A Sample Transform Filter Declaration

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A version of this page is also available for

Windows Embedded CE 6.0 R3

4/8/2010

This sample illustrates a true minimalist filter, which demonstrates the least you must implement for a filter. It uses the transform-inplace classes and derives its filter class from the CTransInPlaceFilterclass.

Following is the class declaration for the derived filter class CNullNull.

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// CNullNull // class CNullNull : public CTransInPlaceFilter { public: static CUnknown CreateInstance(LPUNKNOWN punk, HRESULT phr); DECLARE_IUNKNOWN; LPAMOVIESETUP_FILTER GetSetupData() { return &sudNullNull; } private: // Constructor. Just calls the base class constructor. CNullNull(TCHAR tszName, LPUNKNOWN punk, HRESULT phr) : CTransInPlaceFilter (tszName, punk, CLSID_NullNull, phr) { } // Overrides the pure virtual Transform of CTransInPlaceFilter base lass. // The "real work" of a transform is done by altering pSample. // The Null transform leaves it alone. HRESULT Transform(IMediaSample pSample){ return NOERROR; } // This filter accepts any input type. // (It would return S_FALSE for any it did not accept.) HRESULT CheckInputType(const CMediaType* mtIn) { return S_OK; } };

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// CNullNull
//
class CNullNull
	: public CTransInPlaceFilter
{

public:

	static CUnknown *CreateInstance(LPUNKNOWN punk, HRESULT *phr);

	DECLARE_IUNKNOWN;

	LPAMOVIESETUP_FILTER GetSetupData()
	{
		return &sudNullNull;
}

private:

	// Constructor. Just calls the base class constructor.
	CNullNull(TCHAR *tszName, LPUNKNOWN punk, HRESULT *phr)
		: CTransInPlaceFilter (tszName, punk, CLSID_NullNull, phr)
	{ }

	// Overrides the pure virtual Transform of CTransInPlaceFilter
base 
lass.
	// The "real work" of a transform is done by altering *pSample.

	// The Null transform leaves it alone.
	HRESULT Transform(IMediaSample *pSample){ return NOERROR; }

	// This filter accepts any input type. 
	// (It would return S_FALSE for any it did not accept.)
	HRESULT CheckInputType(const CMediaType* mtIn) { return S_OK; }
};

This example illustrates the following basic member functions required in the base class.

Member function	Description
CreateInstance	Needed by every filter so that it can be instantiated as a COM object.
GetSetupData	Overrides CBaseFilter::GetSetupDataand is used to provide the class with information required to register this particular filter. In this case, it provides the address of a structure defined in the Nullnull.cpp file included in the SDK.
CNullNull	Class constructor, which typically just calls the base class constructor.
Transform	Overrides CTransInPlaceFilter::Transformand does the main work of CNullNull, which in this case is nothing.
CheckInputType	Overrides CTransInPlaceFilter::CheckInputTypeto verify the media type during connection, and in this case accepts any media type offered, because it simply passes it along to the next filter in line.

Note:
GetSetupData is only required if you want your filter to be self-registering. However, because the base classes implement this behavior and it is easy to implement, it is good to include this in your base class.