用QT5的QCamera实现USB摄像头之截图保存功能

通常来说Linux下可以通过V4L2接口及ioctl相关函数直接在底层调用摄像头设备，进行摄像头控制及图像预览和捕获，相对复杂。

QT5.0新增QMultimedia模块提供了更为方便的编程支持，模块涵盖了视，音频及摄像头功能，提供了QML类型和C++类用以处理多媒体内容。

环境： QT5.9.0  Qt Creator Ubuntu 16.04.6 LTS  Linux-4.15.0-133-generic

1.创建Qt Widgets Application工程，并添加QT +=multimedia，QT+=multimediawidgets项

2双击mainwindow.ui启动qt designer可视化布局界面 
首先在布局左侧放置一个Horizental Layout控件，修改对象名为ImageView，用于图像预览显示；再在右侧放一个Vertical Layout 控件，依次在其中放置一个label和4个Push Button，修改label的对象名为ImageCapture，用于显示捕获的图像，修改按钮的显示名称及对象名分别为buttonCapture, buttonOpen,buttonSave, buttonQuit，添加一个ComboBox存放摄像头列表，拖动控件到合适的布局，如图 
   

3. mainwindow.h

#ifndef MAINWINDOW_H
#define MAINWINDOW_H


#include <QMainWindow>


#include <QCamera>


#include <QCameraViewfinder>


#include <QCameraImageCapture>


#include <QFileDialog>




namespace Ui {


class MainWindow;


}




class QCamera;


class QCameraViewfinder;


class QCameraImageCapture;






class MainWindow : public QMainWindow


{


    Q_OBJECT




public:


    explicit MainWindow(QWidget *parent = 0);


    ~MainWindow();


    void openCamera(QString description=QString());


    void enumCamera();


    void bindRecoder(QCamera *camera);


private slots:


    void captureImage();


    void displayImage(int, QImage);


    void saveImage();


    void openCamera_on_clicked();




private:


    Ui::MainWindow *ui;


    QCamera *camera;       //摄像头对象


    QCameraViewfinder *viewfinder;  //摄像头取景器


    QCameraImageCapture *imageCapture;  //截图对象


};


#endif // MAINWINDOW_



4. mainwindow.cpp


#include "mainwindow.h"


#include "ui_mainwindow.h"


#include <QCameraInfo>


#include <QUrl>




MainWindow::MainWindow(QWidget *parent) :


    QMainWindow(parent),


    ui(new Ui::MainWindow)


{


    ui->setupUi(this);


   // camera=new QCamera(this);


    viewfinder = new QCameraViewfinder(this);


    ui->ImageView->addWidget(viewfinder);


    ui->ImageCapture->setScaledContents(true);




    connect(ui->buttonSave,SIGNAL(clicked()),this,SLOT(saveImage()));


    connect(ui->buttonQuit,SIGNAL(clicked()),qApp,SLOT(quit()));


    connect(ui->buttonOpen,SIGNAL(clicked()),this,SLOT(openCamera_on_clicked()));


    enumCamera();


}




MainWindow::~MainWindow()


{


    if(camera != NULL)


        camera->stop();


    delete ui;


}




void MainWindow::openCamera_on_clicked()


{


    openCamera();


   // openCamera(ui->comboBox->currentText());


}


void MainWindow::openCamera(QString description)


{


    if(description.isEmpty())


    {


       QCameraInfo info = QCameraInfo::defaultCamera();


       camera=new QCamera(info, this);


    }else


    {


        QList<QCameraInfo> cameras = QCameraInfo::availableCameras();


        foreach(const QCameraInfo &cameraInfo, cameras)


        {


            if(cameraInfo.description() == description)


            {


                camera = new QCamera(cameraInfo,this);


                break;


            }


        }


    }


    //camera->setCaptureMode(QCamera::CaptureVideo);


    camera->setViewfinder(viewfinder);


    imageCapture = new QCameraImageCapture(camera);




    connect( camera, static_cast<void(QCamera::*)(QCamera::Error)>(&QCamera::error),


             [=](QCamera::Error value){ qDebug()<<value;});


    connect(imageCapture,SIGNAL(imageCaptured(int,QImage)),this,SLOT(displayImage(int,QImage)));




    connect(ui->buttonCapture,SIGNAL(clicked()),this,SLOT(captureImage()));


    camera->start();


}




void MainWindow::captureImage()


{


  if(!camera)


     return;




  ui->statusBar->showMessage(tr("capturing..."),1000);


  imageCapture->capture();


}




void MainWindow::displayImage(int id,QImage image)


{


    qDebug()<<"hello->"<<id;


    ui->ImageCapture->setPixmap((QPixmap::fromImage(image)));


    ui->statusBar->showMessage(tr("capture OK!"),5000);


}




void MainWindow::enumCamera()


{


    QList<QCameraInfo> cameras = QCameraInfo::availableCameras();




    foreach(const QCameraInfo &cameraInfo, cameras)


    {


         qDebug()<<"camera:"<<cameraInfo.description();


        ui->comboBox->addItem(cameraInfo.description());


    }




}




void MainWindow::saveImage()


{


    QString fileName=QFileDialog::getSaveFileName(this,tr("save file"),QDir::homePath(),tr("jpegfile(*.jpg)"));


    if(fileName.isEmpty()){


        ui->statusBar->showMessage(tr("save cancel"), 5000);


        return;


    }


  const QPixmap* pixmap=ui->ImageCapture->pixmap();


  if(pixmap){


    pixmap->save(fileName);


    ui->statusBar->showMessage(tr("save OK"),5000);


    }


}


5.编译运行


  


QCamera类封装了很多底层操作，为了进一步了解Linux下的摄像头的调用机制，我们可以试验下V4L2和ioctl操作摄像头机理。


6.




#include <stdio.h>
#include <stdlib.h>
#include <unistd.h>
#include <error.h>
#include <string.h>
#include <fcntl.h>
#include <linux/videodev2.h>
#include<sys/ioctl.h>


int fd;
const char *input_dev = "/dev/video0";
//const char *input_dev = "/dev/vboxusb/002";
const char *qctrl_name = NULL;
int qctrl_value = 0;


struct v4l2_capability cap;
struct v4l2_queryctrl qctrl;



static void print_qctrl(struct v4l2_queryctrl *qctrl)
{
    struct v4l2_control ctrl;
 
    ctrl.id = qctrl->id;
    if (ioctl(fd, VIDIOC_G_CTRL, &ctrl) < 0) 
    {
        perror("get ctrl failed");
        ctrl.value = -999;
    }
 
    printf("%-14s : id=%08x, type=%d, minimum=%d, maximum=%d
"
            "		 value = %d, step=%d, default_value=%d
", 
            qctrl->name, qctrl->id, qctrl->type, qctrl->minimum, qctrl->maximum,
            ctrl.value, qctrl->step, qctrl->default_value);
}
static void print_menu(struct v4l2_querymenu *menu)
{
    printf("	 %d : %s
", menu->index, menu->name);
}
static int set_qctrl(struct v4l2_queryctrl *qctrl)
{
    struct v4l2_control ctrl;
 
    printf("set %s = %d
", qctrl_name, qctrl_value);
 
    ctrl.id = qctrl->id;
    ctrl.value = qctrl_value;
    return ioctl(fd, VIDIOC_S_CTRL, &ctrl);
}
static void deal_qctrl(struct v4l2_queryctrl *qctrl)
{
    print_qctrl(qctrl);
    if (qctrl_name && !strcmp(qctrl_name, (const char *)qctrl->name))
        set_qctrl(qctrl);
}
 
static void qctrl_get(int id)
{
    qctrl.id = id;
    if (ioctl(fd, VIDIOC_QUERYCTRL, &qctrl) == 0) 
    {
        deal_qctrl(&qctrl);
        if (qctrl.type == V4L2_CTRL_TYPE_MENU) 
        {
            int idx;
            struct v4l2_querymenu menu;
            for (idx = qctrl.minimum; idx <= qctrl.maximum; idx++) 
            {
                menu.id = qctrl.id;
                menu.index = idx;
                if (ioctl(fd, VIDIOC_QUERYMENU, &menu)==0) 
                {
                    print_menu(&menu);
                }
            }
        }
    }
}
 
int main(int argc, char **argv)
{
    int ret, i;
 
    if (argc == 3) 
    {
        qctrl_name = argv[1];
        qctrl_value = atoi(argv[2]);
    }
 
    fd = open(input_dev, O_RDWR);
    if (fd < 0) 
    {
        perror("open video failed");
        return -1;
    }
    printf("open video '%s' success
", input_dev);
 
    ret = ioctl(fd, VIDIOC_QUERYCAP, &cap);
    if (ret < 0) 
    {
        perror("ioctl querycap");
        return -1;
    }
 
    if ((cap.capabilities &  V4L2_CAP_VIDEO_CAPTURE) == 0) 
    {
        printf("video device donot support capture
");
        return -1;
    }
 
    for (i = V4L2_CID_BASE; i < V4L2_CID_LASTP1; i++) 
    {
        qctrl_get(i);
    }
 
    for (i = V4L2_CID_PRIVATE_BASE; i < V4L2_CID_PRIVATE_BASE+25; i++) 
    {
        qctrl_get(i);
    }
 
 
    printf("close video
");
    close(fd);
}






        

	      

	    

	  

	  

	    
      
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          原文地址：https://www.cnblogs.com/7star/p/14408246.html