Using a Screen Capture as a Source

Article
07/05/2006

You can use a screen capture as a video source. The screen capture plug-in enables capture of all or part of the screen, for example, to capture mouse pointer movements to demonstrate a procedure. You can also capture from a card that is connected to a scan converter device.

You can configure the plug-in to specify the area you want to capture. One way is to use the IPropertyBag interface (see the C++ example below). This plug-in also supports a property page that enables configuration of the capture process by end users. To learn how to create a dialog box and display a property page, see Displaying the Plug-In Property Pages.

Consider the following guidelines to improve screen captures:

Use the Windows Media Video 9 Screen codec.
Use a fast graphics card.
Use a fast computer or a computer with a dual processor.
Disable hardware acceleration. From the Control Panel Display Properties, on the Settings tab, click Advanced. On the Troubleshoot tab, slide the Hardware acceleration setting to None.
Decrease color quality to reduce the amount of data that the graphics card must process. From the Control Panel Display Properties, on the Settings tab, click a lower setting for Color quality.
Run your encoding application in 256 colors. Right-click the executable file, click Properties, then on the Compatibility tab, select Run in 256 colors.
Modify the profile to reduce the frame rate and the image capture size. Increasing the bit rate can also result in better performance.
Close all unnecessary applications.
Disable the background picture.

The following Visual Basic example shows how to use a screen capture as a source. The C++ example shows how to configure the screen capture using the property bag. For end-to-end code examples, see Complete Code Examples.

Note When specifying the resource name, use the string "ScreenCap://ScreenCapture1" in the SetInput method of the IWMEncSource object.

Visual Basic Example

' Declare objects and variables.
  Dim Encoder as WMEncoder
  Dim SrcGrpColl As IWMEncSourceGroupCollection
  Dim SrcGrp As IWMEncSourceGroup
  Dim SrcVid As IWMEncVideoSource

' Create a WMEncoder object.
  Set Encoder = New WMEncoder

' Retrieve a source group collection.
  Set SrcGrpColl = Encoder.SourceGroupCollection

' Create a source group called SG_1.
  Set SrcGrp = SrcGrpColl.Add("SG_1")

' Create a video source object and an audio source object.
  Set SrcVid = SrcGrp.AddSource(WMENC_VIDEO)

' Specify the screen capture plug-in to set up the video source.
  SrcVid.SetInput "ScreenCap://ScreenCapture1"

C++ Example

// Include libraries.
#include <windows.h>
#include <atlbase.h>
#include <comdef.h>
#include "C:\WMSDK\WMEncSDK9\include\wmencode.h"

// Define the screen capture properties.
#define WMSCRNCAP_CAPTUREWINDOW     CComBSTR("CaptureWindow")
#define WMSCRNCAP_WINDOWLEFT        CComBSTR("Left")
#define WMSCRNCAP_WINDOWTOP         CComBSTR("Top")
#define WMSCRNCAP_WINDOWRIGHT       CComBSTR("Right")
#define WMSCRNCAP_WINDOWBOTTOM      CComBSTR("Bottom")
#define WMSCRNCAP_FLASHRECT         CComBSTR("FlashRect")
#define WMSCRNCAP_ENTIRESCREEN      CComBSTR("Screen")
#define WMSCRNCAP_WINDOWTITLE       CComBSTR("WindowTitle")

// Declare variables.
HRESULT hr = S_OK;
IWMEncoder* pEncoder;
IWMEncSourceGroupCollection* pSrcGrpColl;
IWMEncSourceGroup* pSrcGrp;
IWMEncSource* pSrc;
IWMEncVideoSource2* pSrcVid;
IPropertyBag* pPropertyBag;
CComVariant varValue;

// Initialize the COM library and retrieve a pointer to an IWMEncoder interface.
hr = CoInitialize(NULL);
if ( SUCCEEDED( hr ) )
{
    hr = CoCreateInstance(CLSID_WMEncoder,
        NULL,
        CLSCTX_INPROC_SERVER,
        IID_IWMEncoder,
        (void**) &pEncoder);
}

// Retrieve the source group collection.
if ( SUCCEEDED( hr ) )
{
    hr = pEncoder->get_SourceGroupCollection(&pSrcGrpColl);
}

// Add a source group to the collection.
if ( SUCCEEDED( hr ) )
{
    hr = pSrcGrpColl->Add(CComBSTR("SG_1"), &pSrcGrp);
}
if ( SUCCEEDED( hr ) )
{
    hr = pSrcGrp->AddSource(WMENC_VIDEO, &pSrc);
}

// Retrieve an IWMEncVideoSource2 pointer.
if ( SUCCEEDED( hr ) )
{
    hr = pSrc->QueryInterface(IID_IWMEncVideoSource2, (void**)&pSrcVid);
}

// Add a video source to the source group.
if ( SUCCEEDED( hr ) )
{
    hr = pSrcVid->SetInput(CComBSTR("ScreenCap://ScreenCapture1"));
}

// Retrieve a pointer to the property bag.
if ( SUCCEEDED( hr ) )
{
    hr = pSrcVid->QueryInterface(IID_IPropertyBag, (void**)&pPropertyBag);
}

// Set full screen capture.
{
    varValue = true;
    if ( SUCCEEDED( hr ) )
    {
        hr = pPropertyBag->Write( WMSCRNCAP_ENTIRESCREEN, &varValue );
    }
}

// Set the capture area.
{
    // nLeft, nRight, nTop, and nBottom are the dimensions to capture
    int nLeft, nRight, nTop, nBottom;

    // Initialize the capture area. The size must be even.
    varValue = false;
    if ( SUCCEEDED( hr ) )
    {
        hr = pPropertyBag->Write( WMSCRNCAP_ENTIRESCREEN, &varValue );
    }

    varValue = nLeft;
    if ( SUCCEEDED( hr ) )
    {
        hr = pPropertyBag->Write( WMSCRNCAP_WINDOWLEFT, &varValue );
    }

    varValue = nRight;
    if ( SUCCEEDED( hr ) )
    {
        hr = pPropertyBag->Write( WMSCRNCAP_WINDOWRIGHT, &varValue );
    }

    varValue = nTop;
    if ( SUCCEEDED( hr ) )
    {
        hr = pPropertyBag->Write( WMSCRNCAP_WINDOWTOP, &varValue );
    }

    varValue = nBottom;
    if ( SUCCEEDED( hr ) )
    {
        hr = pPropertyBag->Write( WMSCRNCAP_WINDOWBOTTOM, &varValue );
    }

    varValue = true;
    if ( SUCCEEDED( hr ) )
    {
        hr = pPropertyBag->Write( WMSCRNCAP_FLASHRECT, &varValue );
    }

    // Continue configuring the encoding session.

    // Release pointers.
    if ( pSrcGrpColl )
    {
        pSrcGrpColl->Release();
        pSrcGrpColl = NULL;
    }
    if ( pSrcGrp )
    {
        pSrcGrp->Release();
        pSrcGrp = NULL;
    }
    if ( pSrcVid )
    {
        pSrcVid->Release();
        pSrcVid = NULL;
    }
    if ( pSrc )
    {
        pSrc->Release();
        pSrc = NULL;
    }
    if ( pPropertyBag )
    {
        pPropertyBag->Release();
        pPropertyBag = NULL;
    }
    if ( pEncoder )
    {
        pEncoder->Release();
        pEncoder = NULL;
    }
}

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Using a Screen Capture as a Source

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