Hi Craig,
On #1, I think we're good -- you can only have one sequence_extension
following a sequence_header. See 13818-2, clause 6.3.1
("sequence_extension() shall only occur immediately following a
sequence_header()").
On #2, I think that's a real bug (that would, as you point out, only
affect bitstreams not conforming to a defined profile). I agree it should
be (((buffer[ 5 ] >> 5) & 0x03) + 1).
-Keith
On Sat, 3 May 2008, Craig Milo Rogers wrote:
> Hello. I'm browsing through the SVN trunk for libmpeg2,
> revision 1171. My reference document is ISO 13818-2. I'm looking at the
> frame rate calculation in "libmpeg2/header.c". I understand that
> libmpeg2 calculates a reciprocal, called frame_period. What bothers
> me is the frame rate extension adjustment in the routine sequence_ext,
> in the following line:
>
> sequence->frame_period =
> sequence->frame_period * ((buffer[5]&31)+1) / (((buffer[5]>>2)&3)+1);
>
>
> 1) It modifies frame_period in place. If it is possible to have
> two or more extension headers after a single sequence header
> (I haven't yet figured out if the ISO spec says anything about
> this possibility), then it seems to me that each frame rate
> (frame_period) adjustment should start with the value from the
> last sequence header (from my reading of section 6.3.3 of the spec).
> The current libmpeg2 code would compound them, and this seems
> incorrect.
>
> 2) Based on section 6.2.2.3 of ISO 13818-2, frame_rate_extension_n
> is a 2-bit field located before the 5-bit frame_rate_extension_d
> field. That implies that the correct way to extract this field
> is:
>
> ((buffer[5]>>5)&3)
>
> not:
>
> ((buffer[5]>>2)&3)
>
>
> According to the standard, these fields should be 0 for
> standard profiles (table 8-5, table E-3), but I imagine that libmpeg2
> should apply these adjustment factors as defined by the specification
> in case there are video streams in which they are nonzero (Sony comes
> to mindm based on patent applications).
>
> Craig Milo Rogers
>
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