Tracking
Tracking is a timed text codec. It allows to localize a time-continuously sampled data file by providing geolocalisation from a GPS device. This geolocalisation is NMEA sentences and multimedia players have to parse them to extract informations (position, speed, orientation, satellites informations, ...).
Tracking is appropriate for use with all Ogg media formats. This description explains how to map Tracking into Ogg.
Tracking specification
Before describing the actual data that goes into a logical Ogg bitstream, we need to understand what the stand-alone "codec" packets contains.
Tracking basically consists of data elements which contains NMEA sentences
An example Tracking file looks like this:
<?xml version="1.0"?> <track xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance" xsi:noNamespaceSchemaLocation="track.xsd"> <data type="NMEA" position="0"> $GPRMC,162254.00,A,3723.02837,N,12159.39853,W,0.820,188.36,110706,,,A*74 $GPVTG,188.36,T,,M,0.820,N,1.519,K,A*3F $GPGGA,162254.00,3723.02837,N,12159.39853,W,1,03,2.36,525.6,M,-25.6,M,,*65 $GPGSA,A,2,25,01,22,,,,,,,,,,2.56,2.36,1.00*02 $GPGSV,4,1,14,25,15,175,30,14,80,041,,19,38,259,14,01,52,223,18*76 $GPGSV,4,2,14,18,16,079,,11,19,312,,14,80,041,,21,04,135,25*7D $GPGSV,4,3,14,15,27,134,18,03,25,222,,22,51,057,16,09,07,036,*79 $GPGSV,4,4,14,07,01,181,,15,25,135,*76 $GPGLL,3723.02837,N,12159.39853,W,162254.00,A,A*7C $GPZDA,162254.00,11,07,2006,00,00*63 </data> <data type="NMEA" position="1"> $GPRMC,162254.00,A,3723.02837,N,12159.39853,W,0.820,188.36,110706,,,A*74 $GPVTG,188.36,T,,M,0.820,N,1.519,K,A*3F $GPGGA,162254.00,3723.02837,N,12159.39853,W,1,03,2.36,525.6,M,-25.6,M,,*65 $GPGSA,A,2,25,01,22,,,,,,,,,,2.56,2.36,1.00*02 $GPGSV,4,1,14,25,15,175,30,14,80,041,,19,38,259,14,01,52,223,18*76 $GPGSV,4,2,14,18,16,079,,11,19,312,,14,80,041,,21,04,135,25*7D $GPGSV,4,3,14,15,27,134,18,03,25,222,,22,51,057,16,09,07,036,*79 $GPGSV,4,4,14,07,01,181,,15,25,135,*76 $GPGLL,3723.02837,N,12159.39853,W,162254.00,A,A*7C $GPZDA,162254.00,11,07,2006,00,00*63 </data> <data> ... </data> </track>
The data elements have two required properties:
- type: the type of data stored (only NMEA is supported yet)
- position: the position of the data in the video (in seconds)
Tracking mapping into Ogg
When Tracking is mapped into an Ogg logical bitstream it needs to be serialised first. XML is a hierarchical file format, so is not generally serialisable. However, Tracking has been designed to be serialised easily.
All of the Tracking bitstream information is text. As it gets encoded into a binary bitstream, an encoding format has to be specified. To simplify things, UTF-8 is defined as the mandatory encoding format for all data in a Tracking binary bitstream.
The media mapping for Tracking into Ogg is as follows:
- The bos page contains a Tracking ident packet.
- The content or data packets contain the "data" tags each encoded in their own packet and inserted at the accurate time.
- The eos page contains a packet with an empty data tag.
The Tracking ident header packet
The Tracking logical bitstream starts with an ident header which is encapsulated into the Tracking bos page. The ident header contains all information required to identify the Tracking bitstream and to set up a Tracking decoder. It has the following format:
0 1 2 3 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1| Byte +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | Identifier 'TRCK\0\0\0\0' | 0-3 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | | 4-7 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | Version major | Version minor | 8-11 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | ...
The Trackingversion as described here is major=0 minor=1.
The Tracking data packets
The data packets of the Tracking bitstream contain the Tracking data elements. Their position attributes however only exist for authoring purposes and are not copied into the bitstream (to avoid contradictory information), but are rather represented through the time mapping of the encapsulation format that interleaves Tracking data with data from other time-continuous bitstreams. Generally the time mapping is done through some timestamp representation and through the position in the stream.
A data tag is encoded with all tags (except for the position attribute) as a string printed into a clip packet. The data tag's position attribute tells the encapsulator at what time to insert the clip packet into the bitstream. It duration is one second.
0 1 2 3 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1| Byte +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | <data ... | 0- +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | ... | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Development
External links
- NMEA sentences: [1]