前言

CaptureFilter就是用dshow采集视频数据，然后添加到filter graph中，获取数据后调用filter来处理。

代码分析

h:\webrtc-20210315\webrtc-20210315\webrtc\webrtc-checkout\src\modules\video_capture\windows\video_capture_ds.cc

VideoCaptureDS::Init

int32_t VideoCaptureDS::Init(const char* deviceUniqueIdUTF8) {
 ***
  _captureFilter = _dsInfo.GetDeviceFilter(deviceUniqueIdUTF8);
  if (!_captureFilter) {
    RTC_LOG(LS_INFO) << "Failed to create capture filter.";
    return -1;
  }
  // Get the interface for DirectShow's GraphBuilder
  HRESULT hr = CoCreateInstance(CLSID_FilterGraph, NULL, CLSCTX_INPROC_SERVER,
                                IID_IGraphBuilder, (void**)&_graphBuilder);
  if (FAILED(hr)) {
    RTC_LOG(LS_INFO) << "Failed to create graph builder.";
    return -1;
  }
  hr = _graphBuilder->QueryInterface(IID_IMediaControl, (void**)&_mediaControl);
  if (FAILED(hr)) {
    RTC_LOG(LS_INFO) << "Failed to create media control builder.";
    return -1;
  }
  hr = _graphBuilder->AddFilter(_captureFilter, CAPTURE_FILTER_NAME);
  if (FAILED(hr)) {
    RTC_LOG(LS_INFO) << "Failed to add the capture device to the graph.";
    return -1;
  }
  _outputCapturePin = GetOutputPin(_captureFilter, PIN_CATEGORY_CAPTURE);
  if (!_outputCapturePin) {
    RTC_LOG(LS_INFO) << "Failed to get output capture pin";
    return -1;
  }
 ***
}

—》 _captureFilter = _dsInfo.GetDeviceFilter(deviceUniqueIdUTF8);

DeviceInfoDS::GetDeviceFilter

IBaseFilter* DeviceInfoDS::GetDeviceFilter(const char* deviceUniqueIdUTF8,
                                           char* productUniqueIdUTF8,
                                           uint32_t productUniqueIdUTF8Length) {
***
  // enumerate all video capture devices
  RELEASE_AND_CLEAR(_dsMonikerDevEnum);
    // 遍历获取视频输入设备
  HRESULT hr = _dsDevEnum->CreateClassEnumerator(CLSID_VideoInputDeviceCategory,
                                                 &_dsMonikerDevEnum, 0);
  if (hr != NOERROR) {
    RTC_LOG(LS_INFO) << "Failed to enumerate CLSID_SystemDeviceEnum, error 0x"
                     << rtc::ToHex(hr) << ". No webcam exist?";
    return 0;
  }
  _dsMonikerDevEnum->Reset();
  ULONG cFetched;
  IMoniker* pM;
  IBaseFilter* captureFilter = NULL;
  bool deviceFound = false;
    // 遍历每个设备，这里只找到一个可用的设备就退出循环了
  while (S_OK == _dsMonikerDevEnum->Next(1, &pM, &cFetched) && !deviceFound) {
    IPropertyBag* pBag;
      // 获取IID_IPropertyBag接口属性
    hr = pM->BindToStorage(0, 0, IID_IPropertyBag, (void**)&pBag);
    if (S_OK == hr) {
      // Find the description or friendly name.
      VARIANT varName;
      VariantInit(&varName);
      if (deviceUniqueIdUTF8Length > 0) {
        hr = pBag->Read(L"DevicePath", &varName, 0);
        if (FAILED(hr)) {
          hr = pBag->Read(L"Description", &varName, 0);
          if (FAILED(hr)) {
            hr = pBag->Read(L"FriendlyName", &varName, 0);
          }
        }
        if (SUCCEEDED(hr)) {
          char tempDevicePathUTF8[256];
          tempDevicePathUTF8[0] = 0;
          WideCharToMultiByte(CP_UTF8, 0, varName.bstrVal, -1,
                              tempDevicePathUTF8, sizeof(tempDevicePathUTF8),
                              NULL, NULL);
          if (strncmp(tempDevicePathUTF8, (const char*)deviceUniqueIdUTF8,
                      deviceUniqueIdUTF8Length) == 0) {
            // We have found the requested device
            deviceFound = true; //找到了可用设备
              // 找到获取数据的filter
            hr =
                pM->BindToObject(0, 0, IID_IBaseFilter, (void**)&captureFilter);
            if
              FAILED(hr) {
                RTC_LOG(LS_ERROR) << "Failed to bind to the selected "
                                     "capture device "
                                  << hr;
              }
            if (productUniqueIdUTF8 &&
                productUniqueIdUTF8Length > 0)  // Get the device name
            {
              GetProductId(deviceUniqueIdUTF8, productUniqueIdUTF8,
                           productUniqueIdUTF8Length);
            }
          }
        }
      }
      VariantClear(&varName);
      pBag->Release();
    }
    pM->Release();
  }
  return captureFilter;
}