IMediaPosition Interface

Applications communicating with the filter graph can call methods on this interface to set or retrieve properties such as the duration of the stream, the start and stop times, the preroll time, the rate, and the current position. The filter graph uses these properties on seekable filters to control the playback of streams within the graph; where there are multiple streams, the filter graph sets them all to play in parallel, beginning at the same position, and will report the duration as being the duration of the longest stream. The REFTIME parameters used in this interface are double value, representing a fractional number of seconds. Internally, filters will store time to an accuracy of 100 nanoseconds.

When to Implement

The filter graph manager exposes the IMediaPosition interface if any of the filters within the graph are seekable (can seek to an arbitrary position in the stream). This normally means a seekable file source filter. Filters, such as a file source filter, will expose IMediaPosition if they can seek their data or if their output pin represents a seekable stream. The renderer filter should also expose this interface. Output pins of transform filters expose this interface to pass the positioning information upstream from the renderer through each intermediate filter to the seekable filter.

Use the CMediaPosition class to help implement this interface on a filter. Use the CPosPassThru base class to implement this interface on output pins of transform filters used to pass media positioning information upstream. This is enabled by default in the pin base classes.

When to Use

Applications can use this interface to set or retrieve media positioning properties. Most commonly, an application will use the methods on this interface to play a media stream for some duration starting at some set position in the stream (for example, 10 seconds from the start).

Methods in Vtable Order

IUnknown methods	Description
QueryInterface	Returns pointers to supported interfaces.
AddRef	Increments the reference count.
Release	Decrements the reference count.

IDispatch methods	Description
GetTypeInfoCount	Determines whether there is type information available for this dispinterface.
GetTypeInfo	Retrieves the type information for this dispinterface if GetTypeInfoCount returned successfully.
GetIDsOfNames	Converts text names of properties and methods (including arguments) to their corresponding DISPIDs.
Invoke	Calls a method or accesses a property in this dispinterface if given a DISPID and any other necessary parameters.

IMediaPosition methods	Description
get_Duration	Retrieves the total duration of the media stream.
put_CurrentPosition	Sets the time that the media stream begins.
get_CurrentPosition	Retrieves the current position in terms of the total length of the media stream.
get_StopTime	Retrieves the position within the media stream at which playback should stop.
put_StopTime	Sets the position within the media stream at which playback should stop.
get_PrerollTime	Retrieves the time prior to the start position that the filter graph begins any nonrandom access device rolling.
put_PrerollTime	Sets the time prior to the start position that the filter graph begins any nonrandom access device rolling.
put_Rate	Sets the playback rate, relative to normal playback of the media stream.
get_Rate	Retrieves the playback rate, relative to normal playback of the media stream.
CanSeekForward	Determines if the current position can be moved forward in the media stream.
CanSeekBackward	Determines if the current position can be moved backward in the media stream.

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IMediaPosition::CanSeekBackward

IMediaPosition Interface

Determines if the current position can be moved backward in the media stream.

HRESULT CanSeekBackward(
  LONG *pCanSeekBackward
  );

Parameters

pCanSeekBackward

[out] Set to OATRUE if able to seek backward; otherwise set to OAFALSE.

Return Values

Returns an HRESULT value.

---

IMediaPosition::CanSeekForward

IMediaPosition Interface

Determines if the current position can be moved forward in the media stream.

HRESULT CanSeekForward(
  LONG *pCanSeekForward
  );

Parameters

pCanSeekForward

[out] Set to OATRUE if able to seek forward; otherwise set to OAFALSE.

Return Values

Returns an HRESULT value.

---

IMediaPosition::get_CurrentPosition

IMediaPosition Interface

Retrieves the current position in terms of the total length of the media stream.

HRESULT get_CurrentPosition(
  REFTIME* pllTime
  );

Parameters

pllTime

[out] Reference time of the current position.

Return Values

Returns an HRESULT value.

Remarks

This is the position that playback has reached. It is a value between zero and the total duration of the media (that is, it does not take into account the rate and start time). If the filter graph is paused, this is the position at which it will restart.

When the filter graph is stopped or paused, this method returns the position at which playback will recommence. When the filter graph is running, the filter graph manager returns the position according to the reference clock. If an individual filter implements this, it should return the stream time of the sample it is processing (that is, the offset time from the beginning) when paused or running. If you implement this using stream time or the reference clock, remember to adjust the value you return for start position and playback rate so that the value returned is in terms of the media's total duration.

After stopping or pausing, a run command causes playback to begin at the current position. This will be where playback stopped or paused, unless there has been an IMediaPosition::put_CurrentPosition call in the meantime.

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IMediaPosition::get_Duration

IMediaPosition Interface

Retrieves the total duration of the media stream.

HRESULT get_Duration(
  REFTIME* plength
  );

Parameters

plength

[out] Returned length of the media stream.

Return Values

Returns an HRESULT value.

Remarks

The duration assumes normal playback speed, and it is therefore unaffected by the rate.

---

IMediaPosition::get_PrerollTime

IMediaPosition Interface

Retrieves the time prior to the start position that any nonrandom access device should start rolling.

HRESULT get_PrerollTime(
  REFTIME* pllTime
  );

Parameters

pllTime

[out] Returned preroll time as a double pointer value.

Return Values

Returns an HRESULT value.

Remarks

Preroll time is the time prior to the start position at which nonrandom access devices, such as tape players, should start rolling.

---

IMediaPosition::get_Rate

IMediaPosition Interface

Retrieves the rate of playback relative to normal playback speed.

HRESULT get_Rate(
  double * pdRate
  );

Parameters

pdRate

[out] Returned rate.

Return Values

Returns an HRESULT value.

Remarks

A rate of 1.0 indicates normal playback speed. A rate of 0.5 indicates half speed. A rate of –1.0 indicates normal speed in reverse.

---

IMediaPosition::get_StopTime

IMediaPosition Interface

Retrieves the time at which the media stream stops.

HRESULT get_StopTime(
  REFTIME* pllTime
  );

Parameters

pllTime

[out] Returned stop time as a double pointer value.

Return Values

Returns an HRESULT value.

Remarks

The stop time is a position between zero and the duration of the media at which playback should stop.

The stop position is applied before the rate and therefore is the position at typical playback speed.

---

IMediaPosition::put_CurrentPosition

IMediaPosition Interface

Sets the time that the media stream begins.

HRESULT put_CurrentPosition(
  REFTIME llTime
  );

Parameters

llTime

[in] Start time expressed as a double value.

Return Values

Returns an HRESULT value.

Remarks

The start time is a position between zero and the duration of the media at which playback should begin when the next run command is issued.

If this method is called when the filter graph manager is running, the filter graph manager will pause the graph, run the method, and then issue a new run command.

If called when the filter graph is paused, this method must flush existing data by using IPin::BeginFlush and IPin::EndFlush before pushing the new data (at the new current position).

Setting the current position when paused or stopped causes playback to resume from the new start position when the run command is issued.

The current position is applied before the rate and therefore is the position at typical playback speed.

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IMediaPosition::put_PrerollTime

IMediaPosition Interface

Sets the time prior to the start position that any nonrandom access device should start rolling.

HRESULT put_PrerollTime(
  REFTIME llTime
  );

Parameters

llTime

[in] Preroll time to be set.

Return Values

Returns an HRESULT value.

Remarks

Preroll time is the time prior to the start position at which nonrandom access devices, such as tape players, should start rolling.

---

IMediaPosition::put_Rate

IMediaPosition Interface

Sets the rate of playback relative to normal speed.

HRESULT put_Rate(
  double dRate
  );

Parameters

dRate

[in] Rate to set.

Return Values

Returns an HRESULT value.

Remarks

This method allows an application to speed up or slow down playback relative to the normal default playback speed. A rate of 1.0 indicates normal playback speed. Specifying 2.0 causes playback at twice the normal rate: a video created for 10 frames per second (fps) will be played back at 20 fps, if resources permit. Audio streams played back at above-normal speed increase the pitch rather than drop samples.

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IMediaPosition::put_StopTime

IMediaPosition Interface

Sets the time at which the media stream will stop.

HRESULT put_StopTime(
  REFTIME llTime
  );

Parameters

llTime

[in] Stop time as a double value.

Return Values

Returns an HRESULT value.

Remarks

The stop time is a position between zero and the duration of the media at which playback should stop.

The stop position is applied before the rate and therefore is the position at typical playback speed.

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