I have been having a tough time with an IP camera that serves out MPEG4. I have dug into the headers and all that bit parseing and I think I understand why the timestamps are messed up. It has no "Group Of Video Object Plane" headers, but that is ok, they are "optional" But the vop_time_increment rolls over and the modulo_time_base is ALWAYS 0. :(
My question is about the if condition in
MPEG4VideoStreamParser::parseVideoObjectPlane() that handles these kinds of
"buggy" mpeg 4 streams.
It reads:
...
} else {
if (newTotalTicks < fPrevNewTotalTicks && vop_coding_type != 2/*B*/
&& modulo_time_base == 0 && vop_time_increment == 0 && !fJustSawTimeCode)
{
// This is another kind of buggy MPEG-4 video stream, in which
// "vop_time_increment" wraps around, but without
// "modulo_time_base" changing (or just having had a new time code).
// Overcome this by pretending that "vop_time_increment" *did* wrap around:
#ifdef DEBUG
fprintf(stderr, "Buggy MPEG-4 video stream: \"vop_time_increment\" wrapped
around, but without \"modulo_time_base\" changing!\n");
#endif
++fSecondsSinceLastTimeCode;
newTotalTicks += vop_time_increment_resolution;
...
In my case vop_time_increment is never (or very rarely) exactly 0. It is
usually a number below 100. Also the ticks case is never true.
When I add a last_vop_time_increment variable and change the test I can get it
to execute this code with improved results.
...
if ((vop_time_increment - last_vop_time_increment_ < 0) && vop_coding_type !=
2/*B*/
&& modulo_time_base == 0 && !fJustSawTimeCode_)
...
It still has a negative jump in the timestamps, but they are at least now very
predictable and patterned.
Is MPEG4 just that way? Messy group of non-compliant streams?
Jeff Shanab, Manager, Software Engineering
D 630.633.4515 | C 630.453.7764 | F 630.633.4815 | jsha...@smartwire.com
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