store a dummy (empty) metadata packet
I would vote for 1 or 3.
I disagreed with doing 2 in the RTP draft, but didn't insist because the payload implementation already has to be codec-specific. For general container use like this, I think it's important to require the metadata packet where the codec requires it. This is less surprising for implementors.
You do still have to know how the vorbis headers are packed, but for playback there is still a difference between these two procedures:
# option 1 or 3 for header in global_headers: submit_packet_to_codec(header);
# option 2 header_number = 0; for header in global_headers: submit_packet_to_codec(header) header_number += 1 if (header_number == 1): header = construct_dummy_metadata_packet() submit_packet_to_codec(header)
submit_packet_to_codec(header) submit_packet_to_codec(dummy_defined_in_a_static_char_array) submit_packet_to_codec(header) but in reality the demuxer will likely pass the single global header unchanged around, and the vorbis decoder (or wraper) would then option 1 or 3 parse_header1(); skip_header2(); parse_header3(); vs. option 2 parse_header1(); parse_header3(); your method might depening on API end up looking like demuxer_open(){ blah blah for all streams if stream is one of vorbis, theora, ... stream->xiph_glob= alloc(3) stream->xiph_glob_len= alloc(3) stream->xiph_glob_index=0; for i in 3 find len of packet stream->xiph_glob_len[i]= len stream->xiph_glob[i]= alloc(len) read packet into stream->xiph_glob[i] } demuxer_get_packet() if(stream->xiph_glob_index<3 && stream->xiph_glob[stream->xiph_glob_index]){ packet.dts= dunno has none packet.pts= dunno has none packet.duration= 0 packet.flags= ? packet.len= stream->xiph_glob_len[stream->xiph_glob_index] memcpy(packet.data, stream->xiph_glob[stream->xiph_glob_index], packet.len); stream->xiph_glob_index++ stream->xiph_glob_len[stream->xiph_glob_index]=0 free(stream->xiph_glob[stream->xiph_glob_index]) return } considering that this would be needed for every non ogg demuxer i wouldnt implement it like that ... [...]
Therefore I'd propose, "The global headers MUST consist of the normal sequence of header packets required for codec initialization, in the order expected by the codec. An implementation MAY strip metadata and other redundant information not necessary for correct playback from the global headers to save space in the bitstream."
i would s/expected by the codec/defined in the codec spec/ otherwise its highly unclear, think about mpeg and its headers, the spec is clear what is allowed and what not, specific implementations might be more forgiving but still its the mpegvideo spec which we should follow in that case also i would rather say An implementation MAY/SHOULD/MUST strip metadata and other redundant information not necessary for correct playback from the global headers as long as no incorrect values are stored and as long as the striped result is not less valid per codec spec as before striping (probably that could be improved too ...) anyway after thinking about that a little option 3 with the encoder name and version in the metadata packet seems like the best solution ... [...] -- Michael GnuPG fingerprint: 9FF2128B147EF6730BADF133611EC787040B0FAB In the past you could go to a library and read, borrow or copy any book Today you'd get arrested for mere telling someone where the library is