Use ltmmCapture to Perform Simple Video and Audio Capture

The ltmmCapture object allows the user to capture data from video and audio hardware devices.

  1. To begin simple capturing you will first need to create an instance of the ltmmCapture class. This is accomplished using the Win32 CoCreateInstance function as follows:

C Source 

IltmmCapture* pCapture; 
CoCreateInstance(&CLSID_ltmmCapture, NULL, CLSCTX_INPROC_SERVER, &IID_IltmmCapture, (void**)&pCapture);

C++ Source 

IltmmCapture* pCapture; 
CoCreateInstance(CLSID_ltmmCapture, NULL, CLSCTX_INPROC_SERVER, IID_IltmmCapture, (void**)&pCapture);

  1. Define a window to send capture status messages:

C Source 

HWND hwndNotify; 
#define WM_CAPTURENOTIFY (WM_USER + 1000)  
IltmmCapture_SetNotifyWindow(pCapture, (long)hwndNotify, WM_CAPTURENOTIFY);

C++ Source 

HWND hwndNotify; 
#define WM_CAPTURENOTIFY (WM_USER + 1000)  
pCapture->SetNotifyWindow((long)hwndNotify, WM_CAPTURENOTIFY);

The above code instructs the capture object to send WM_CAPTURENOTIFY messages to the window procedure for hwndNotify. The wParam parameter of the window message will contain the notification code.

Note: A capture can be accomplished without the use of the notification window, but the user would be required to poll the objects state to determine when the capture has finished.

  1. Define a frame window for the video preview:

C Source 

HWND  hwndFrame; 
IltmmCapture_put_VideoWindowFrame(pCapture, (long)hwndFrame);

C++ Source 

HWND hwndFrame; 
pCapture->put_VideoWindowFrame((long)hwndFrame);

The user is required to create the window that serves as the video frame. ltmmCapture will subclass this window, so there is no need to forward any messages to the ltmmCapture object. By default, ltmmCapture will automatically maximize the video within the frame window. The video will automatically resize when the frame window size changes.

Note: The notification window and the video frame window can be the same.

  1. Define audio and video devices to be used as the capture source. The ltmmCapture object contains audio and video device collection objects. These objects allow the user to enumerate the registered audio and video devices and to select which devices to capture.

  2. Enumerate the registered video devices:

C Source 

IltmmDevices* pDevices; 
long lCount; 
long i; 
// get an interface into video devices collection 
IltmmCapture_get_VideoDevices(pCapture, &pDevices); 
// get the total number of registered devices 
IltmmDevices_get_Count(pDevices, &lCount); 
// enumerate all of the devices 
for (i = 0; i < lCount; i++) 
{ 
   IltmmDevice* pDevice; 
   BSTR bstrName; 
   BSTR bstrFriendlyName; 
   VARIANT_BOOL fSelected; 
   // get an interface for the ith device 
   IltmmDevices_Item(pDevices, i, &pDevice); 
   // get the devices "full name" (used to uniquely identify the device) 
   IltmmDevice_get_Name(pDevice, &bstrName); 
   // get the devices "friendly name"  (used for readable display) 
   IltmmDevice_get_FriendlyName(pDevice, &bstrFriendlyName); 
   // query whether the device is currently selected 
   IltmmDevice_get_Selected(pDevice, &fSelected); 
   // 
   // add additional code here 
   // 
   // free strings 
   SysFreeString(bstrName); 
   SysFreeString(bstrFriendlyName); 
   // release the device interface 
   IUnknown_Release(pDevice); 
} 
// release the devices interface 
IUnknown_Release(pDevices);

C++ Source 

IltmmDevices* pDevices; 
long lCount; 
long i; 
// get an interface into video devices collection 
pCapture->get_VideoDevices(&pDevices); 
// get the total number of registered devices 
pDevices->get_Count(&lCount); 
// enumerate all of the devices 
for (i = 0; i < lCount; i++) 
{ 
   IltmmDevice* pDevice; 
   BSTR bstrName; 
   BSTR bstrFriendlyName; 
   VARIANT_BOOL fSelected; 
   // get an interface for the ith device 
   pDevices->Item(i, &pDevice); 
   // get the devices "full name" (used to uniquely identify the device) 
   pDevice->get_Name(&bstrName); 
   // get the devices "friendly name"  (used for readable display) 
   pDevice->get_FriendlyName(&bstrFriendlyName); 
   // query whether the device is currently selected 
   pDevice->get_Selected(&fSelected); 
   // 
   // add additional code here 
   // 
   // free strings 
   SysFreeString(bstrName); 
   SysFreeString(bstrFriendlyName); 
   // release the device interface 
   pDevice->Release(); 
} 
// release the devices interface 
pDevices->Release();

  1. The registered audio devices can be enumerated with the following code:

C Source 

IltmmDevices* pDevices; 
long lCount; 
long i; 
// get an interface into audio devices collection 
IltmmCapture_get_AudioDevices(pCapture, &pDevices); 
// get the total number of registered devices 
IltmmDevices_get_Count(pDevices, &lCount); 
// enumerate all of the devices 
for (i = 0; i < lCount; i++) 
{ 
   IltmmDevice* pDevice; 
   BSTR bstrName; 
   BSTR bstrFriendlyName; 
   VARIANT_BOOL fSelected; 
   // get an interface for the   ith device 
   IltmmDevices_Item(pDevices, i, &pDevice); 
   // get the devices "full name" (used to uniquely identify the device) 
   IltmmDevice_get_Name(pDevice, &bstrName); 
   // get the devices "friendly name"  (used for readable display) 
   IltmmDevice_get_Selected(pDevice, bstrFriendlyName); 
   // query whether the device is currently selected 
   IltmmDevice_get_Selected(pDevice, &fSelected); 
   // 
   // add additional code here 
   // 
   // free strings 
   SysFreeString(bstrName); 
   SysFreeString(bstrFriendlyName); 
   // release the device interface 
   IUnknown_Release(pDevice); 
} 
// release the devices interface 
IUnknown_Release(pDevices);

C++ Source 

IltmmDevices* pDevices; 
long lCount; 
long i; 
// get an interface into audio devices collection 
pCapture->get_AudioDevices(&pDevices); 
// get the total number of registered devices 
pDevices->get_Count(&lCount); 
// enumerate all of the devices 
for (i = 0; i < lCount; i++) 
{ 
   IltmmDevice* pDevice; 
   BSTR bstrName; 
   BSTR bstrFriendlyName; 
   VARIANT_BOOL fSelected; 
   // get an interface for the ith device 
   pDevices->Item(i, &pDevice); 
   // get the devices "full name" (used to uniquely identify the device) 
   pDevice->get_Name(&bstrName); 
   // get the devices "friendly name"  (used for readable display) 
   pDevice->get_Name(&bstrFriendlyName); 
   // query whether the device is currently selected 
   pDevice->get_Selected(&fSelected); 
   // 
   // add additional code here 
   // 
   // free strings 
   SysFreeString(bstrName); 
   SysFreeString(bstrFriendlyName); 
   // release the device interface 
   pDevice->Release(); 
} 
// release the devices interface 
pDevices->Release();

Note: The only difference between enumerating video and audio devices is the initial collection object retrieved.

  1. An individual device can be selected for capture by calling the device collections put_Selection function:

C Source 

IltmmDevices* pDevices; 
// get an interface into video devices collection 
IltmmCapture_get_AudioDevices(pCapture, &pDevices); 
// select device 
IltmmDevices_put_Selection(10); 
// release collection 
IUnknown_Release(pDevices);

C++ Source 

IltmmDevices* pDevices; 
// get an interface into video devices collection 
pCapture->get_AudioDevices(&pDevices); 
// select device 
pDevices->put_Selection(10); 
// release collection 
pDevices->Release();

The previous code selects the 10th device to be captured. You can also deselect any device by passing a 1 as the item index.

  1. Define the target or output file. As demonstrated with the following code:

C Source 

BSTR bstr; 
// create a string containing the target file path 
bstr = SysAllocString(L"c:\\target.avi"); 
// assign the target file path to the capture object 
IltmmCapture_put_TargetFile(pCapture, bstr); 
// free the string 
SysFreeString(bstr);

C++ Source 

BSTR bstr; 
// create a string containing the target file path 
bstr = SysAllocString(L"c:\\target.avi"); 
// assign the target file path to the capture object 
pCapture->put_TargetFile(bstr); 
// free the string 
SysFreeString(bstr);

The code above allocates a string containing an AVI file path and assigns the path to the capture object with a call to put_TargetFile.

  1. You can now, optionally, set capture and preview properties using the ShowDialog function:

C Source 

// capture properties 
IltmmCapture_ShowDialog(pCapture, ltmmCapture_Dlg_Capture, (long)hwndMain); 
// preview properties 
IltmmCapture_ShowDialog(pCapture, ltmmCapture_Dlg_Preview, (long)hwndMain);

C++ Source 

// capture properties 
pCapture->ShowDialog(ltmmCapture_Dlg_Capture, (long)hwndMain); 
// preview properties 
pCapture->ShowDialog(ltmmCapture_Dlg_Preview, (long)hwndMain);

Use the HasDialog function to test for the existence of any dialog.

  1. You are now ready to start the capturing data. This is accomplished with the following code:

C Source 

IltmmCapture_StartCapture (pCapture, ltmmCapture_Mode_VideoOrAudio);

C++ Source 

pCapture->StartCapture (ltmmCapture_Mode_VideoOrAudio);

  1. The notification window will be sent messages indicating the current capture progress: 

    LRESULT CaptureWndProc(HWND hwnd, UINT msg, WPARAM wParam, LPARAM lParam) 
{ 
   if (msg == WM_CAPTURENOTIFY) 
   { 
      switch (wParam) 
      { 
      case ltmmCapture_Notify_Started: 
         // capture started 
         break; 
      case ltmmCapture_Notify_Complete: 
         // capture successfully completed 
         break; 
      case ltmmCapture_Notify_ErrorAbort: 
         // a capture error occurred 
         // the error code is contained in lParam 
         break; 
      case ltmmCapture_Notify_Progress: 
         // the current capture time (milliseconds) is in lParam 
         break; 
      } 
   } 
   return 0; 
}

  2. To stop capturing the data, call the StopCapture function as follows:

C Source 

IltmmCapture_StopCapture (pCapture);

C++ Source 

pCapture->StopCapture ();

  1. You can, optionally, capture a manual sequence of frames. To start a manual frame capture sequence call StartCapture as follows:

C Source 

IltmmCapture_StartCapture (pCapture, ltmmCapture_Mode_ManualFrames);

C++ Source 

pCapture->StartCapture (ltmmCapture_Mode_ManualFrames);

  1. To trigger the capture of each frame call the CaptureFrame as follows:

C Source 

IltmmCapture_CaptureFrame (pCapture);

C++ Source 

pCapture->CaptureFrame ();

  1. If you would like the capture object to automatically trigger the single frame captures, then you would start the capture as follows:

C Source 

// set the trigger time to 5 seconds 
IltmmCapture_put_FrameDelay(pCapture, 5.0); 
// start capturing frame sequence 
IltmmCapture_StartCapture(pCapture, ltmmCapture_Mode_AutoFrames);

C++ Source 

// set the trigger time to 5 seconds 
pCapture->put_FrameDelay(5.0); 
// start capturing frame sequence 
pCapture->StartCapture(ltmmCapture_Mode_AutoFrames);

The above code will schedule a frame to be captured every 5 seconds.

  1. Another variation captures a single still image as a device independent bitmap or an automation picture object. A DIB can be captured as follows:

C Source 

HGLOBAL hDIB; 
IltmmCapture_CaptureDIB(pCapture, (long*)&hDIB);

C++ Source 

HGLOBAL hDIB; 
pCapture->CaptureDIB((long*)&hDIB);

The above code can be used without assigning the TargetFile. The user is responsible for freeing the returned bitmap.

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