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
-------------------------------------------------------------------------
This SF.net email is sponsored by the 2008 JavaOne(SM) Conference
Don't miss this year's exciting event. There's still time to save $100.
Use priority code J8TL2D2.
http://ad.doubleclick.net/clk;198757673;13503038;p?http://java.sun.com/javaone
_______________________________________________
Libmpeg2-devel mailing list
Libmpeg2-devel@lists.sourceforge.net
https://lists.sourceforge.net/lists/listinfo/libmpeg2-devel
