Implementing Callback Objects for Windows Media

Implementing Callback Objects for Windows Media

The callback class, CASFCallback, is implemented entirely inside its class definition. It inherits interfaces from both IWMStatusCallback, which is used during frame indexing, and IAMWMBufferPassCallback, which is used when adding SMPTE timecode metadata to the file:

class CASFCallback : public IWMStatusCallback, public IAMWMBufferPassCallback { public: // We create the object with a ref count of zero // because we know that no one // will try to use it before we call QI. CASFCallback(): m_refCount(0) { phr = NULL ; hEvent = NULL; } ~CASFCallback() { DbgLog((LOG_TRACE, 3, _T("Deleting CASFCallback! refCount=0x%x\n"), m_refCount)); } // IAMWMBufferPassCallback // This method is called by the WM ASF Writer's video input pin // after it receives each sample but before the sample is encoded. // In this example we only set the timecode property. // You can extend this to set or get any number of properties // on the sample. virtual HRESULT STDMETHODCALLTYPE Notify(INSSBuffer3* pNSSBuffer3, IPin* pPin, REFERENCE_TIME* prtStart, REFERENCE_TIME* prtEnd ) { WMT_TIMECODE_EXTENSION_DATA SMPTEExtData; ZeroMemory( &SMPTEExtData, sizeof( SMPTEExtData ) ); // wRange is already zero, // but we set it explicitly here to show that // in this example, we just have one range for the entire file. SMPTEExtData.wRange = 0; DWORD carryOver = 0; // Convert REFERENCE_TIME to seconds for convenience. DWORD dwStartTimeInSeconds = (DWORD)(*prtStart / 10000000 ); DWORD dwSeconds = dwStartTimeInSeconds % 60; DWORD dwHours = dwStartTimeInSeconds / 3600; DWORD dwMinutes = dwStartTimeInSeconds / 60; // If we are at > 60 minutes then we do one additional calculation // so that minutes correctly starts back at zero after 59. if (dwHours) { dwMinutes = dwMinutes % 60; } // SMPTE frames are 0-based. // Also, our sample is hard-coded for 30 fps. DWORD dwFrames = (DWORD) ((*prtStart % 10000000) / 333333); // // The timecode values are stored in the ANSI SMPTE format. // // BYTE MSB LSB // ---------------------------------------------- // 1 Tens of hour Hour // 2 Tens of minute Minute // 3 Tens of second Second // 4 Tens of frame Frame // // For example, 01:19:30:01 would be represented as 0x01193001. SMPTEExtData.dwTimecode = ( ( dwFrames / 10 ) << 4 ) ( dwFrames % 10 ); SMPTEExtData.dwTimecode = ( ( dwSeconds / 10 ) << 12 ) ( ( dwSeconds % 10 ) << 8 ); SMPTEExtData.dwTimecode = ( ( dwMinutes / 10 ) << 20 ) ( ( dwMinutes % 10 ) << 16 ); SMPTEExtData.dwTimecode = ( ( dwHours / 10 ) << 28 ) ( ( dwHours % 10 ) << 24 ); HRESULT hr = pNSSBuffer3->SetProperty(WM_SampleExtensionGUID_Timecode, (void*) &SMPTEExtData, WM_SampleExtension_Timecode_Size); if(FAILED(hr)) { DbgLog((LOG_TRACE, 3, _T("Failed to SetProperty! hr=0x%x\n"), hr)); } // The calling pin ignores the HRESULT. return hr; } // IWMStatusCallback virtual HRESULT STDMETHODCALLTYPE OnStatus( /* [in] */ WMT_STATUS Status, /* [in] */ HRESULT hr, /* [in] */ WMT_ATTR_DATATYPE dwType, /* [in] */ BYTE __RPC_FAR *pValue, /* [in] */ void __RPC_FAR *pvContext) { switch ( Status ) { case WMT_INDEX_PROGRESS: // Display the indexing progress as a percentage. // Use "carriage return" (\r) to reuse the status line. DbgLog((LOG_TRACE, 3, _T("Indexing in progress (%d%%)\r"), *pValue)); break ; case WMT_CLOSED: *phr = hr; SetEvent(hEvent) ; DbgLog((LOG_TRACE, 3, _T("\n"))); break; case WMT_ERROR: *phr = hr; SetEvent(hEvent) ; DbgLog((LOG_TRACE, 3, _T("\nError during indexing operation! hr=0x%x\n"), hr)); break; // Ignore these messages. case WMT_OPENED: case WMT_STARTED: case WMT_STOPPED: break; } return S_OK; } ULONG STDMETHODCALLTYPE AddRef( void ) { m_refCount++; DbgLog((LOG_TRACE, 3, _T("CASFCallback::AddRef! refCount=0x%x\n"), m_refCount)); return 1; } ULONG STDMETHODCALLTYPE Release( void ) { m_refCount--; DbgLog((LOG_TRACE, 3, _T("CASFCallback::Release! refCount=0x%x\n"), m_refCount)); if(m_refCount == 0) delete this; return 1; } HRESULT STDMETHODCALLTYPE QueryInterface( /* [in] */ REFIID riid, /* [iid_is][out] */ void __RPC_FAR *__RPC_FAR *ppvObject) { if ( riid == IID_IWMStatusCallback ) { *ppvObject = ( IWMStatusCallback * ) this; } else if (riid == IID_IAMWMBufferPassCallback) { *ppvObject = ( IAMWMBufferPassCallback * ) this; } else { return E_NOINTERFACE; } AddRef(); return S_OK; } public: HANDLE hEvent ; HRESULT *phr ; int m_refCount; };

Because the CASFCallback object has ancestors in both IWMStatus Callback and IAMWMBufferPassCallback, it must implement its QueryInterface method so it can disambiguate between calls on each interface, returning the correct interface while maintaining the reference count. The IWMStatus Callback interface must have an implementation of its callback method, On Status, and IAMWMBufferPassCallback must implement the Notify method. The OnStatus method tracks the progress of the indexing operation, informing the application when the index has been completed (or, alternately, if it fails).

The Notify method handles the SMPTE timecode data unit extension generation. The callback receives a pointer to an INSSBuffer3 interface. This interface allows you to set or read properties on an individual sample (in this case, a video frame). The timecode string is generated (in the weird ANSI format discussed in Chapter 6) and then applied to the sample with the INSSBuffer3 method SetProperty. This method is supplied with the GUID of the property (which matches the GUID of the property as initialized in AddSmpteDataUnit Extention), a pointer to the SMPTE data, and the length of the data. Every time the callback executes that is, every time a sample is received by the WM ASF Writer another SMPTE timecode data packet is inserted into the stream.