CBaseMediaFilter Class

This is an abstract base class that provides support for the IMediaFilter interface. The CBaseMediaFilter class handles State_Stopped, State_Paused, and State_Running state transitions. Typically, this class is used for plug-in distributors rather than filters with pins. Derive your filter classes from the CBaseFilter class (or base classes derived from this) instead of from this class.

All member functions in this class that return HRESULT and accept a pointer as a parameter return E_POINTER when passed a null pointer.

Protected Data Members

Name	Description
m_clsid	Class identifier (CLSID) used for serialization using IPersist.
m_pClock	Pointer to a reference clock used for synchronization. The reference count of the clock object must be incremented using AddRef. Pass NULL if no reference clock is available.
m_State	Current state of the filter, which can be State_Stopped, State_Paused, or State_Running.
m_tStart	Offset from the stream time to the reference time.

Member Functions

Name	Description
CBaseMediaFilter	Constructs a CBaseMediaFilter object.
IsActive	Determines if the filter is currently active (running or paused) or stopped.

Overridable Member Functions

Name	Description
StreamTime	Returns the current stream time.

Implemented IPersist Methods

Name	Description
GetClassID	Returns the class identifier of this filter.

Implemented IMediaFilter Methods

Name	Description
GetState	Retrieves the current state of the filter.
GetSyncSource	Retrieves the current reference clock in use by this filter.
Pause	Instructs the filter to transition to the new (paused) state.
Run	Instructs the filter to transition to the new (running) state.
SetSyncSource	Informs the filter of the reference clock with which it should synchronize activity.
Stop	Instructs the filter to transition to the new (stopped) state.

Implemented INonDelegatingUnknown Methods

Name	Description
NonDelegatingQueryInterface	Passes out references to interfaces supported by CBaseFilter. Override this to pass out pointers to interfaces supported in a derived filter class.

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CBaseMediaFilter::CBaseMediaFilter

CBaseMediaFilter Class

Constructs a CBaseMediaFilter object.

CBaseMediaFilter(
  TCHAR *pName,
  LPUNKNOWN pUnk,
  CCritSec *pLock,
  REFCLSID clsid
  );

Parameters

pName

Name of the CBaseMediaFilter class.

pUnk

IUnknown interface of the delegating object.

pLock

Pointer to the object that maintains the lock.

clsid

Class identifier used to serialize this filter.

Return Values

No return value.

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CBaseMediaFilter::GetClassID

CBaseMediaFilter Class

Fills the pClsID parameter with the class identifier of this filter (from m_clsid).

HRESULT GetClassID(
  CLSID *pClsID
  );

Parameters

pClsID

Pointer to the class identifier to be filled out.

Return Values

Returns an HRESULT value.

---

CBaseMediaFilter::GetState

CBaseMediaFilter Class

Retrieves the current state of the filter.

HRESULT GetState(
  DWORD dwMilliSecsTimeout,
  FILTER_STATE * State
  );

Parameters

dwMilliSecsTimeout

Duration of the time-out, in milliseconds.

State

Returned state of the filter.

Return Values

Returns S_OK.

Remarks

This member function implements the IMediaFilter::GetState method. It returns the value of the m_State data member.

Filters should derive their filters from CBaseFilter and not from CBaseMediaFilter, so filters will not likely use this member function. Use CBaseFilter::GetState instead.

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CBaseMediaFilter::GetSyncSource

CBaseMediaFilter Class

Retrieves the current reference clock in use by this filter.

HRESULT GetSyncSource(
  IReferenceClock ** pClock
  );

Parameters

pClock

Pointer to a reference clock; will be set to the IReferenceClock interface.

Return Values

Returns an HRESULT value

Remarks

This member function implements the IMediaFilter::GetSyncSource method. It returns the value of m_pClock after adding a reference to it. Be sure to release the interface by calling the IUnknown::Release method when finished with the pointer.

Filters should derive their filters from CBaseFilter and not from CBaseMediaFilter, so filters will not likely use this member function. Use CBaseFilter::GetSyncSource instead.

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CBaseMediaFilter::IsActive

CBaseMediaFilter Class

Determines if the filter is currently active (running or paused) or stopped.

BOOL IsActive(void);

Return Values

Returns TRUE if the filter is paused or running, or FALSE if it is stopped.

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CBaseMediaFilter::NonDelegatingQueryInterface

CBaseMediaFilter Class

Retrieves an interface and increments the reference count.

HRESULT NonDelegatingQueryInterface(
  REFIID riid,
  void ** ppv
  );

Parameters

riid

Reference identifier.

ppv

Pointer to the interface.

Return Values

Returns E_POINTER if ppv is invalid. Returns NOERROR if the query is successful or E_NOINTERFACE if it is not.

Remarks

This member function implements the INonDelegatingUnknown::NonDelegatingQueryInterface method and passes out references to the IMediaFilter, IPersist, and IUnknown interfaces. Override this class to return other interfaces on the object in the derived class.

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CBaseMediaFilter::Pause

CBaseMediaFilter Class

Transitions the filter to State_Paused state if it is not in this state already.

HRESULT Pause (void);

Return Values

Returns an HRESULT return value (S_OK by default).

Remarks

This member function implements the IMediaFilter::Pause method. It sets the value of m_State to State_Paused.

Note that filters should derive their filters from CBaseFilter and not from CBaseMediaFilter, so this member function will not likely be used by filters. Use CBaseFilter::Pause instead.

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CBaseMediaFilter::Run

CBaseMediaFilter Class

Transitions the filter to State_Running state if it is not in this state already.

HRESULT Run (
  REFERENCE_TIME tStart
  );

Parameters

tStart

Reference time value corresponding to stream time 0.

Return Values

Returns an HRESULT value.

Remarks

If the filter is in State_Stopped state, the CBaseMediaFilter::Pause member function is called first to transition the filter to State_Paused state, which has the effect of activating any of the filter's connected pins. If any pin returns a failure return code from its CBasePin::Active member function, the function fails and the state is not changed. If this member function succeeds, the filter's m_State member variable is set to State_Running. This member function holds the filter's lock.

Filters should derive their filters from CBaseFilter and not from CBaseMediaFilter, so filters will not likely use this member function. Use CBaseFilter::Run instead.

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CBaseMediaFilter::SetSyncSource

CBaseMediaFilter Class

Identifies the reference clock to which the filter should synchronize activity.

HRESULT SetSyncSource(
  IReferenceClock * pClock
  );

Parameters

pClock

Pointer to the IReferenceClock interface.

Return Values

Returns an HRESULT value.

Remarks

This member function implements the IMediaFilter::SetSyncSource method. It sets the m_pClock data member to the pClock parameter and increments the reference count on the IReferenceClock interface passed in.

This member function is most important to rendering filters and might not apply to other filters.

Filters should derive their filters from CBaseFilter and not from CBaseMediaFilter, so filters will not likely use this member function. Use CBaseFilter::SetSyncSource instead.

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CBaseMediaFilter::Stop

CBaseMediaFilter Class

Transitions the filter to State_Stopped state if it is not in this state already.

HRESULT Stop(void);

Return Values

Returns an HRESULT value.

Remarks

This member function implements the IMediaFilter::Stop method. It sets the m_State member variable to State_Stopped.

Note that filters should derive their filters from CBaseFilter and not from CBaseMediaFilter, so this member function will not likely be used by filters. Use CBaseFilter::Stop instead.

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CBaseMediaFilter::StreamTime

CBaseMediaFilter Class

Retrieves the current stream time.

virtual HRESULT StreamTime(
  CRefTime& rtStream
  );

Parameters

rtStream

Current stream time.

Return Values

Returns an HRESULT value, which can include the following values.

Value	Meaning
E_FAIL	Unable to get time from clock.
S_OK	Stream time returned in the rtStream parameter.
VFW_E_NO_CLOCK	No reference clock is available.

Remarks

Current stream time is the reference clock time minus the stream time offset. All samples with time stamps less than or equal to this time should have been presented.

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