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Topic: header packet length computaion in .ogm (Read 3300 times) previous topic - next topic
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header packet length computaion in .ogm

quick question about .ogm files (ie the changes made by tobias to .ogg to support multi a/v streams).

there's something I'm not clear about in the "specs": at the beginning of non-vorbis streams, for data packets, some bytes are added (see http://tobias.everwicked.com/packfmt.htm/)
In this header are specified, for example, for video if the frame is a key frame or not...

From the spec (ie the link specified before), it looks like it is a fixed size... well, in practice it is not...

My question: how can we know how many bytes are used by this header: it seems to vary from packets to packets ?

By checking, the source code it seems it is based on the packet length.

Is there a way to know the header length without having to find out the packet length ?

header packet length computaion in .ogm

Reply #1 –
You've just found out why AVI-Mux GUI will most likely never support OGM files, not even for reading 

A really simple question, and no one able to help you within over 40 days. What a discontinued format...
 

header packet length computaion in .ogm

Reply #2 –
Quote
My question: how can we know how many bytes are used by this header: it seems to vary from packets to packets ?


One byte for the flags, 'duration_len' bytes for the 'duration of this packets in media units'. duration_len is...

Code: [Select]
/// Some defines from OggDS
#define PACKET_TYPE_HEADER       0x01
#define PACKET_TYPE_COMMENT      0x03
#define PACKET_TYPE_BITS         0x07
#define PACKET_LEN_BITS01        0xc0
#define PACKET_LEN_BITS2         0x02
#define PACKET_IS_SYNCPOINT      0x08

duration_len = (*op.packet & PACKET_LEN_BITS01) >> 6;
duration_len |= (*op.packet & PACKET_LEN_BITS2) << 1;


with op being the Ogg packet (so *op.packet is the packet's first byte). So the duration itself is...

Code: [Select]
if ((duration_len > 0) && (op.bytes >= (duration_len + 1)))
  for (i = 0, duration = 0; i < duration_len; i++) {
    duration = duration << 8;
    duration += *((unsigned char *)op.packet + duration_len - i);
  }