Windows平台使用DirectShow获取UVC摄像头上按键后的抓拍图

什么是UVC摄像头，UVC是什么意思？

UVC是一种标准的USB视频设备协议，就是传说中的免驱摄像头。UVC是Usb Video Class的简称。
Windows平台上UVC驱动文档

DirectShow简介

DirectShow是微软公司在ActiveMovie和Video for Windows的基础上推出的新一代基于COM(Component Object Model)的流媒体处理的开发包，与DirectX开发包一起发布。DirectShow使用一种叫Filter Graph的模型来管理整个数据流的处理过程，运用DirectShow，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。这样使在多媒体数据库管理系统（MDBMS）中多媒体数据的存取变得更加方便。它广泛地支持各种媒体格式，包括Asf、Mpeg、Avi、Dv、Mp3、Wave等，为多媒体流的捕捉和回放提供了强有力的支持。

调用DirectShow获取摄像头按键后的抓拍图片

MSDN文档：Video Capture

Filter

DirectShow使用模块化体系结构，其中每个处理阶段都由称为filter的COM对象完成。 DirectShow为应用程序提供了一组标准filter，开发人员可以编写自己的自定义过滤器来扩展DirectShow的功能。

Graph

有多个Filter连接，构成Graph。Filter Graph Manager用来构建和控制Graph中的Filter

代码实战

初始化各个Com接口

HRESULT GetInterfaces(void)
{
    HRESULT hr;

    //创建Filter Graph Manager.
    hr = CoCreateInstance (CLSID_FilterGraph, NULL, CLSCTX_INPROC,
        IID_IGraphBuilder, (void **) &pGraphBuilder);
    if (FAILED(hr))
        return hr;
    //创建Capture Graph Builder.
    hr = CoCreateInstance (CLSID_CaptureGraphBuilder2 , NULL, CLSCTX_INPROC,
        IID_ICaptureGraphBuilder2, (void **) &pCaptureGraphBuilder2);
    if (FAILED(hr))
        return hr;

    // IMediaControl接口，用来控制流媒体在Filter Graph中的流动，例如流媒体的启动和停止；
    hr = pGraphBuilder->QueryInterface(IID_IMediaControl,(LPVOID *) &pMediaControl);
    if (FAILED(hr))
        return hr;

    // IVideoWindow,用来显示预览视频
    hr = pGraphBuilder->QueryInterface(IID_IVideoWindow, (LPVOID *) &pVideoWindow);
    if (FAILED(hr))
        return hr;

    // IMediaEvent接口，该接口在Filter Graph发生一些事件时用来创建事件的标志信息并传送给应用程序
    hr = pGraphBuilder->QueryInterface(IID_IMediaEvent,(LPVOID *) &pMediaEvent);
    if (FAILED(hr))
        return hr;

    //创建用于预览的Sample Grabber Filter.
    hr = CoCreateInstance(CLSID_SampleGrabber, NULL, CLSCTX_INPROC_SERVER,
        IID_IBaseFilter, (void**)&pGrabberF);
    if (FAILED(hr))
        return hr;

    //获取ISampleGrabber接口，用于设置回调等相关信息
    hr = pGrabberF->QueryInterface(IID_ISampleGrabber, (void**)&pSampleGrabber);
    if (FAILED(hr)) 
        return hr;

    //创建用于抓拍的Sample Grabber Filter.
    hr = CoCreateInstance(CLSID_SampleGrabber, NULL, CLSCTX_INPROC_SERVER,IID_IBaseFilter, (void**)&pGrabberStill);
    if (FAILED(hr))
        return hr;

    //获取ISampleGrabber接口，用于设置回调等相关信息
    hr = pGrabberStill->QueryInterface(IID_ISampleGrabber, (void**)&pSampleGrabberStill);
    if (FAILED(hr)) 
        return hr;

    //创建Null Filter
    hr = CoCreateInstance(CLSID_NullRenderer,NULL,CLSCTX_INPROC_SERVER,IID_IBaseFilter,(void**)&pNull);
    if (FAILED(hr))
        return hr;
    return hr;
}

检测设备

//遍历设备，找到指定vidpid设备后打开设备
HRESULT InitMonikers()
{
    USES_CONVERSION;
    HRESULT hr;
    ULONG cFetched;

    //https://docs.microsoft.com/zh-cn/windows/desktop/DirectShow/selecting-a-capture-device
    //遍历设备
    ICreateDevEnum *pCreateDevEnum;
    hr = CoCreateInstance(CLSID_SystemDeviceEnum, NULL, CLSCTX_INPROC_SERVER, IID_ICreateDevEnum, (void**)&pCreateDevEnum);
    if (FAILED(hr))
    {
        printf("Failed to enumerate all video and audio capture devices!  hr=0x%x\n", hr);
        return hr;
    }

    IEnumMoniker *pEnumMoniker;
    hr = pCreateDevEnum->CreateClassEnumerator(CLSID_VideoInputDeviceCategory, &pEnumMoniker, 0);
    if (FAILED(hr) || !pEnumMoniker)
    {
        printf("Failed to create ClassEnumerator!  hr=0x%x\n", hr);
        return -1;
    }

    while (hr = pEnumMoniker->Next(1, &pMonikerVideo, &cFetched), hr == S_OK)
    {
        IPropertyBag *pPropBag;
        //BindToStorage之后就可以访问设备标识的属性集了。
        HRESULT hr = pMonikerVideo->BindToStorage(0, 0, IID_PPV_ARGS(&pPropBag));
        if (FAILED(hr))
        {
            pMonikerVideo->Release();
            continue;
        }

        VARIANT var;
        VariantInit(&var);
        //DevicePath中包含vidpid
        hr = pPropBag->Read(L"DevicePath", &var, 0);
        if (FAILED(hr))
        {
            VariantClear(&var);
            pMonikerVideo->Release();
            continue;
        }

        //比较是否是要使用的设备
        //TRACE("Device path: %S\n", var.bstrVal);
        std::string devpath = std::string(W2A(var.bstrVal));
        if (devpath.find("vid_06f8&pid_3015") == -1)
        {
            VariantClear(&var);
            pMonikerVideo->Release();
            continue;
        }
        VariantClear(&var);

        // BindToObject将某个设备标识绑定到一个DirectShow Filter，
        //     然后调用IFilterGraph::AddFilter加入到Filter Graph中，这个设备就可以参与工作了
        // 调用IMoniker::BindToObject建立一个和选择的device联合的filter，
        //      并且装载filter的属性(CLSID,FriendlyName, and DevicePath)。
        hr = pMonikerVideo->BindToObject(0, 0, IID_IBaseFilter, (void**)&pVideoCaptureFilter);
        if (FAILED(hr))
        {
            printf("Couldn't bind moniker to filter object!  hr=0x%x\n", hr);
            return hr;
        }

        pPropBag->Release();
        pMonikerVideo->Release();
    }

    pEnumMoniker->Release();
    return hr;
}

构建预览Graph

//开始构建预览graph
    //加入设备抓拍filter
    hr = pGraphBuilder->AddFilter(pVideoCaptureFilter, L"Video Capture");
    if (FAILED(hr))
    {
        printf("Couldn't add video capture filter to graph!  hr=0x%x\n", hr);
        pVideoCaptureFilter->Release();
        return hr;
    }

    //加入DirectShow中自带的SampleGrabber Filter
    hr = pGraphBuilder->AddFilter(pGrabberF, L"Sample Grabber");
    if (FAILED(hr))
    {
        printf("Couldn't add sample grabber to graph!  hr=0x%x\n", hr);
        // Return an error.
    }

    //使用Capture Graph Builder构建预览 Graph
    hr = pCaptureGraphBuilder2->RenderStream(&PIN_CATEGORY_CAPTURE, &MEDIATYPE_Video, pVideoCaptureFilter, pGrabberF, 0 );
    if (FAILED(hr))
    {
        printf("Couldn't render video capture stream. The device may already be in use.  hr=0x%x\n", hr);
        pVideoCaptureFilter->Release();
        return hr;
    }

构建抓拍Graph

    //加入DirectShow中自带的SampleGrabber Filter
    hr = pGraphBuilder->AddFilter(pGrabberStill, L"Still Sample Grabber");
    if (FAILED(hr))
    {
        printf("Couldn't add sample grabber to graph!  hr=0x%x\n", hr);
        // Return an error.
    }

    //加入DirectShow中自带的NullRender Filter
    hr = pGraphBuilder->AddFilter(pNull, L"NullRender");
    if (FAILED(hr))
    {
        printf("Couldn't add null to graph!  hr=0x%x\n", hr);
        return hr;
    }

    //使用Capture Graph Builder构建抓拍 Graph
    hr = pCaptureGraphBuilder2->RenderStream(&PIN_CATEGORY_STILL, &MEDIATYPE_Video, pVideoCaptureFilter, pGrabberStill, pNull);
    if (FAILED(hr))
    {
        printf("Couldn't render video capture stream. The device may already be in use.  hr=0x%x\n", hr);
        pVideoCaptureFilter->Release();
        return hr;
    }

    //configure the Sample Grabber so that it buffers samples :
    hr = pSampleGrabberStill->SetOneShot(FALSE);
    hr = pSampleGrabberStill->SetBufferSamples(TRUE);

    //获取设备输出格式信息
    AM_MEDIA_TYPE mt;
    hr = pSampleGrabber->GetConnectedMediaType(&mt);
    if (FAILED(hr))
    {
        return -1;
    }
    VIDEOINFOHEADER * vih = (VIDEOINFOHEADER*)mt.pbFormat;
    CSampleGrabberCB *CB = new CSampleGrabberCB();
    if (!FAILED(hr))
    {
        CB->Width = vih->bmiHeader.biWidth;
        CB->Height = vih->bmiHeader.biHeight;
    }

    //设置触发抓拍后，调用的回调函数 0-调用SampleCB 1-BufferCB
    hr = pSampleGrabberStill->SetCallback(CB, 1);
    if (FAILED(hr))
    {
        printf("set still trigger call back failed\n");
    }

启动Graph

    //设置预览窗口大小位置
    SetupVideoWindow();

    //启动设备
    hr = pMediaControl->Run();
    if (FAILED(hr))
    {
        printf("Couldn't run the graph!  hr=0x%x\n", hr);
        return hr;
    }
    else 
        psCurrent = Running;

使用GraphEdt进行调试

GraphEdt.exe是windows sdk中的graph调试工具，使用方法简介如下：

1. 查看设备
   在Graph菜单中点击Insert Filters，找到Video Capture Sources

   
   
   微信图片_20181018093403.png
   
   

   可以看到设备信息，点击Insert Filter可以将设备Filter加入到Graph中。

2. 构建Graph
   如果不清楚设备FIlter后边应该假如什么Filter，可以在Filter的Pin上右键，选择Render Filter，会自动连接一个Video Render。
   
   微信图片_20181018093830.png
   
   此时点击播放按钮，可以看到视频

源码

github