[ogg-dev] Seeking to granules in discontinuous streams

Conrad Parker conrad at metadecks.org
Fri Feb 22 15:52:49 PST 2008


do you still think you need all this, if you are allowed to have equal
granulepos on subsequent pages?


On 18/02/2008, ogg.k.ogg.k at googlemail.com <ogg.k.ogg.k at googlemail.com> wrote:
> Hi,
>  I've now got another way of encoding granule (oh, not *again*, I hear
>  you cry). I believe it's an improvement over the existing "generic"
>  method, so I'll outline it here for your consideration. Again, it's
>  different from what Skeleton can handle, but it's a simple superset and
>  would be easy to add to Skeleton (and liboggz). It is also compatible
>  with other existing codecs (eg, theora). It is implemented in libkate,
>  so it works and the changes needed are minimal. I'm happy to make the
>  necessary changes in oggmerge/liboggz if needed.
>  The two advantages this change brings are increased usable time (for the
>  same timestamping precision) and the ability to map different granules
>  to the same time. Both are adjustable by client code.
>  That said, I see the interest of having a shared method of mapping time
>  and granules, and I'll use the Theora/CMML method if this proposed
>  method doesn't convince you.
>  See the end of the mail for the trivial code difference between the two
>  methods.
>  The proposed mapping is based on the existing granule shift based
>  system, and adds two more quantities:
>  - a second granule rate fraction
>  - a second granule shift
>  If the second granule rate fraction is equal to the main granule rate
>  fraction, and the second granule shift is zero, the system is equivalent
>  to the existing one, ensuring backwards compatibility (eg, for those
>  older streams, one can set the second fraction to the first one, and the
>  second granule shift to zero).
>  The current system uses the same granule rate for both the base (the
>  high bits) and the offset (the low bits) of the granulepos. Here, a
>  separate granule rate (typically coarser) is applied to the base,
>  allowing the base to span a larger timespace. Since the offset added to
>  that base uses the finer grained granule rate, the precision of the
>  resulting timestamping remains (as long as the time than an offset can
>  span is less or equal to the granularity of the base, but that does
>  leave quite a bit of leeway). In effect, the existing method maps a lot
>  of different granules to the same time (though not contiguous granules).
>  In other words, a particular timestamp can be represented by a lot of
>  different granules. This is where the new method gets bits back without
>  sacrificing timestamping precision.
>  Additionally, a second granule shift can be used to reserve a few low
>  bits so that they will not be used in the timestamping computation. This
>  allows a codec to use those bits as a counter which will not advance the
>  time (eg, to allow multiple events at the same time, which is not
>  possible with the current system). Setting this shift to zero reverts to
>  the existing system.
>  Attached is a simple program that shows the two different methods and
>  the granules generated by base/offset pairs, along with the number of
>  bits each requires.
>  For a timestamping precision of 1ms, as you can see by running the
>  program, the existing method can't handle a base+offset of more than
>  about 50 days. If you move the granuleshift around, since the two are
>  added together, you just move the granulespace from base to offset or
>  the other way round, without getting more actual space.
>  With the method I've described, one can choose to lengthen the usable
>  time space by losing precision on the base. As shown by running the
>  program, the default setup (25 bits for the base, 2 kill bits) can
>  encode both a 1157 day base and a 1157 offset at the same time while
>  keeping the base precision to 10 seconds (the timestamping precision
>  still being 1 ms, so all times between 0 and 2300 days or so can be
>  represented at millisecond precision), and seek to any of those times
>  will be only at most 10 seconds before the intended time. Of course, for
>  smaller time ranges (such as a two hour movie), the base precision can
>  be kept to the same millisecond precision.
>  I believe that the replacement of the current "generic" description of
>  granule mapping, as described in skeleton, by this sytem should be
>  considered for the following reasons:
>  - For the same timestamping precision, that new mapping extends greatly
>   the range of times that can be represented
>  - The new mapping allows the low bits of the granulepos to map to the
>   same time, allowing multiple events at the same time
>  - The user code can choose to extend (to all intents and purposes)
>   arbitrarily the representable range of times, at the expense of base
>   precision (though the base+offset precision remains the same)
>  - Ogg does not mandate any particular mapping, so the current system
>   overly constrains the range of mappings (the proposed one still
>   constrains to a linear mapping, but constrains less).
>  - Skeleton isn't widely used yet (AFAIK), and CMML is being changed, so
>   now is a good time for such a change (well, that's not a reason to
>   change, but rather a lack of reason not to change).
>  - Finally, the proposed mapping is a superset of the currently used one,
>   and resolves to it if the kill bits granule shift is zero and the
>   granule rate fraction is the same for both base and offset, so
>   backwards compatibility is kept for streams encoded with the existing
>   method.
>  For reference, the code difference between the two methods is:
>  Current method (from libtheora's toplevel.c):
>     ogg_int64_t iframe=granulepos>>pbi->keyframe_granule_shift;
>     ogg_int64_t pframe=granulepos-(iframe<<pbi->keyframe_granule_shift);
>     return (iframe+pframe)*
>       ((double)pbi->info.fps_denominator/pbi->info.fps_numerator);
>  Proposed method (from libkate's kate_granule.c):
>   base=granulepos>>ki->granule_shift;
>   offset=(granulepos-(base<<ki->granule_shift))>>ki->granule_shift2;
>   return base*(kate_float)ki->gps_base_denominator/ki->gps_base_numerator
>       +offset*(kate_float)ki->gps_offset_denominator/ki->gps_offset_numerator;
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