[FFmpeg-devel] [PATCH] Enforce ogg granule position monotonicity

[Alex Converse]

On Sat, Feb 14, 2009 at 7:34 PM, David Conrad <lessen42 at gmail.com> wrote:
> On Feb 14, 2009, at 7:01 PM, Alex Converse wrote:
>
>> Hi,
>>
>> I'm new to Ogg but had problems with certain streams seeming to loop
>> forever. These streams were corrupted and had negative granule
>> positions. See inf-loop.ogv. This patch adds a gp sanity check to
>> ogg_read_page.
>>
>> Here is what I know for sure:
>> *"The Granule Position is a signed 64 bit field appearing in the
>> header of every Ogg page." [1] (The fact that gp variables in ffmpeg
>> are almost always uint64_t bothers me.)
>
> vs.
>
>> *"Granule Position must always increase forward or remain equal from
>> page to page, be unset, or be zero for a header page. The absolute
>> time to which any correct sequence of granule position maps must
>> similarly always increase forward or remain equal." Ibid.
>
> Given that the header pages are at the start of a stream (with a
> granule of 0), how can later pages have a negative granule, given that
> they cannot decrease? The special '-1' case is just the simplest way
> to say all 64 bits are 1 (as opposed to saying 0xFFFFFFFFFFFFFFFF).
>

The fact that they specifically mention two's complement on the '-1'
leads me to believe me that this is the case.

>
>
>> *"A special value of '-1' (in two's complement) indicates that no
>> packets finish on this page." [2]
>> *Libtheora rejects negative granule positions [3]
>>
>> What I am uncertain of is what the implications of "unset" are.
>>
>> [1] http://www.xiph.org/ogg/doc/ogg-multiplex.html
>> [2] http://www.xiph.org/ogg/doc/framing.html
>> [3] http://libtheora.sourcearchive.com/documentation/1.0~beta3/
>> group__basefuncs_g95b10e76fc4c05d0240ea2dfd9fd62bd
>> .html#g95b10e76fc4c05d0240ea2dfd9fd62bd
>>
>> Your thoughts would be greatly appreciated, but I know no one here
>> likes Ogg and don't want to get a lot of whining about how
>> "broken"/"strange"/"ass backwards" it is.
>>
>> Regards,
>> Alex Converse
>> <ogg-monotonic-gp.diff>_______________________________________________
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