Directshow中的视频捕捉[8]

然后通过renderstream将still pin ，sample grabber ，null renderer连接起来

 hr = pbuild->renderstream(

  &pin_category_still, // connect this pin ...

  &mediatype_video, // with this media type ...

  pcap, // on this filter ...

  psg_filter, // to the sample grabber ...

  pnull); // ... and finally to the null renderer.

然后调用isamplegrabber指针，来通过这个指针可以分配内存。

  // configure the sample grabber.

  isamplegrabber *psg;

  hr = psg_filter->queryinterface(iid_isamplegrabber, (void**)&psg);

  psg->setoneshot(false);

  psg->setbuffersamples(true);

设置你的回调对象

 psg->setcallback(&g_stillcapcb, 0); // 0 = use the samplecb callback 
  method

获取静态pin和sample grabber之间连接所用的媒体类型

 // store the media type for later use.

  am_media_type g_stillmediatype;

  hr = psg->getconnectedmediatype(&g_stillmediatype);

  psg->release();

媒体类型包含一个bitmapinfoheader结构来定义图片的格式，在程序退出前一定要释放媒体类型

 // on exit, remember to release the media type.

  freemediatype(g_stillmediatype);

看看下面的回调类吧。这个类从isamplegrabber接口派生，但是它没有保持引用计数，因为应用程序在堆上创建这个对象，在整个graph的生存周期它都存在。
所有的工作都在buffercb函数里完成，当有一个新的sample到来的时候，这个函数就会被sample grabber调用到。在下面的例子里，bitmap被写入到一个文件中

 class samplegrabbercallback : public isamplegrabbercb

  {

  public:

  // fake referance counting.

  stdmethodimp_(ulong) addref() { return 1; }

  stdmethodimp_(ulong) release() { return 2; }

 stdmethodimp queryinterface(refiid riid, void **ppvobject)

  {

  if (null == ppvobject) return e_pointer;

  if (riid == __uuidof(iunknown))

  {

  *ppvobject = static_cast<iunknown*>(this);

  return s_ok;

  }

  if (riid == __uuidof(isamplegrabbercb))

  {

  *ppvobject = static_cast<isamplegrabbercb*>(this);

  return s_ok;

  }

  return e_notimpl;

  }
 stdmethodimp samplecb(double time, imediasample *psample)

  {

  return e_notimpl;

  }
 stdmethodimp buffercb(double time, byte *pbuffer, long bufferlen)

  {

  if ((g_stillmediatype.majortype != mediatype_video) ||

  (g_stillmediatype.formattype != format_videoinfo) ||

  (g_stillmediatype.cbformat < sizeof(videoinfoheader)) ||

  (g_stillmediatype.pbformat == null))

  {

  return vfw_e_invalidmediatype;

  }

  handle hf = createfile("c:\\example.bmp", generic_write, 

  file_share_write, null, create_always, 0, null);

  if (hf == invalid_handle_value)

  {

  return e_fail;

  }

  long cbbitmapinfosize = g_stillmediatype.cbformat - size_preheader;

  videoinfoheader *pvideoheader =

  (videoinfoheader*)g_stillmediatype.pbformat;
 bitmapfileheader bfh;

  zeromemory(&bfh, sizeof(bfh));

  bfh.bftype = 'mb'; // little-endian for "mb".

  bfh.bfsize = sizeof( bfh ) + bufferlen + cbbitmapinfosize;

  bfh.bfoffbits = sizeof( bitmapfileheader ) + cbbitmapinfosize;

  

  // write the file header.

  dword dwwritten = 0;

  writefile( hf, &bfh, sizeof( bfh ), &dwwritten, null );

  writefile(hf, header(pvideoheader), cbbitmapinfosize, &dwwritten, null); 
  

  writefile( hf, pbuffer, bufferlen, &dwwritten, null );

  closehandle( hf );

  return s_ok;
 }

  };