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OGGZ Seek API


Detailed Description

The seeking semantics of the Ogg file format were outlined by Monty in a post to theora-dev in September 2002.

Quoting from that post, we have the following assumptions:

(For more details on the structure of Ogg streams, see the Ogg Basics section).

For data such as media, for which it is possible to provide a mapping such as 'time', OGGZ can efficiently navigate through an Ogg stream by use of an OggzMetric callback, thus allowing automatic seeking to points in 'time'. For common codecs you can ask Oggz to set this for you automatically by instantiating the OGGZ handle with the OGGZ_AUTO flag set. For others you can specify a multiplier with oggz_set_metric_linear(), or a generic non-linear metric with oggz_set_metric().


Typedefs

typedef ogg_int64_t(* OggzMetric )(OGGZ *oggz, long serialno, ogg_int64_t granulepos, void *user_data)
 This is the signature of a function to correlate Ogg streams.


Functions

int oggz_set_metric_linear (OGGZ *oggz, long serialno, ogg_int64_t granule_rate_numerator, ogg_int64_t granule_rate_denominator)
 Specify that a logical bitstream has a linear metric.

int oggz_set_metric (OGGZ *oggz, long serialno, OggzMetric metric, void *user_data)
 Set the OggzMetric to use for an OGGZ handle.

ogg_int64_t oggz_seek_units (OGGZ *oggz, ogg_int64_t units, int whence)
 Seek to a number of units corresponding to the Metric function.

int oggz_set_data_start (OGGZ *oggz, off_t offset)
 Tell OGGZ to remember the given offset as the start of data.

off_t oggz_tell (OGGZ *oggz)
 Provide the file offset in bytes corresponding to the data read.

off_t oggz_seek (OGGZ *oggz, off_t offset, int whence)
 Seek to a specific byte offset.


Typedef Documentation

typedef ogg_int64_t(* OggzMetric)(OGGZ * oggz, long serialno, ogg_int64_t granulepos, void * user_data)
 

This is the signature of a function to correlate Ogg streams.

If every position in an Ogg stream can be described by a metric (eg. time) then define this function that returns some arbitrary unit value. This is the normal use of OGGZ for media streams. The meaning of units is arbitrary, but must be consistent across all logical bitstreams; for example a conversion of the time offset of a given packet into nanoseconds or a similar stream-specific subdivision may be appropriate.

Parameters:
oggz An OGGZ handle
serialno Identifies a logical bitstream within oggz
granulepos A granulepos within the logical bitstream identified by serialno
user_data Arbitrary data you wish to pass to your callback
Returns:
A conversion of the (serialno, granulepos) pair into a measure in units which is consistent across all logical bitstreams within oggz


Function Documentation

off_t oggz_seek OGGZ oggz,
off_t  offset,
int  whence
 

Seek to a specific byte offset.

Parameters:
oggz An OGGZ handle
offset a byte offset
whence As defined in <stdio.h>: SEEK_SET, SEEK_CUR or SEEK_END
Returns:
the new file offset, or -1 on failure.

ogg_int64_t oggz_seek_units OGGZ oggz,
ogg_int64_t  units,
int  whence
 

Seek to a number of units corresponding to the Metric function.

Parameters:
oggz An OGGZ handle
units A number of units
whence As defined in <stdio.h>: SEEK_SET, SEEK_CUR or SEEK_END
Returns:
the new file offset, or -1 on failure.

int oggz_set_data_start OGGZ oggz,
off_t  offset
 

Tell OGGZ to remember the given offset as the start of data.

This informs the seeking mechanism that when seeking back to unit 0, go to the given offset, not to the start of the file, which is usually codec headers. The usual usage is:

oggz_set_data_start (oggz, oggz_tell (oggz));
Parameters:
oggz An OGGZ handle previously opened for reading
offset The offset of the start of data
Returns:
0 on success, -1 on failure.

int oggz_set_metric OGGZ oggz,
long  serialno,
OggzMetric  metric,
void *  user_data
 

Set the OggzMetric to use for an OGGZ handle.

Parameters:
oggz An OGGZ handle
serialno Identify the logical bitstream in oggz to attach this metric to. A value of -1 indicates that this metric should be attached to all unattached logical bitstreams in oggz.
metric An OggzMetric callback
user_data arbitrary data to pass to the metric callback
Returns:
0 Success
Return values:
OGGZ_ERR_BAD_SERIALNO serialno does not identify an existing logical bitstream in oggz, and is not -1
OGGZ_ERR_BAD_OGGZ oggz does not refer to an existing OGGZ
Note:
Specifying values of serialno other than -1 allows you to pass logical bitstream specific user_data to the same metric.

Alternatively, you may use a different metric for each serialno, but all metrics used must return mutually consistent unit measurements.

int oggz_set_metric_linear OGGZ oggz,
long  serialno,
ogg_int64_t  granule_rate_numerator,
ogg_int64_t  granule_rate_denominator
 

Specify that a logical bitstream has a linear metric.

Parameters:
oggz An OGGZ handle
serialno Identify the logical bitstream in oggz to attach this linear metric to. A value of -1 indicates that the metric should be attached to all unattached logical bitstreams in oggz.
granule_rate_numerator The numerator of the granule rate
granule_rate_denominator The denominator of the granule rate
Returns:
0 Success
Return values:
OGGZ_ERR_BAD_SERIALNO serialno does not identify an existing logical bitstream in oggz.
OGGZ_ERR_BAD_OGGZ oggz does not refer to an existing OGGZ

off_t oggz_tell OGGZ oggz  ) 
 

Provide the file offset in bytes corresponding to the data read.

Parameters:
oggz An OGGZ handle
Returns:
The current offset of oggz.
Note:
When reading, the value returned by oggz_tell() reflects the data offset of the start of the most recent packet processed, so that when called from an OggzReadPacket callback it reflects the byte offset of the start of the packet. As OGGZ may have internally read ahead, this may differ from the current offset of the associated file descriptor.


Generated on Wed Jan 21 15:05:41 2004 for liboggz by doxygen 1.3.4