[ogg-dev] Fwd: Skeleton 4.0 draft, help with Dirac fields please!

Ralph Giles giles at thaumas.net
Tue May 11 10:05:13 PDT 2010

On 10 May 2010 23:20, Chris Pearce <chris at pearce.org.nz> wrote:

> The granulepos radix was something that Conrad and Ralph were talking about
> at FOMS2010. I don't know how it's supposed to be used, or why we need it.
> It was supposed to be needed for Dirac? Maybe Ralph or Conrad can remember?
> If not, we should remove it. There's no point in adding a poorly spec'd
> field that no one will use.

Right, so adding the 'granulepos radix' was based on my recollection
of a draft of the Ogg Dirac mapping. That version split the
granlulepos value into three parts to represent the reordering depth
as well as the offset to the start of the access unit. Since we record
the (variable) split position between the two granulepos parts in the
theora scheme, I thought we should add a field to document both for
Dirac. The field would just be zero for non-reordering codecs like

However, looking at David's most recent draft* the current scheme is
more complicated than that, with the three parts split into four
pieces to achieve better compatibility with muxers which only know the
theora scheme. In any case, everything is fixed width, so we can just
record the fixed 'granule shift' of 22 bits in the current skeleton
field and things will be fine as far as Dirac goes.

The question then becomes how well the Dirac mapping would work for
other reordering codecs, and whether we want to add two (or three!)
additional granulepos interpretation fields to the fishbone spec in
case other codecs want to make different resolution tradeoffs.


* http://diracvideo.org/download/mapping-specs/dirac-mapping-ogg-1.0.pdf

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