why show error "mainwindow.obj:-1: error: LNK2019: unresolved external symbol _cvLoadImage referenced in function "private: void __thiscall MainWindow::openImage(void)"...

#include "mainwindow.h"
#include "ui_mainwindow.h"
#include <opencv/highgui.h>
#include <opencv/cv.h>
#include <opencv2/core/core.hpp>
#include <opencv2/highgui/highgui.hpp>
#include <opencv2/core/mat.hpp>
#include <opencv2/objdetect/objdetect.hpp>

MainWindow::MainWindow(QWidget *parent) :
    QMainWindow(parent),
    ui(new Ui::MainWindow)
{
    ui->setupUi(this);
}
using namespace cv;
using namespace std;

MainWindow::~MainWindow()
{
    setAttribute(Qt::WA_QuitOnClose);
    delete ui;
}
void MainWindow::openImage ()
{
    fileName = QFileDialog::getOpenFileName(this,tr("Open Image"),
                                            QDir::currentPath(),
                                            tr("Image Files [ *.jpg , *.jpeg , *.bmp , *.png , *.gif]"));
    charFileName = fileName.toLocal8Bit().data();
    iplImg = cvLoadImage(charFileName);

    qimgNew = QImage((const unsigned char*)iplImg->imageData,iplImg->width,iplImg->height,QImage::Format_RGB888).rgbSwapped();
    ui->PictureOriginal->setPixmap(QPixmap::fromImage(qimgNew));

}


void MainWindow::on_OpenImage_clicked()
{
    openImage();
}