[Icecast] Icecast WebM Support Patch Second Edition

[Dennis Heerema]

Hi David,

Streaming live WebM would also give a boost to this open format.
Whitch Client do you use to stream the webm format to icecast with?

Regards,

Dennis


-----Original Message-----
From: Krad Radio <kradradio at gmail.com>
To: icecast-dev at xiph.org, icecast at xiph.org
Date: Wed, 8 Feb 2012 23:53:05 -0500
Subject: [Icecast] Icecast WebM Support Patch Second Edition

Howdy,
tl;dr:  Nothing new or interesting to non-developers
Attached is a newer Icecast WebM support patch for Icecast SVN, there is 
simply aesthetic changes. Whitespace has been altered to match Icecast 
project style, 
some functions have been renamed and moved around.
Some discussion. 
The format_ebml.c file lines 0-296 operates much the same as the 
format_ogg.c file does. It uses a parsing library api to feed in and pull 
out bytes via 
the _get_buffer function. In the case of Ogg this is of course libogg, in 
the case of ebml, the "parsing library" consists of the functions below line 
296.
This mini-included library comes from some debugging code that I wrote 
during my work on a much more robust EBML muxing library, originally I had 
thought
icecast would need to do some manipulation of the EBML segment header in all 
cases, but it turns out in most cases this is better done by the source 
client. 
This mini-included library implements the same function calls that Icecast 
would need to use if it was using this EBML muxing library, but "parse" is a 
very 
generous descriptor of whats actually happening. If you are familiar with 
the internals of Ogg streaming, you know that the stream starts out with a 
few header pages
that are then followed by pages containing the acutual video and audio 
packets. EBML doctype WebM and MKV (The only currently known) work in a 
similar way. 
There is a "Segment Header" followed by "Clusters" (Its actually more 
nuanced than this, but this oversimplification will suffice for the moment). 
Clusters are comparable to
Ogg pages. The connecting client needs to be sent the header, and then it 
can start on any cluster as long as it starts exactly on a cluster. (This is 
actually not a format requirement per se but all media players I am aware of 
lack the ability to re-sync themselves if started on a random byte, 
something I intend not to be a limitation of my own work). So, in a live 
WebM stream, everything before the first cluster is the "Header" and then 
the rest is the clusters, of which the boundary between them is marked with 
4 specific bytes. So at every cluster the refbuf is marked as a sync point. 
The size of clusters could vary significantly during a stream or from stream 
to stream, but on a properly constructed one would indicate a keyframe in 
the case of a stream with video. The 'mini-library' doesn't actually 'parse' 
the stream at all, all it does is look for the four byte sequence indicating 
a cluster boundary and informs the format functions appropriately. I am a 
fraudulent mathematician at best, but I calculate that there is a 1 in 4.2 
billion chance of this happening for any given 4 bytes, and 1 in 4294 per 
megabyte, and likely once per 4.2 gigabytes. This is not a problem once the 
stream has started for the client however, it only matters when the source 
connects and the header is stored, and when a client connects and needs a 
proper starting point. I suppose that makes it very unlikely to cause a 
problem, even though its technically wrong. It also means that each byte is 
being compared, whilst when properly parsing most would be skipped, but 
computers are so damn fast that its moot. At any rate proper parsing could 
be added this this mini-library or provided by the external library as it 
matures.
Enjoy,
David
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