hlsdemux: using iframe playlists when seeking

[Sebastian Dröge]

On Fr, 2014-12-05 at 15:55 +1000, Duncan Palmer wrote:
> Hi,
> 
> I'm finding that seek performance with hlsdemux is poor when seeking
> to a time near the end of a segment. This is because the entire
> segment is pushed through the pipeline - most of it is not decoded,
> but with HD streams, the volume of data is high enough that it takes 2
> or 3 seconds for data at the correct offset to arrive at the decoders,
> if I seek to somewhere near the end of a 10 second segment.
> 
> I'm looking at using iframe playlists in hlsdemux to enable it to seek
> to the correct offset within a segment. I think this can be
> implemented by
> 
> Modifying gst_hls_demux_src_event() so that it does something like the
> following:
> - switch to the iframe playlist
> - determine the offset within the current segment corresponding to the
> seek position. Store this in demux->client->partial_range_start (new
> variable).
> - switch back to the regular playlist
> 
> Modifying gst_m3u8_client_get_next_fragment() so that rather than
> using the offset it's just read from the m3u8 file, use
> demux->client->partial_range_start, just once.
> 
> I've attached a patch which implements the core part of this idea (it
> compiles, but it untested, and undoubtedly has bugs and missing bits).
> 
> Would anyone like to comment on this approach? Can you think of a
> neater way to do it?

While the idea is good, the problem is that this requires the I-frame
playlist to point to the I-frame offsets in the normal playlist. This is
not required by the spec, it can be a completely different MPEG TS
stream.

I think there's not much we can do better here. I guess in theory you
could estimate the byte offset inside the TS stream by looking at the
bitrate and underestimate it a bit but that's also not going to work
reliable.

-- 
Sebastian Dröge, Centricular Ltd · http://www.centricular.com
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