前言
DirectShow是微软公司提供的一套在Windows平台上进行流媒体处理的开发包，与DirectX开发包一起发布。DirectShow为多媒体流的捕捉和回放提供了强有力的支持。用DirectShow开发应用程序，我们可以很方便地从支持WDM驱动模型的采集卡上捕获数据，并且进行相应的后期处理乃至存储到文件中。
DirectShow是基于COM的，为了编写DirectShow应用程序，需要了解COM客户程序编写的基础知识。DirectShow提供了大量的接口，但在编程中发现还是不够方便，如果能构建一个视频捕捉类把常用的一些动作封装起来，那么就更方便了。
编程思路
为了更加容易建立视频捕捉应用程序，DirectShow提供了一个叫做Capture Graph Builder的对象，Capture Graph Builder提供IcaptureGraphBuilder2接口，该接口可以建立和控制Capture Graph。
建立视频捕捉程序，必须首先获取并初始化IcaptureGraphBuilder2接口，然后选择一个适当的视频捕捉设备。选择好设备后，为该设备创建Capture filter，然后调用AddFilter把Capture filter添加到Filter Graph。
如果仅仅希望用摄像头来进行实时监控的话，只需要在上面的基础上调用ICaptureGraphBuilder2::RenderStream就可以了：
ICaptureGraphBuilder2 *pBuild; // Capture Graph Builder
//省略初始化部分代码
IBaseFilter *pCap; // Video capture filter.
//省略初始化和添加到Filter Graph部分代码
pBuild->RenderStream(&PIN_CATEGORY_PREVIEW, &MEDIATYPE_Video, pCap, NULL, NULL);
DirectShow提供了一个捕捉静态图像的方法：使用Sample Grabber filter。依次按照以下三个步骤就可以了：
第一步, 定义一个类实现Sample Grabber的回调接口IsampleGrabberCB：
class CSampleGrabberCB : public ISampleGrabberCB
{
//在后面提供的类中具体完成
}
CSampleGrabberCB mCB;
第二步、调用RenderStream依次把Still pin、Sample Grabber和系统默认Renderer Filter连接起来。
第三步、配置Sample Grabber以捕获数据。
视频捕捉类CCaptureVideo的具体实现
// CCaptureVideo视频捕捉类头文件/////////////////////////////////////////////////////////////////////#if !defined(AFX_CAPTUREVIDEO_H__F5345AA4_A39F_4B07_B843_3D87C4287AA0__INCLUDED_)#define AFX_CAPTUREVIDEO_H__F5345AA4_A39F_4B07_B843_3D87C4287AA0__INCLUDED_/////////////////////////////////////////////////////////////////////// CaptureVideo.h : header file/////////////////////////////////////////////////////////////////////#if _MSC_VER > 1000#pragma once#endif // _MSC_VER > 1000#include <atlbase.h>#include <windows.h>#include <dshow.h>#ifndef SAFE_RELEASE#define SAFE_RELEASE( x ) if ( NULL != x ) { x->Release( ); x = NULL; }#endifclass CSampleGrabberCB;class CCaptureVideo : public CWnd {friend class CSampleGrabberCB;public:void GrabOneFrame(BOOL bGrab);HRESULT Init(int iDeviceID,HWND hWnd);int EnumDevices(HWND hList);CCaptureVideo();virtual ~CCaptureVideo();private:HWND m_hWnd;IGraphBuilder *m_pGB;ICaptureGraphBuilder2* m_pCapture;IBaseFilter* m_pBF;IMediaControl* m_pMC;IVideoWindow* m_pVW;CComPtr<ISampleGrabber> m_pGrabber;protected:void FreeMediaType(AM_MEDIA_TYPE& mt);bool BindFilter(int deviceId, IBaseFilter **pFilter);void ResizeVideoWindow();HRESULT SetupVideoWindow();HRESULT InitCaptureGraphBuilder();};#endif // !defined(AFX_CAPTUREVIDEO_H__F5345AA4_A39F_4B07_B843_3D87C4287AA0__INCLUDED_)//-------------------------------------------------------------------// CCaptureVideo视频捕捉类实现文件CaptureVideo.cpp//-------------------------------------------------------------------// CaptureVideo.cpp: implementation of the CCaptureVideo class.///////////////////////////////////////////////////////////////////////#include "stdafx.h"#include "CaptureVideo.h"#ifdef _DEBUG#undef THIS_FILEstatic char THIS_FILE[]=__FILE__;#define new DEBUG_NEW#endifBOOL bOneShot=FALSE;//全局变量class CSampleGrabberCB : public ISampleGrabberCB {public:long lWidth;long lHeight;TCHAR m_szFileName[MAX_PATH];// 位图文件名称CSampleGrabberCB( ){strcpy(m_szFileName, "c:\donaldo.bmp");} STDMETHODIMP_(ULONG) AddRef() { return 2; }STDMETHODIMP_(ULONG) Release() { return 1; }STDMETHODIMP QueryInterface(REFIID riid, void ** ppv){if( riid == IID_ISampleGrabberCB || riid == IID_IUnknown ){ *ppv = (void *) static_cast<ISampleGrabberCB*> ( this );return NOERROR;} return E_NOINTERFACE;}STDMETHODIMP SampleCB( double SampleTime, IMediaSample * pSample ){return 0;}STDMETHODIMP BufferCB( double dblSampleTime, BYTE * pBuffer, long lBufferSize ){if( !bOneShot )return 0;if (!pBuffer)return E_POINTER;SaveBitmap(pBuffer, lBufferSize);bOneShot = FALSE;return 0;}//创建位图文件BOOL SaveBitmap(BYTE * pBuffer, long lBufferSize ){HANDLE hf = CreateFile(m_szFileName, GENERIC_WRITE, FILE_SHARE_READ, NULL,CREATE_ALWAYS, NULL, NULL );if( hf == INVALID_HANDLE_VALUE )return 0;// 写文件头 BITMAPFILEHEADER bfh;memset( &bfh, 0, sizeof( bfh ) );bfh.bfType = ’MB’;bfh.bfSize = sizeof( bfh ) + lBufferSize + sizeof( BITMAPINFOHEADER );bfh.bfOffBits = sizeof( BITMAPINFOHEADER ) + sizeof( BITMAPFILEHEADER );DWORD dwWritten = 0;WriteFile( hf, &bfh, sizeof( bfh ), &dwWritten, NULL );// 写位图格式BITMAPINFOHEADER bih;memset( &bih, 0, sizeof( bih ) );bih.biSize = sizeof( bih );bih.biWidth = lWidth;bih.biHeight = lHeight;bih.biPlanes = 1;bih.biBitCount = 24;WriteFile( hf, &bih, sizeof( bih ), &dwWritten, NULL );// 写位图数据WriteFile( hf, pBuffer, lBufferSize, &dwWritten, NULL );CloseHandle( hf );return 0;}};CSampleGrabberCB mCB;//////////////////////////////////////////////////////////////////////// Construction/Destruction//////////////////////////////////////////////////////////////////////CCaptureVideo::CCaptureVideo(){//COM Library Intializationif(FAILED(CoInitialize(NULL))) /*, COINIT_APARTMENTTHREADED)))*/{AfxMessageBox("CoInitialize Failed!rn"); return;}m_hWnd = NULL;m_pVW = NULL;m_pMC = NULL;m_pGB = NULL;m_pCapture = NULL; }CCaptureVideo::~CCaptureVideo(){// Stop media playbackif(m_pMC)m_pMC->Stop();if(m_pVW){m_pVW->put_Visible(OAFALSE);m_pVW->put_Owner(NULL);}SAFE_RELEASE(m_pCapture);SAFE_RELEASE(m_pMC);SAFE_RELEASE(m_pGB);SAFE_RELEASE(m_pBF);CoUninitialize( );}int CCaptureVideo::EnumDevices(HWND hList){if (!hList)return -1;int id = 0;//枚举视频扑捉设备ICreateDevEnum *pCreateDevEnum;HRESULT hr = CoCreateInstance(CLSID_SystemDeviceEnum, NULL, CLSCTX_INPROC_SERVER,IID_ICreateDevEnum, (void**)&pCreateDevEnum);if (hr != NOERROR)return -1;CComPtr<IEnumMoniker> pEm;hr = pCreateDevEnum->CreateClassEnumerator(CLSID_VideoInputDeviceCategory,&pEm, 0);if (hr != NOERROR)return -1;pEm->Reset();ULONG cFetched;IMoniker *pM;while(hr = pEm->Next(1, &pM, &cFetched), hr==S_OK){IPropertyBag *pBag;hr = pM->BindToStorage(0, 0, IID_IPropertyBag, (void **)&pBag);if(SUCCEEDED(hr)) {VARIANT var;var.vt = VT_BSTR;hr = pBag->Read(L"FriendlyName", &var, NULL);if (hr == NOERROR) {TCHAR str[2048]; id++;WideCharToMultiByte(CP_ACP,0,var.bstrVal, -1, str, 2048, NULL, NULL);::SendMessage(hList, CB_ADDSTRING, 0,(LPARAM)str);SysFreeString(var.bstrVal);}pBag->Release();}pM->Release();}return id;}HRESULT CCaptureVideo::Init(int iDeviceID, HWND hWnd){HRESULT hr;hr = InitCaptureGraphBuilder();if (FAILED(hr)){AfxMessageBox("Failed to get video interfaces!");return hr;}// Bind Device Filter. We know the device because the id was passed inif(!BindFilter(iDeviceID, &m_pBF))return S_FALSE;hr = m_pGB->AddFilter(m_pBF, L"Capture Filter");// hr = m_pCapture->RenderStream(&PIN_CATEGORY_PREVIEW, &MEDIATYPE_Video, // m_pBF, NULL, NULL);// create a sample grabberhr = m_pGrabber.CoCreateInstance( CLSID_SampleGrabber );if( !m_pGrabber ){AfxMessageBox("Fail to create SampleGrabber, maybe qedit.dll is not registered?");return hr;}CComQIPtr< IBaseFilter, &IID_IBaseFilter > pGrabBase( m_pGrabber );//设置视频格式AM_MEDIA_TYPE mt; ZeroMemory(&mt, sizeof(AM_MEDIA_TYPE));mt.majortype = MEDIATYPE_Video;mt.subtype = MEDIASUBTYPE_RGB24;hr = m_pGrabber->SetMediaType(&mt);if( FAILED( hr ) ){AfxMessageBox("Fail to set media type!");return hr;}hr = m_pGB->AddFilter( pGrabBase, L"Grabber" );if( FAILED( hr ) ){AfxMessageBox("Fail to put sample grabber in graph");return hr;}// try to render preview/capture pinhr = m_pCapture->RenderStream(&PIN_CATEGORY_PREVIEW, &MEDIATYPE_Video,m_pBF,pGrabBase,NULL);if( FAILED( hr ) )hr = m_pCapture->RenderStream(&PIN_CATEGORY_CAPTURE, &MEDIATYPE_Video,m_pBF,pGrabBase,NULL);if( FAILED( hr ) ){AfxMessageBox("Can’t build the graph");return hr;}hr = m_pGrabber->GetConnectedMediaType( &mt );if ( FAILED( hr) ){AfxMessageBox("Failt to read the connected media type");return hr;}VIDEOINFOHEADER * vih = (VIDEOINFOHEADER*) mt.pbFormat;mCB.lWidth = vih->bmiHeader.biWidth;mCB.lHeight = vih->bmiHeader.biHeight;FreeMediaType(mt);hr = m_pGrabber->SetBufferSamples( FALSE );hr = m_pGrabber->SetOneShot( FALSE );hr = m_pGrabber->SetCallback( &mCB, 1 );//设置视频捕捉窗口m_hWnd = hWnd ; SetupVideoWindow();hr = m_pMC->Run();//开始视频捕捉if(FAILED(hr)){AfxMessageBox("Couldn’t run the graph!");return hr;}return S_OK;}bool CCaptureVideo::BindFilter(int deviceId, IBaseFilter **pFilter){if (deviceId < 0)return false;// enumerate all video capture devicesCComPtr<ICreateDevEnum> pCreateDevEnum;HRESULT hr = CoCreateInstance(CLSID_SystemDeviceEnum, NULL, CLSCTX_INPROC_SERVER,IID_ICreateDevEnum, (void**)&pCreateDevEnum);if (hr != NOERROR){return false;}CComPtr<IEnumMoniker> pEm;hr = pCreateDevEnum->CreateClassEnumerator(CLSID_VideoInputDeviceCategory,&pEm, 0);if (hr != NOERROR) {return false;}pEm->Reset();ULONG cFetched;IMoniker *pM;int index = 0;while(hr = pEm->Next(1, &pM, &cFetched), hr==S_OK, index <= deviceId){IPropertyBag *pBag;hr = pM->BindToStorage(0, 0, IID_IPropertyBag, (void **)&pBag);if(SUCCEEDED(hr)) {VARIANT var;var.vt = VT_BSTR;hr = pBag->Read(L"FriendlyName", &var, NULL);if (hr == NOERROR) {if (index == deviceId){pM->BindToObject(0, 0, IID_IBaseFilter, (void**)pFilter);}SysFreeString(var.bstrVal);}pBag->Release();}pM->Release();index++;}return true;}HRESULT CCaptureVideo::InitCaptureGraphBuilder(){HRESULT hr;// 创建IGraphBuilder接口hr=CoCreateInstance(CLSID_FilterGraph, NULL, CLSCTX_INPROC_SERVER, IID_IGraphBuilder, (void **)&m_pGB);// 创建ICaptureGraphBuilder2接口hr = CoCreateInstance (CLSID_CaptureGraphBuilder2 , NULL, CLSCTX_INPROC,IID_ICaptureGraphBuilder2, (void **) &m_pCapture);if (FAILED(hr))return hr;m_pCapture->SetFiltergraph(m_pGB);hr = m_pGB->QueryInterface(IID_IMediaControl, (void **)&m_pMC);if (FAILED(hr))return hr;hr = m_pGB->QueryInterface(IID_IVideoWindow, (LPVOID *) &m_pVW);if (FAILED(hr))return hr;return hr;}HRESULT CCaptureVideo::SetupVideoWindow(){HRESULT hr;hr = m_pVW->put_Owner((OAHWND)m_hWnd);if (FAILED(hr))return hr;hr = m_pVW->put_WindowStyle(WS_CHILD | WS_CLIPCHILDREN);if (FAILED(hr))return hr;ResizeVideoWindow();hr = m_pVW->put_Visible(OATRUE);return hr;}void CCaptureVideo::ResizeVideoWindow(){if (m_pVW){//让图像充满整个窗口CRect rc;::GetClientRect(m_hWnd,&rc);m_pVW->SetWindowPosition(0, 0, rc.right, rc.bottom);} }void CCaptureVideo::GrabOneFrame(BOOL bGrab){bOneShot = bGrab;}void CCaptureVideo::FreeMediaType(AM_MEDIA_TYPE& mt){if (mt.cbFormat != 0) {CoTaskMemFree((PVOID)mt.pbFormat);// Strictly unnecessary but tidiermt.cbFormat = 0;mt.pbFormat = NULL;}if (mt.pUnk != NULL) {mt.pUnk->Release();mt.pUnk = NULL;}} 如何使用视频捕捉类CCaptureVideo
构建CCaptureVideo类以后，使用就方便多了，我们在编程中只需要是要下面三个类成员函数就可以实现用摄像头进行视频捕捉：
①int EnumDevices(HWND hList); //hList是下拉列表框的句柄，本函数用于枚举当前系统安装的所有视频捕捉设备
②HRESULT Init(int iDeviceID,HWND hWnd);//iDeviceID是视频捕捉设备序号，hWnd是视频捕捉窗口的句柄
③void GrabOneFrame(BOOL bGrab);//调用GrabOneFrame(true)就可以捕获当前的静态图像并保存到硬盘上
具体示例：用MFC AppWizard(exe)创建一个对话框应用程序，取名为ds，给对话框添加一个下拉列表框(IDC_COMBO1)、两个按钮（IDC_PHOTO、IDC_HAVEALOOK）和一个Picture控件（ID:IDC_STATIC_SCREEN，Type: Rectangle，Color:Gray）。
1、使用向导添加成员变量
CStatic m_staticScreen; // IDC_STATIC_SCREEN
CComboBox m_ListCtrl; // IDC_COMBO1
CCaptureVideo m_cap;
2、为BOOL CDsDlg::OnInitDialog()添加如下代码：
// TODO: Add extra initialization here
m_cap.EnumDevices (m_ListCtrl);
m_ListCtrl.SetCurSel (0);
3、为确定按钮添加代码如下：
void CDsDlg::OnOK()
{
//只需要四行代码就可以进行视频捕捉了
UpdateData();
HWND hWnd = m_staticScreen.GetSafeHwnd() ;
HRESULT hr = m_cap.Init(m_ListCtrl.GetCurSel (),hWnd);
GetDlgItem(IDOK)->EnableWindow(FALSE);
}
4、如果希望捕捉静态图像，为照相按钮添加如下代码：
void CDsDlg::OnPhoto()
{
m_cap.GrabOneFrame(true);
}
运行程序时，选定摄像头后只需要按确定就可以了,实际效果如下图所示：
结束语
本文提供的视频捕捉类CcaptureVideo和示例，在Win2K + DirectX9 SDK + VC6 环境下调试通过。注意：编译时需要Strmiids.lib Quartz.lib两个库文件(DirectX9 SDK自带)。