若改变图像适应label，则最后一行代码的src改为： src . scaled ( ui -> label -> size ())

若改变label适应图像，则在最后添加一行代码：   ui -> label -> resize ( ui -> label -> pixmap ()-> size ());

    QString fileName = QFileDialog::getOpenFileName(this, tr("打开图像"), ".", tr("*.jpg *.bmp *.png"));

    QPixmap src(fileName);//QPixmap src;   src.load(fileName);

    ui->label->setPixmap(src);//若改变图像适应label，则 ui->label->setPixmap(src.scaled(ui->label->size()));

【方法二】QImage转换为 QPixmap显示

    QString fileName = QFileDialog::getOpenFileName(this, tr("打开图像"), ".", tr("*.jpg *.bmp *.png"));

    QImage src(fileName);//QImage src;   src.load(fileName);

    ui->label->setPixmap(QPixmap::fromImage(src));

【方法三】Mat转换QImage再转换QPixmap显示

说明：注意imread读取路径名的格式；Mat类转换为QImage类的格式；

    QString fileName = QFileDialog::getOpenFileName(this, tr("打开图像"), ".", tr("*.jpg *.bmp *.png"));

    Mat openImage = imread(fileName.toLatin1().data());//imread读取路径名

    Mat openImage1;

    cvtColor(openImage1, openImage1, CV_BGR2RGB);

    QImage src = QImage((const unsigned char*)(openImage1.data), openImage1.cols, openImage1.rows, QImage::Format_RGB888);//Mat类转换为QImage类

    ui->label->setPixmap(QPixmap::fromImage(src.scaled(ui->label->size())));

【头文件】

#include <QMainWindow>

#include<QFileDialog>

#include<opencv2/opencv.hpp>

using namespace cv;

using namespace std;