c++ - 如何使用 DirectShow 和网络摄像头预览图像

Question

我正在尝试使用 C++ 和 DirectShow 来显示我的网络摄像头...但是我遇到了一些麻烦。下面的代码给我一个段错误:

m_pDF->EnumPins(&pinEnum);

我的 cpp 代码:

#include <tchar.h>
#include <strsafe.h>
#include <dshow.h>
#include <atlbase.h>
#include <d3d9.h>
#include <vmr9.h>

#pragma comment(lib,"Strmiids.lib")

#define SAFE_RELEASE(x) { if (x) x->Release(); x = NULL; }

HRESULT EnumerateDevices(REFGUID category, IEnumMoniker **ppEnum)
{
    // Create the System Device Enumerator.
    ICreateDevEnum *pDevEnum;   // Video and Audio interface object
    HRESULT hr = CoCreateInstance(CLSID_SystemDeviceEnum, NULL, CLSCTX_INPROC_SERVER, IID_PPV_ARGS(&pDevEnum)); // Creates the system device enumerator

    if (SUCCEEDED(hr))
    {
        // Create an enumerator for the category.
        hr = pDevEnum->CreateClassEnumerator(category, ppEnum, 0);  // Enumeration of 'category' objects
        if (hr == S_FALSE)
        {
            hr = VFW_E_NOT_FOUND;  // The category is empty. Treat as an error.
        }
        pDevEnum->Release(); // Deletes Enumeration object
    }
    return hr;
}

void main(void)
{
    IGraphBuilder *pGraph = NULL;
    IMediaControl *pControl = NULL; // contains methods for stopping and starting the graph
    IMediaEvent   *pEvent = NULL;   // methods for getting events from the Filter Graph Manager

    IPin *m_pCamOutPin;
    IBaseFilter *m_pDF=NULL;
    IMoniker *pM;
    IEnumMoniker *pEnum; // Enumerator object

    // Initialize the COM library.
    HRESULT hr = CoInitialize(NULL);
    if (FAILED(hr))
    {
        printf("ERROR - Could not initialize COM library");
        return;
    }

    // Create the filter graph manager and query for interfaces.
    hr = CoCreateInstance(CLSID_FilterGraph, NULL, CLSCTX_INPROC_SERVER, 
                        IID_IGraphBuilder, (void **)&pGraph);
    if (FAILED(hr))
    {
        printf("ERROR - Could not create the Filter Graph Manager.");
        return;
    }

    // Bind
    hr = EnumerateDevices(CLSID_VideoInputDeviceCategory, &pEnum);
    pEnum->Next(1, &pM, NULL);
    pM->BindToObject(0, 0, IID_IBaseFilter, (void**)m_pDF);
    pM->Release();

    hr=pGraph->AddFilter(m_pDF, L"Video Capture");

    CComPtr<IEnumPins> pinEnum;
    m_pDF->EnumPins(&pinEnum);

    hr = pinEnum->Reset();
    hr = pinEnum->Next(1, &m_pCamOutPin, NULL); 

    if (FAILED(hr))
    return;

    // control
    hr = pGraph->QueryInterface(IID_IMediaControl, (void **)&pControl);
    hr = pGraph->QueryInterface(IID_IMediaEvent, (void **)&pEvent);

    // Build the graph. IMPORTANT: Change this string to a file on your system.
    hr = pGraph->Render(m_pCamOutPin);
    if (SUCCEEDED(hr))
    {
        // Run the graph.
        hr = pControl->Run();
        if (SUCCEEDED(hr))
        {
            // Wait for completion.
            long evCode;
            pEvent->WaitForCompletion(INFINITE, &evCode);

            // Note: Do not use INFINITE in a real application, because it
            // can block indefinitely.
        }
    }
    SAFE_RELEASE(pControl);
    SAFE_RELEASE(pEvent);
    SAFE_RELEASE(pGraph);
    CoUninitialize();
}

关于我做错了什么以及我必须做什么才能让它发挥作用有什么想法吗？

Answer 1

m_pDF 是指向 IBaseFilter 的指针:

IBaseFilter *m_pDF=NULL;

BindToObject 最后一个参数是指向指针的指针。这意味着这个函数调用是错误的:

pM->BindToObject(0, 0, IID_IBaseFilter, (void**)m_pDF);

相反，您需要传递 m_pDF 的地址以获取指向指针的指针:

pM->BindToObject(0, 0, IID_IBaseFilter, (void**)&m_pDF);

将IBaseFilter** 转换为void** 没有问题，但是不能将IBaseFilter* 转换为无效**。