1．加入Sample Grabber Filter
当我们加入Sample Grabber Filter的时候，我们可以直接调用其接口（interface）ISampleGrabber。该接口可以获取经过该Filter的单独的Media Samples。详情请参见DXSDK。
1.1 派生出自己的Sample Grabber
从ISampleGrabberCB中派生出自己的类，然后实现其虚函数，详情请参见SDK中的示例程序(DXSDK ROOT\Samples\C++\DirectShow\Editing\GrabBitmaps)。
1.2 直接调用Sample Grabber Filter的接口
假如我们在播放的过程中动态的加入Filter的话，操作和效率都不乐观。所以我采用下面的方法：
该方法传递的是时间，不是在播放的时候动态加入Filter然后截图，而是另外打开源文件进行操作。
A）申明以下接口：
#001 IGraphBuilder *pGraph = NULL; //for graph builder
#002 IMediaControl *pControl = NULL; //media control
#003 IMediaSeeking *pSeeking = NULL; //media seeking
#004 IMediaEventEx *pEvent = NULL; //media envent
#005 IBaseFilter *pNullFilter =NULL;//for holding the Sample grabber Filter
B）初始化接口：
#001 JIF(CoCreateInstance(CLSID_FilterGraph, NULL, CLSCTX_INPROC,
#002 IID_IGraphBuilder, (void **)&pGraph));
#003
#004 JIF(CoCreateInstance(CLSID_NullRenderer, NULL, CLSCTX_INPROC,
#005 IID_IBaseFilter, (void **)&pNullFilter));
#006 JIF(pGraph->QueryInterface(IID_IMediaControl,(void **)&pControl));
#007 JIF(pGraph->QueryInterface(IID_IMediaSeeking, (void **)&pSeeking));
#008 JIF(pGraph->QueryInterface(IID_IMediaEvent, (void **)&pEvent));
C）创建Sample Grabber
#001 // Create the Sample Grabber.
#002 IBaseFilter *pGrabberF = NULL;
#003 JIF(CoCreateInstance(CLSID_SampleGrabber,NULL, CLSCTX_INPROC_SERVER,
#004 IID_IBaseFilter, (void**)&pGrabberF));
#005
#006 JIF(pGraph->AddFilter(pGrabberF, L"Sample Grabber"));
#007 JIF(pGraph->AddFilter(pNullFilter, L"Null Render Filter"));
#008
#009 ISampleGrabber *pGrabber;
#010 JIF(pGrabberF->QueryInterface(IID_ISampleGrabber, (void**)&pGrabber));
设置Sample Grabber的媒体格式：调用SetMediaType,该函数接受一个AM_MEDIA_TYPE的结构，主要是设置该结构中的majortype，和suBType域。
D）添加Source Filter：
#001 IBaseFilter *pSrc;
#002 JIF(pGraph->AddSourceFilter(T2W(m_szFile), L"Source", &pSrc));
E）连接Grabber 和 NullRender两个Filter：
#001 IPin *pOutPin;
#002 hr = GetPin(pGrabberF, PINDIR_OUTPUT, &pOutPin);
#003
#004 IPin *pInPin;
#005 hr = GetPin(pNullFilter, PINDIR_INPUT, &pInPin);
#006
#007 pGraph->Connect(pOutPin, pInPin);
F）取得当前所连接媒体的类型
#001 AM_MEDIA_TYPE mt;
#002 hr = pGrabber->GetConnectedMediaType(&mt);
#003 // Examine the format block.
#004 VIDEOINFOHEADER *pVih;
#005 if ((mt.formattype == FORMAT_VideoInfo) &&
#006 (mt.cbFormat >= sizeof(VIDEOINFOHEADER)) &&
#007 (mt.pbFormat != NULL) )
#008 {
#009 pVih = (VIDEOINFOHEADER*)mt.pbFormat;
#010 }
#011 else
#012 {
#013 // Wrong format. Free the format block and return an error.
#014 FreeMediaType(mt);
#015 return VFW_E_INVALIDMEDIATYPE;
#016 }
#017
#018 // Do buffer the samples as they pass through
#019 //
#020 hr = pGrabber->SetBufferSamples(TRUE);
#021
#022 // Only grab one at a time, stop stream after
#023 // grabbing one sample
#024 //
#025 hr = pGrabber->SetOneShot( TRUE );
G）Seeking文件，使其到达要截图的时间帧
#001 pSeeking->SetPositions(pCurrentPos,
#002 AM_SEEKING_AbsolutePositioning，
#003 NULL, AM_SEEKING_NoPositioning );
#004
#005 pControl->Run();
#006
#007 long EvCode = 0;
#008
#009 hr = pEvent->WaitForCompletion( INFINITE, &EvCode );
H）取得当前的buffer数据
#001 // Find the required buffer size.
#002 long cbBuffer = 0;
#003 hr = pGrabber->GetCurrentBuffer(&cbBuffer, NULL);
#004 LONGLONG currentPos;
#005 pSeeking->GetCurrentPosition(¤tPos);
#006 BYTE *pBuffer = new BYTE[cbBuffer];
#007 if (!pBuffer)
#008 {
#009 // Out of memory. Return an error code.
#010 Msg("Out of Memory");
#011 }
#012 hr = pGrabber->GetCurrentBuffer(&cbBuffer, (long*)pBuffer);
I）写入文件
#001 // Create a file to hold the bitmap
#002 HANDLE hf = CreateFile(szFilename, GENERIC_WRITE, FILE_SHARE_READ,
#003 NULL, CREATE_ALWAYS, NULL, NULL );
#004
#005 if( hf == INVALID_HANDLE_VALUE )
#006 {
#007 // Failed to create file
#008 return 0;
#009 }
#010
#011 // Write out the file header
#012 //
#013 BITMAPFILEHEADER bfh;
#014 memset( &bfh, 0, sizeof( bfh ) );
#015 bfh.bfType = 'MB';
#016 bfh.bfSize = sizeof( bfh ) + cbBuffer + sizeof( BITMAPINFOHEADER );
#017 bfh.bfOffBits = sizeof(BITMAPINFOHEADER)+sizeof( BITMAPFILEHEADER );
#018
#019 DWord Written = 0;
#020 WriteFile( hf, &bfh, sizeof( bfh ), &Written, NULL );
#021
#022 // Write the bitmap format
#023 //
#024 BITMAPINFOHEADER bih;
#025 memset( &bih, 0, sizeof( bih ) );
#026 bih.biSize = sizeof( bih );
#027 bih.biWidth = pVih->bmiHeader.biWidth;
#028 bih.biHeight = pVih->bmiHeader.biHeight;
#029 bih.biPlanes = pVih->bmiHeader.biPlanes;
#030 bih.biBitCount = pVih->bmiHeader.biBitCount;
#031
#032 Written = 0;
#033
#034 WriteFile( hf, &bih, sizeof( bih ), &Written, NULL );
#035
#036 // Write the bitmap bits
#037 //
#038 Written = 0;
#039 WriteFile( hf, pBuffer, cbBuffer, &Written, NULL );
#040 FreeMediaType(mt);
#041 CloseHandle(hf);
J）释放资源
#001 pControl->Stop();
#002 SAFE_RELEASE(pControl);
#003 SAFE_RELEASE(pSeeking);
#004 SAFE_RELEASE(pEvent);
#005 SAFE_RELEASE(pSrc);
#006 SAFE_RELEASE(pNullFilter);
#007 SAFE_RELEASE(pGrabber);
#008 SAFE_RELEASE(pGrabberF);
#009 SAFE_RELEASE(pGraph);
K）其实我们可以不用NullRender，而是用IVideoWindow接口来实现。假如是那样的话，首先申明IVideoWindow *pVideo = NULL;将pVideo加入到Filter Graph中
#001 JIF(pGraph->QueryInterface(IID_IVideoWindow,(void**)&pVideo));
#002 hr = pGraph->Render(pOutPin);
#003 if (pVideo)
#004 {
#005 hr = pVideo->put_AutoShow(OAFALSE);
#006 }
通过IBasicVideo：：GetCurrentImage接口
对于该接口，Video Renderer和Video Mixing Renderer（VMR）有不同的实现。
A）Video Renderer
假如该Renderer使用了DDraw加速的话，该调用会失败。在调用该接口的时候，必须首先暂停Renderer（可以通过IMediaControl：：Pause（）暂停，假如不能确信该操作是否成功，应该调用IMediaControl::GetState()判定状态）。
B）Video Mixing Renderer
对于VMR，该方法都会成功（不管是否运用了DDraw加速，也不管是否是暂停状态），此时对于它所有的状态（running， stopped， or paused）都适用。
函数Grabber代码如下（调用该函数的时候应该先将媒体文件暂停，原因上面已经说了）：
#001 bool Grabber(IBasicVideo mBasicVideo, TCHAR *szFilename)
#002 {
#003 if (mBasicVideo)
#004 {
#005 long bitmapSize = 0;
#006 if(SUCCEEDED(mBasicVideo->GetCurrentImage(&bitmapSize, 0)))
#007 {
#008 //if语句里面的操作时取得buffer的size。
#009 //当我们在布确定image buffer的大小的情况下，我们给
#010 //GetCurrentImage的第二个参数传递0或者NULL，取得buffer的
#011 //大小供以后使用。
#012 bool pass = false;
#013 unsigned char * buffer = new unsigned char[bitmapSize];
#014 if(SUCCEEDED(mBasicVideo->GetCurrentImage(&bitmapSize,(long*)buffer)))
#015 {
#016 //此时已经用到刚才所取得的大小（分配空间）
#017 BITMAPFILEHEADER hdr; //Bitmap的头信息
#018 LPBITMAPINFOHEADER lpbi; // Bitmap的文件信息（包括数据）
#019
#020 lpbi = (LPBITMAPINFOHEADER)buffer;
#021
#022 int nColors = 1 << lpbi->biBitCount;
#023 if (nColors > 256)
#024 nColors = 0;
#025
#026 hdr.bfType = ((WORD) ('M' << 8) 'B'); //always is "BM"
#027 hdr.bfSize = bitmapSize + sizeof( hdr );
#028 hdr.bfReserved1 = 0;
#029 hdr.bfReserved2 = 0;
#030 hdr.bfOffBits = (DWORD) (sizeof(BITMAPFILEHEADER) + lpbi->biSize
#031 CFile bitmapFile(outFile, CFile::modeReadWrite CFile::modeCreate CFile::typeBinary);
#032 bitmapFile.Write(&hdr, sizeof(BITMAPFILEHEADER));
#033 bitmapFile.Write(buffer, bitmapSize);
#034 bitmapFile.Close();
#035 pass = true;
#036 }
#037 delete [] buffer; //数据用过之后记得要释放空间
#038 return true;
#039 }
#040 }
#041
#042 return false;
#043 }
IMediaDet接口
IMediaDet接口可以取得媒体文件的信息。SDK里面的示例代码（没有写入文件）：
#001 long size;
#002 //取得图像帧的大小，给GetBitmapBits的第三个参数传递0 or NULL
#003 hr = pDet->GetBitmapBits(0, &size, 0, width, height);
#004 if(SUCCEEDED(hr))
#005 {
#006 char *pBuffer = new char[size];
#007 if(!pBuffer)
#008 {
#009 return E_OUTOFMEMORY;
#010 }
#011
#012 try
#013 {
#014 hr = pDet->GetbitmapsBits(0, 0, pBuffer, width, height);
#015 }
#016 catch(...)
#017 {
#018 delete [] pBuffer;
#019 throw;
#020 }
#021
#022 if(SUCCEEDED(hr))
#023 {
#024 BITMAPINFOHEADER *bmih = (BITMAPINFOHEADER*)pBuffer;
#025 HDC hdcDest = GetDC(0);
#026
#027 //Find the address of the start of the image data
#028 void *pData = pBuffer + sizeof(BITMAPINFOHEADER);
#029
#030 //Note: In general a BITMAPINFOHEADER can include extra color
#031 //information at the end, so calculating the offset to the image
#032 //data i snot generally correct. However, the IMediaDet interface
#033 //always returns an RGB-24 image with no extra color information
#034
#035 BITMAPINFO bmi;
#036 ZeroMemory(&bmi, sizeof(BITMAPINFO));
#037 CopyMemory(&(bmi.bmiHeader), bmih, sizeof(BITMAPINFOHEADER));
#038 HBITMAP hBitmap = CreateDIBitmap(hdcDect, bmih, CBM_INIT,
#039 pData, &bmi, DIB_RGB_COLORS);
#040 }
#041
#042 delete [] pBuffer;
#043 }
该方法并没有写入bitmap，具体的写入过程可以参加上面的几种方法。
