Show QImage in a QLabel

QImage enables to construct images from file paths on the disk. Constructed QImages can be displayed in a QLabel. By setting the pixmap value of QLabel from setPixmap function, QLabel displays content of QImage.
Sample project started as a QT Gui Application and contains main.cpp, ShowQImageDialog.h and ShowQImageDialog.cpp files.
ShowQImageDialog constructor creates a QGridLayout instance that is containing a QScrollArea inside it and displays the image in this area.
Project Directory Structure in Qt Creator IDE :


ShowQImageDialog.pro is the project configuration file and contains:

TARGET = ShowQImageDialog
CONFIG   += core gui
TEMPLATE = app
SOURCES += main.cpp \
    ShowQImageDialog.cpp
HEADERS += \
    ShowQImageDialog.h

main.cpp file contains the custom QDialog instance and shows it.

#include <QApplication>
#include "ShowQImageDialog.h"
int main(int argc, char *argv[]) {
    QApplication a(argc, argv);
    ShowQImageDialog w;
    w.show();
    return a.exec();
}

ShowQImageDialog.h file is the extended QDialog class header file.

#ifndef SHOWQIMAGEDIALOG_H
#define SHOWQIMAGEDIALOG_H
#include <QDialog>
#include <QGridLayout>
#include <QScrollArea>
#include <QLabel>
#include <QImage>

class ShowQImageDialog : public QDialog
{
    Q_OBJECT

public:
    ShowQImageDialog(QWidget *parent = 0);
    ~ShowQImageDialog();
private:
    QGridLayout* gridLayout;
    QImage* inputImg;
    QLabel* imgDisplayLabel;
    QScrollArea* scrollArea;
};

#endif // SHOWQIMAGEDIALOG_H

ShowQImageDialog.cpp file contains implementation details for extended QDialog class.

#include "ShowQImageDialog.h"
ShowQImageDialog::ShowQImageDialog(QWidget *parent) : QDialog(parent)
{
    gridLayout = new QGridLayout();
    inputImg = new QImage("/home/tufan/wallpaper.png");
    imgDisplayLabel = new QLabel("");
    imgDisplayLabel->setPixmap(QPixmap::fromImage(*inputImg));
    imgDisplayLabel->adjustSize();
    scrollArea = new QScrollArea();
    scrollArea->setWidget(imgDisplayLabel);
    scrollArea->setMinimumSize(256,256);
    scrollArea->setMaximumSize(512,512);
    gridLayout->addWidget(scrollArea,0,0);
    setLayout(gridLayout);
}

ShowQImageDialog::~ShowQImageDialog()
{
    delete inputImg;
    delete imgDisplayLabel;
    delete scrollArea;
    delete gridLayout;
}

ShowQImageDialog:

Application created by QT Creator and uses Qt4.7 .

Check for valid file extension provided by QFileDialog

QFileDialog is used to select files from a specific directory. getOpenFileName() static function of QFileDialog returns the name of the selected file. Also, a filter can be set for the type of the file selected by the user by separating types with ';;'.
Ex:
"All Files (*.*);;JPEG (*.jpeg *.jpg);;PNG (*.png)"
In order to check for file extension of a file name provided by BrowseDialog which is extending QDialog and containing QFileDialog in its sl_browseImage() slot,
bool isValidImageFile(const QString& str)
function is implemented.
isValidImageFile function takes the full path of the file and splits it into pieces to find the file extension from the original QString instance.
File extension is placed after the last period character, so the last member of the QStringList contains the file extension that we are looking for.
By getting the upper case of the file extension which is provided by the client, we can search for a suitable match in our accepted file extension list. If the provided file extension is not in our list, then the derived custom QDialog opens a QMessageBox and gives a warning to the client.
Sample project started as a QT Gui Application and contains main.cpp, BrowseFileDialog.h and BrowseFileDialog.cpp files.

Project Directory Structure in Qt Creator IDE :

BrowseFileDialog.pro is the project configuration file contains:

TARGET = BrowseFileDialog
CONFIG   += core gui
TEMPLATE = app
SOURCES += main.cpp \
    BrowseFileDialog.cpp
HEADERS += \
    BrowseFileDialog.h

main.cpp file contains the custom QDialog instance and shows it.

#include <QApplication>
#include "BrowseFileDialog.h"
int main(int argc, char *argv[]) {
    QApplication a(argc, argv);
    BrowseFileDialog w;
    w.show();
    return a.exec();
}

BrowseFileDialog.h file is the derived QDialog class header file. Custom QDialog contains signals and slots to be associated with buttons on the dialog.

#ifndef BROWSEFILEDIALOG_H
#define BROWSEFILEDIALOG_H
#include <QDialog>
#include <QLineEdit>
#include <QPushButton>
#include <QLabel>

class BrowseFileDialog : public QDialog
{
    Q_OBJECT
public:
    BrowseFileDialog(QWidget *parent = 0);
    ~BrowseFileDialog();
public slots:
    void sl_browseImage();
private:
    void initLayout();
    void initConnections();
    bool isValidImageFile(const QString& str);
    QLabel* label;
    QLineEdit* lineEdit;
    QPushButton* pushButtonBrowse;
    QPushButton* pushButtonOK;
    QPushButton* pushButtonCancel;
};
#endif // BROWSEFILEDIALOG_H

BrowseFileDialog.cpp file contains implementation details for extended QDialog class.

#include "BrowseFileDialog.h"
#include <QHBoxLayout>
#include <QFileDialog>
#include <QString>
#include <QMessageBox>
#include <QSpacerItem>
BrowseFileDialog::BrowseFileDialog(QWidget *parent) : QDialog(parent)
{
    initLayout();
    initConnections();
}

BrowseFileDialog::~BrowseFileDialog()
{
    delete label;
    delete lineEdit;
    delete pushButtonBrowse;
    delete pushButtonOK;
    delete pushButtonCancel;
}

void BrowseFileDialog::initLayout()
{
    QGridLayout* mainLayout = new QGridLayout(this);
    QHBoxLayout* hBoxLayout = new QHBoxLayout();
    hBoxLayout->setSpacing(0);
    hBoxLayout->setMargin(0);
    label = new QLabel("Image File Path :");
    lineEdit = new QLineEdit(this);
    lineEdit->setReadOnly(true);
    pushButtonBrowse = new QPushButton("Browse...");
    hBoxLayout->addWidget(label);
    hBoxLayout->addWidget(lineEdit);
    hBoxLayout->addWidget(pushButtonBrowse);
    QHBoxLayout* actionWidgetsHBoxLayout = new QHBoxLayout();
    pushButtonCancel = new QPushButton("Cancel");
    pushButtonOK = new QPushButton("OK");
    actionWidgetsHBoxLayout->addStretch();
    actionWidgetsHBoxLayout->addWidget(pushButtonCancel);
    actionWidgetsHBoxLayout->addWidget(pushButtonOK);
    mainLayout->addLayout(hBoxLayout,0,0);
    mainLayout->addLayout(actionWidgetsHBoxLayout,1,0);
    setLayout(mainLayout);
}

void BrowseFileDialog::initConnections()
{
    connect(pushButtonBrowse, SIGNAL(clicked()), this, SLOT(sl_browseImage()));
    connect(pushButtonOK, SIGNAL(clicked()), this, SLOT(accept()));
    connect(pushButtonCancel, SIGNAL(clicked()), this, SLOT(reject()));
}

void BrowseFileDialog::sl_browseImage()
{
    QString inputImagePath=QFileDialog::getOpenFileName(this,tr("Select Image File"),QDir::currentPath(),
                                                        tr("All Files (*.*);;JPEG (*.jpeg *.jpg);;PNG (*.png)"));
    if(isValidImageFile(inputImagePath))
        lineEdit->setText(inputImagePath);
    else {
            QMessageBox::information(this, tr("Error!"),
                    tr("Please Provide a valid input image with JPG,JPEG or PNG extension!"));
    }
}

bool BrowseFileDialog::isValidImageFile(const QString& strImageFileName)
{
    QStringList acceptedImageFileTypeList;
    acceptedImageFileTypeList << "JPG"<< "JPEG" << "PNG";
    QStringList splittedStrList = strImageFileName.split(".");
    if( ((splittedStrList.size())-1) >= 0) {
        QString fileExtensionToCheck = splittedStrList[((splittedStrList.size())-1)];
        if( acceptedImageFileTypeList.contains(fileExtensionToCheck.toUpper()) )
            return true;
        else
            return false;
    }
    else
        return false;
}

BrowseFileDialog:

Application created by QT Creator and uses Qt4.7 .

Create Custom QDialogs With QLayout Classes

Qt Designer is used to design user interfaces for QT applications. In addition to using QT Designer to generate user interfaces, you can populate your dialogs with widgets on them dynamically at runtime by using layout classes. To achieve this, you need to extend qdialog class and add widgets to your derived qdialog instance dynamically at runtime.
Qt's layout classes are used to place widgets on parent dialogs or widgets. Instead of manually placing widgets on main containers and creating static user interfaces, layout classes help to place widgets easily. There are different QT layout classes but QHBoxLayout and QVBoxLayout are the most popular of them. By using the addWidget() method of layout class, you place items on the layout. After adding widgets to the layout properly, it is required to call setLayout() method of the main container to make layout visible on it.
Sample project started as a QT Gui Application and contains main.cpp, DynamicQDialog.h and DynamicQDialog.cpp files.
DynamicDialog.pro is the project configuration file contains:

TARGET = DynamicDialog
CONFIG   += qt gui
TEMPLATE = app
SOURCES += main.cpp \
    DynamicQDialog.cpp
HEADERS += \
    DynamicQDialog.h

main.cpp file contains the custom QDialog instance and shows it.

#include <QApplication>
#include "DynamicQDialog.h"
int main(int argc, char *argv[]) {
    QApplication app(argc, argv);
    DynamicQDialog dialog;
    dialog.show();
    return app.exec();
}

DynamicQDialog.h file is the derived QDialog class header file. Custom QDialog contains signals and slots to be associated with buttons on the dialog.

#ifndef DYNAMICQDIALOG_H
#define DYNAMICQDIALOG_H
#include <QDialog>
class DynamicQDialog : public QDialog
{
    Q_OBJECT
public:
    DynamicQDialog(QWidget * parent = 0, Qt::WindowFlags f = 0);
    ~DynamicQDialog();
public slots:
    void sl_accept();
    void sl_cancel();
signals:
};
#endif // DYNAMICQDIALOG_H

DynamicQDialog.cpp file contains implementation details for derived QDialog class. 2 QPushButtons added to the QHBoxLayout dynamically at runtime and displayed on the DynamicQDialog instance.

#include "DynamicQDialog.h"
#include <QHBoxLayout>
#include <QPushButton>
#include <QTextEdit>
#include <QMessageBox>

DynamicQDialog::DynamicQDialog(QWidget* parent, Qt::WindowFlags flags): QDialog( parent, flags )
{
    QGridLayout* mainGrid = new QGridLayout;
    QVBoxLayout* topLayout = new QVBoxLayout;
    topLayout->addWidget(new QTextEdit);
    mainGrid->addLayout(topLayout,0,0);

    QHBoxLayout* hLayout = new QHBoxLayout;
    QPushButton* btn;
    for( int i=0; i<2 br="" i="">        if( i == 0 ) {
            btn = new QPushButton("OK");
            connect(btn, SIGNAL(clicked()), this, SLOT(sl_accept()));
        }
        else if( i == 1 ) {
            btn = new QPushButton("Cancel");
            connect(btn, SIGNAL(clicked()), this, SLOT(sl_cancel()));
        }
        hLayout->addWidget(btn);
    }
    mainGrid->addLayout(hLayout,1,0);

    setLayout(mainGrid);
}

DynamicQDialog::~DynamicQDialog(){}

void DynamicQDialog::sl_accept(){
    QMessageBox msgBox;
    msgBox.setText("OK Clicked!");
    msgBox.exec();
}

void DynamicQDialog::sl_cancel(){
    QMessageBox msgBox;
    msgBox.setText("Cancel Clicked!");
    msgBox.exec();
}

Application created by QT Creator and uses Qt4.7 .