"将 QPainter 与 QCoreApplication 一起使用"

Question

We have an application (QCoreApplication) that takes some images as input, does something to them, and exports them again. We now need to add some text to the images, and tried to do this with the QPainter class. It all worked well when using it in one of our other apps (using QApplication), but not in our main QCoreApplication app.

void drawTextOnImage(QImage* image, const QString& text, const QFont& font)
{
    QPainter p;
    if (!p.begin(image)) return;

    p.setFont(font);
    p.drawText(image->rect(), Qt::AlignLeft | Qt::AlignTop | Qt::TextWordWrap, text);

    p.end();
}

Answer 1

When using classes from "Qt Gui" like QPainter , you are supposed to use a QGuiApplication , not a QCoreApplication .

You might get lucky and be able to make some GUI stuff works while using only a QCoreApplication . But as you have discovered, it makes your application very brittle. Some classes like QPixmap will print an error message, but others will just crash.

The same is applicable with "Qt Widget": if you use a widget related class, you must use a QApplication .

Note that since QApplication inherits QGuiApplication , if you have a QApplication you can use "Qt Gui".

Answer 2

In case you need to run a non-GUI application on something without a windowing system, you need, aside from creating an instance of QGuiApplication<\/code> , to also choose an appropriate Qt platform.

#include <stdlib.h>
#include <QImage>
#include <QPainter>
#include <QGuiApplication>

int main(int argc, char** argv)
{
    setenv("QT_QPA_PLATFORM","offscreen",1);
    QGuiApplication app(argc,argv);

    QImage img(128,128, QImage::Format_RGB888);
    img.fill(Qt::white);
    QPainter p(&img);
    p.drawText(QPoint(0,64), "Works!");
    img.save("/tmp/test.png");
}