DirectShow 视频捕获(3)

 Capture Graph Builder

原文：http://msdn.microsoft.com/en-us/library/ms940047.aspx

执行视频或者音频捕获的过滤图表叫做捕获图表。捕获图表经常比 文件回放图表复杂。为了应用程序更简单的创建捕获图表，DirectShow提供了一个Capture Graph Builder 帮助对象。Capture Graph Builder暴露了ICaptureGraphBuilder2接口，这些接口包含创建和控制捕获图表的方法。

开始时调用CoCreateInstance创建一个新Capture Graph Builder和 Filter Graph Manager实例。然后使用指向过滤器图表管理器的IGraphBuilder 接口的指针调用ICaptureGraphBuilder2::SetFiltergraph来初始化Capture Graph Builder。

下面的代码显示了执行这些步骤的相关函数

HRESULT InitCaptureGraphBuilder(

  IGraphBuilder **ppGraph,         // Receives the pointer.

  ICaptureGraphBuilder2 **ppBuild  // Receives the pointer.

)

{

    if (!ppGraph || !ppBuild) {

        return E_POINTER;

    }

    IGraphBuilder *pGraph = NULL;

    ICaptureGraphBuilder2 *pBuild = NULL;

 

    // Create the Capture Graph Builder.

    HRESULT hr = CoCreateInstance(CLSID_CaptureGraphBuilder, NULL,

        CLSCTX_INPROC_SERVER, IID_IcaptureGraphBuilder2, (void**)&pBuild);

    if (SUCCEEDED(hr)) {

        // Create the Filter Graph Manager.

        hr = CoCreateInstance(CLSID_FilterGraph, 0, CLSCTX_INPROC_SERVER,

            IID_IGraphBuilder, (void**)&pGraph);

        if (SUCCEEDED(hr)) {

            // Initialize the Capture Graph Builder.

            pBuild->SetFiltergraph(pGraph);

 

            // Return both interface pointers to the caller.

            *ppBuild = pBuild;

            *ppGraph = pGraph; // The caller must release both interfaces.

            return S_OK;

        }

        else {

            pBuild->Release();

        }

    }

    return hr; // Failed

}

在这段视频捕获的代码中，假设你使用Capture Graph Builder来创建捕获图表。并且可以使用IGraphBuilder方法来创建完整的捕获图表。这被认为是一个高级的方法，而且Capture Graph Builder 方法要优先选择。要获取更多信息参加Advanced Capture Topics.