A Sample Transform Filter Declaration

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This sample illustrates a true minimalist filter, which does nothing except demonstrate the least you must implement for a filter. It uses the transform-inplace classes and derives its filter class from the CTransInPlaceFilter class. Following is the class declaration for the derived filter class CNullNull.

// 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::GetSetupData and 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::Transform and does the main work of CNullNull, which in this case is nothing.
CheckInputType Overrides CTransInPlaceFilter::CheckInputType to 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   **Strictly speaking, GetSetupData is required only if you want your filter to be self-registering. However, because the base classes implement this feature and it is easy to implement, it is a good idea to include this in your base class.

Last updated on Wednesday, April 13, 2005

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