Windows平臺使用DirectShow獲取UVC攝像頭上按鍵後的抓拍圖

什麼是UVC攝像頭，UVC是什麼意思？

UVC是一種標準的USB視頻設備協議，就是傳說中的免驅攝像頭。UVC是Usb Video Class的簡稱。
Windows平臺上UVC驅動文檔

DirectShow簡介

DirectShow是微軟公司在ActiveMovie和Video for Windows的基礎上推出的新一代基於COM(Component Object Model)的流媒體處理的開發包，與DirectX開發包一起發佈。DirectShow使用一種叫Filter Graph的模型來管理整個數據流的處理過程，運用DirectShow，我們可以很方便地從支持WDM驅動模型的採集卡上捕獲數據，並且進行相應的後期處理乃至存儲到文件中。這樣使在多媒體數據庫管理系統（MDBMS）中多媒體數據的存取變得更加方便。它廣泛地支持各種媒體格式，包括Asf、Mpeg、Avi、Dv、Mp3、Wave等，爲多媒體流的捕捉和回放提供了強有力的支持。

調用DirectShow獲取攝像頭按鍵後的抓拍圖片

MSDN文檔：Video Capture

Filter

DirectShow使用模塊化體系結構，其中每個處理階段都由稱爲filter的COM對象完成。 DirectShow爲應用程序提供了一組標準filter，開發人員可以編寫自己的自定義過濾器來擴展DirectShow的功能。

Graph

有多個Filter連接，構成Graph。Filter Graph Manager用來構建和控制Graph中的Filter

代碼實戰

初始化各個Com接口

HRESULT GetInterfaces(void)
{
	HRESULT hr;

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

	// IMediaControl接口，用來控制流媒體在Filter Graph中的流動，例如流媒體的啓動和停止；
	hr = pGraphBuilder->QueryInterface(IID_IMediaControl,(LPVOID *) &pMediaControl);
	if (FAILED(hr))
		return hr;

	// IVideoWindow,用來顯示預覽視頻
	hr = pGraphBuilder->QueryInterface(IID_IVideoWindow, (LPVOID *) &pVideoWindow);
	if (FAILED(hr))
		return hr;

	// IMediaEvent接口，該接口在Filter Graph發生一些事件時用來創建事件的標誌信息並傳送給應用程序
	hr = pGraphBuilder->QueryInterface(IID_IMediaEvent,(LPVOID *) &pMediaEvent);
	if (FAILED(hr))
		return hr;

	//創建用於預覽的Sample Grabber Filter.
	hr = CoCreateInstance(CLSID_SampleGrabber, NULL, CLSCTX_INPROC_SERVER,
		IID_IBaseFilter, (void**)&pGrabberF);
	if (FAILED(hr))
		return hr;

	//獲取ISampleGrabber接口，用於設置回調等相關信息
	hr = pGrabberF->QueryInterface(IID_ISampleGrabber, (void**)&pSampleGrabber);
	if (FAILED(hr)) 
		return hr;

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

	//獲取ISampleGrabber接口，用於設置回調等相關信息
	hr = pGrabberStill->QueryInterface(IID_ISampleGrabber, (void**)&pSampleGrabberStill);
	if (FAILED(hr)) 
		return hr;

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

####檢測設備

//遍歷設備，找到指定vidpid設備後打開設備
HRESULT InitMonikers()
{
	USES_CONVERSION;
	HRESULT hr;
	ULONG cFetched;

	//https://docs.microsoft.com/zh-cn/windows/desktop/DirectShow/selecting-a-capture-device
	//遍歷設備
	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之後就可以訪問設備標識的屬性集了。
		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;
		}

		//比較是否是要使用的設備
		//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將某個設備標識綁定到一個DirectShow Filter，
		//     然後調用IFilterGraph::AddFilter加入到Filter Graph中，這個設備就可以參與工作了
		// 調用IMoniker::BindToObject建立一個和選擇的device聯合的filter，
		//      並且裝載filter的屬性(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;
}

####構建預覽Graph

//開始構建預覽graph
	//加入設備抓拍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中自帶的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構建預覽 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;
	}

####構建抓拍Graph

	//加入DirectShow中自帶的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中自帶的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構建抓拍 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);

	//獲取設備輸出格式信息
	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;
	}

	//設置觸發抓拍後，調用的回調函數 0-調用SampleCB 1-BufferCB
	hr = pSampleGrabberStill->SetCallback(CB, 1);
	if (FAILED(hr))
	{
		printf("set still trigger call back failed\n");
	}

####啓動Graph

	//設置預覽窗口大小位置
	SetupVideoWindow();

	//啓動設備
	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進行調試
GraphEdt.exe是windows sdk中的graph調試工具，使用方法簡介如下：

1. 查看設備
   在Graph菜單中點擊Insert Filters，找到Video Capture Sources
   
   可以看到設備信息，點擊Insert Filter可以將設備Filter加入到Graph中。
2. 構建Graph
   如果不清楚設備FIlter後邊應該假如什麼Filter，可以在Filter的Pin上右鍵，選擇Render Filter，會自動連接一個Video Render。
   
   此時點擊播放按鈕，可以看到視頻
   ##源碼
   github