OpenCV中的VideoCapture不僅可以開啟視訊、usb攝像頭，還可以做很多事，例如讀取流媒體檔案，網路攝像頭，影象序列等。

下面我簡單介紹一個在Qt中用VideoCapture類開啟膝上型電腦自帶攝像頭。

(我用的是VS2015+QT5.8+Opencv3.2)

主要用到下面兩個函式

 //獲取預設攝像頭
videocapture = new VideoCapture(0);

//把攝像頭獲取到的某一幀影象傳給 Mat matFrame
videocapture->read(matFrame);　　

然後 我們用Qt裡面的Qtimer類定時獲取影象，實現連續的每一幀影象的獲取，再把Mat顯示在介面裡面就好啦。

MainWindow.cpp

#pragma execution_character_set("utf-8")
#include "MainWindow.h"

//#include <QCameraViewfinder>

//QCamera：系統攝像裝置（攝像頭）
//QCameraViewfinder ：攝像取景器部件
//QCameraImageCapture：截圖部件
using namespace cv;

QtGuiApplication1::QtGuiApplication1(QWidget *parent)
	: QMainWindow(parent)
{
	ui.setupUi(this);
	this->setFixedSize(300, 400);
	//setWindowState(Qt::WindowMaximized);//max
	timer = new QTimer(this);
	timer->stop();
	connect(timer, SIGNAL(timeout()), this, SLOT(readFarme()));  // 時間到，讀取當前攝像頭資訊
	ok = true;

	pushButton = new QPushButton(tr("開始/暫停"),this);
	pushButton->setGeometry(QRect(0, 0, 300, 100));
	pushButton->setFont(QFont("Times", 32, QFont::Bold));
	connect(pushButton, SIGNAL(clicked()),this, SLOT(on_pushButton_clicked()));

	clickLabel = new QLabel(this);
	clickLabel->setGeometry(0, 100, 300, 300);

	//開啟攝像頭，從攝像頭中獲取視訊
	videocapture = new VideoCapture(0);
	//videocapture = new VideoCapture(0);
	//timer->start(33);
	
}

QImage QtGuiApplication1::cvMat2QImage(const Mat& mat)    // Mat 改成 QImage
{
	if (mat.type() == CV_8UC1)					// 單通道
	{
		QImage image(mat.cols, mat.rows, QImage::Format_Indexed8);
		image.setColorCount(256);				// 灰度級數256
		for (int i = 0; i < 256; i++)
		{
			image.setColor(i, qRgb(i, i, i));
		}
		uchar *pSrc = mat.data;					// 複製mat資料
		for (int row = 0; row < mat.rows; row++)
		{
			uchar *pDest = image.scanLine(row);
			memcpy(pDest, pSrc, mat.cols);
			pSrc += mat.step;
		}
		return image;
	}

	else if (mat.type() == CV_8UC3)				// 3通道
	{
		const uchar *pSrc = (const uchar*)mat.data;			// 複製畫素
		QImage image(pSrc, mat.cols, mat.rows, mat.step, QImage::Format_RGB888);	// R, G, B 對應 0,1,2
		return image.rgbSwapped();				// rgbSwapped是為了顯示效果色彩好一些。
	}
	else if (mat.type() == CV_8UC4)
	{
		const uchar *pSrc = (const uchar*)mat.data;			// 複製畫素
															// Create QImage with same dimensions as input Mat  
		QImage image(pSrc, mat.cols, mat.rows, mat.step, QImage::Format_ARGB32);		// B,G,R,A 對應 0,1,2,3
		return image.copy();
	}
	else
	{
		return QImage();
	}
}

Mat QtGuiApplication1::QImage2cvMat(QImage image)			// QImage改成Mat
{
	Mat mat;
	switch (image.format())
	{
	case QImage::Format_ARGB32:
	case QImage::Format_RGB32:
	case QImage::Format_ARGB32_Premultiplied:
		mat = Mat(image.height(), image.width(), CV_8UC4, (void*)image.constBits(), image.bytesPerLine());
		break;
	case QImage::Format_RGB888:
		mat = Mat(image.height(), image.width(), CV_8UC3, (void*)image.constBits(), image.bytesPerLine());
		cv::cvtColor(mat, mat, CV_BGR2RGB);
		break;
	case QImage::Format_Indexed8:
		mat = Mat(image.height(), image.width(), CV_8UC1, (void*)image.constBits(), image.bytesPerLine());
		break;
	}
	return mat;
}

////////////////////////////////////事件驅動///////////////////////////////////////////////////////
//開啟攝像頭
void QtGuiApplication1::on_pushButton_clicked()
{
	// 開始計時，超時則發出timeout()訊號
	if(ok)timer->start(33);
	else timer->stop();
	ok = !ok;
}
//讀取Frame影象 when timeout()
void QtGuiApplication1::readFarme()
{
	videocapture->read(matFrame);
	QImage imgg = cvMat2QImage(matFrame);
	QPixmap qpixmap = QPixmap::fromImage(imgg);
	
	// 將圖片顯示到label上
	clickLabel->setPixmap(qpixmap);
	
}

//exit
void QtGuiApplication1::bnClose()
{
	timer->stop();         // 停止讀取資料。
	videocapture->release();
	//exit
	QApplication* app;
	app->exit(0);
}

　　MainWindow.h

#pragma once

#include <QtWidgets/QMainWindow>
#include "ui_QtGuiApplication1.h"
#include <opencv2/opencv.hpp>
#include <opencv2/core/core.hpp>
#include <opencv2/highgui/highgui.hpp>
#include <opencv2/imgproc/imgproc.hpp>
#include "qimage.h"

#include <QFileDialog>
#include <QLabel>
#include <QTimer>
#include <QPushButton>
#include <QCamera>
#include <QCameraImageCapture>
using namespace cv;          //OpenCV名稱空間

class QtGuiApplication1 : public QMainWindow
{
	Q_OBJECT

public:
	QtGuiApplication1(QWidget *parent = Q_NULLPTR);

private:
	Ui::QtGuiApplication1Class ui;
	QImage cvMat2QImage(const Mat & mat);
	Mat QImage2cvMat(QImage image);

	QTimer *timer;	
	bool ok;
	VideoCapture *videocapture;
	Mat matFrame;
	QLabel *clickLabel;
	QPushButton *pushButton;

private slots:
	void on_pushButton_clicked();
	void readFarme();
	void bnClose();
};