无法存储（保存）QPainter图像

Question

I'm combining 2 QImages ("firstImage" and "secondImage") vertically with the help of QPainter. Combined image displays properly, with no error, as seen from the screen shot:

Combined Image Displays Properly

First Image

Second Image

But if I want to store the "combinedImage" by using .save, program halts and I get assert error.

How can I store the combined image into the harddisk?

mainwindow.cpp:

#include "mainwindow.h"
#include "ui_mainwindow.h"
#include <QPainter>
#include <QImage>
#include <QPaintEvent>

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

MainWindow::~MainWindow()
{
    delete ui;
}

void  MainWindow::paintEvent(QPaintEvent *)
{

    QString firstImagePath = "/home/nvidia/Desktop/TestFolder/firstImage.jpg";
    QString secondImagePath = "/home/nvidia/Desktop/TestFolder/secondImage.jpg";
    QImage firstImage(firstImagePath);
    QImage secondImage(secondImagePath);
    QImage combinedImage;
    QPainter paint(this);

    paint.begin(&combinedImage);
    paint.drawImage(0, 0, firstImage);
    paint.drawImage(0, firstImage.height()*1.1, secondImage);
    paint.end();

    bool i = firstImage.save("/home/nvidia/Desktop/TestFolder/firstImage-Copy.bmp");
    Q_ASSERT(i);

    bool j = combinedImage.save("/home/nvidia/Desktop/TestFolder/combinedImage-Copy.bmp");
    Q_ASSERT(j);
}   

mainwindow.h:

#ifndef MAINWINDOW_H
#define MAINWINDOW_H

#include <QMainWindow>

namespace Ui {
class MainWindow;
}

class MainWindow : public QMainWindow
{
    Q_OBJECT

public:
    explicit MainWindow(QWidget *parent = 0);
    ~MainWindow();
    void paintEvent(QPaintEvent *);

private:
    Ui::MainWindow *ui;
};

#endif // MAINWINDOW_H

Here is the Qt output:

Starting /home/nvidia/qtprojects/build-CombinedSave-Jetson_TX2-Debug/CombinedSave...
QPainter::begin: Painter already active
ASSERT: "j" in file ../CombinedSave/mainwindow.cpp, line 39
The program has unexpectedly finished.
/home/nvidia/qtprojects/build-CombinedSave-Jetson_TX2-Debug/CombinedSave crashed

firstImage-Copy.bmp is stored in the TestFolder but combinedImage-Copy.bmp is not..

Some of my system and program properties:

ARMv8 Processor rev 3 (v8l) × 4 ARMv8 Processor rev 0 (v8l) × 2
Ubuntu 16.04 LTS
Qt Creator 3.5.1 Based on Qt 5.5.1 (GCC 5.2.1 20151129, 64 bit)

Answer 1

Consider the code...

QPainter paint(this);
paint.begin(&combinedImage);

The first line is essentially...

QPainter paint;
paint.begin(this);

and makes the QPainter active on this (your MainWindow instance). But you then have...

paint.begin(&combinedImage);

which makes a second call to QPainter::begin on the already active QPainter . That's probably the source of the error message...

QPainter::begin: Painter already active

In addition, you also initialize combinedImage using the default QImage constructor...

QImage combinedImage;

At that point combinedImage is a null image -- it has no size or format associated with it and cannot be used as-is.

To initialise and render combinedImage try...

QImage combinedImage(std::max(firstImage.width(), secondImage.width()), /* Width */
                     firstImage.height() + secondImage.height(),        /* Height */
                     QImage::Format_ARGB32_Premultiplied);              /* Format */
{
    QPainter paint(&combinedImage);
    paint.drawImage(0, 0, firstImage);
    paint.drawImage(0, firstImage.height(), secondImage);
}