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Using OggzMetric


Detailed Description

An OggzMetric is a helper function for Oggz's seeking mechanism.

Introduction

If every position in an Ogg stream can be described by a metric such as time, then it is possible to define a function that, given a serialno and granulepos, returns a measurement in units such as milliseconds. Oggz will use this function repeatedly while seeking in order to navigate through the Ogg stream.

The meaning of the units is arbitrary, but must be consistent across all logical bitstreams. This allows Oggz to seek accurately through Ogg bitstreams containing multiple logical bitstreams such as tracks of media.

How to set metrics

You don't need to set metrics for Speex, Vorbis, FLAC, Theora, CMML or Annodex. These can be handled automatically by Oggz.

For most others it is simply a matter of providing a "granulerate": a frame or sampling rate, if each packet's granulepos represents a sample number.

Some codecs use a "granuleshift" to divide a granulepos into two halves; the first describing a dependency on a previous packet, the second giving the offset since that packet. This is used to mark keyframes and intermediate frames.

Custom Metrics

For streams with non-linear granulepos, you need to set a custom metric:

Seeking with OggzMetrics

To seek, use oggz_seek_units(). Oggz will perform a ratio search through the Ogg bitstream, using the OggzMetric callback to determine its position relative to the desired unit.

Note:
Many data streams begin with headers describing such things as codec setup parameters. One of the assumptions Monty makes is:

Thus, the first action taken by applications dealing with such data is to read in and cache the decode headers; thereafter the application can safely seek to arbitrary points in the data.

This impacts seeking because the portion of the bitstream containing decode headers should not be considered part of the metric space. To inform Oggz not to seek earlier than the end of the decode headers, use oggz_set_data_start().


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_get_granuleshift (OGGZ *oggz, long serialno)
 Retrieve the granuleshift of a logical bitstream.
int oggz_set_granuleshift (OGGZ *oggz, long serialno, int granuleshift)
 Specify the granuleshift of a logical bitstream.
int oggz_get_granulerate (OGGZ *oggz, long serialno, ogg_int64_t *granulerate_n, ogg_int64_t *granulerate_d)
 Retrieve the granulerate of a logical bitstream.
int oggz_set_granulerate (OGGZ *oggz, long serialno, ogg_int64_t granule_rate_numerator, ogg_int64_t granule_rate_denominator)
 Specify the granulerate of a logical bitstream.
int oggz_set_metric (OGGZ *oggz, long serialno, OggzMetric metric, void *user_data)
 Set the OggzMetric to use for an OGGZ handle.
int oggz_set_data_start (OGGZ *oggz, off_t offset)
 Tell OGGZ to remember the given offset as the start of data.


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

int oggz_get_granulerate OGGZ oggz,
long  serialno,
ogg_int64_t *  granulerate_n,
ogg_int64_t *  granulerate_d
 

Retrieve the granulerate of a logical bitstream.

Parameters:
oggz An OGGZ handle
serialno Identify the logical bitstream in oggz
granulerate_n Return location for the granulerate numerator
granulerate_d Return location for the granulerate denominator
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

int oggz_get_granuleshift OGGZ oggz,
long  serialno
 

Retrieve the granuleshift of a logical bitstream.

Parameters:
oggz An OGGZ handle
serialno Identify the logical bitstream in oggz
Returns:
The granuleshift of the specified logical bitstream.
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

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_granulerate OGGZ oggz,
long  serialno,
ogg_int64_t  granule_rate_numerator,
ogg_int64_t  granule_rate_denominator
 

Specify the granulerate of a logical bitstream.

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

int oggz_set_granuleshift OGGZ oggz,
long  serialno,
int  granuleshift
 

Specify the granuleshift of a logical bitstream.

Parameters:
oggz An OGGZ handle
serialno Identify the logical bitstream in oggz to attach this granuleshift metric to. A value of -1 indicates that the metric should be attached to all unattached logical bitstreams in oggz.
granuleshift The granuleshift
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

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.


Generated on Mon Feb 14 18:00:08 2005 for liboggz by  doxygen 1.4.0