A Sample Transform Filter Declaration (Windows CE 5.0)
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 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 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.
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