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);
}