How to paint on pixmaps inside a QList<QPixmap>? - c++

Question

I'm trying to generate pixmaps with the following code
QList<QPixmap> pixmapList;
for (int i=0;i<50;++i){
QPixmap pixmap = QPixmap(10050,10050);
pixmap.fill(Qt::transparent);
pixmapList<<pixmap;
}
The above part works find. And I would like to paint on those pixmaps later, e.g.
QPixmap pixmap = pixmapList[10];
QPainter painter(&pixmap);
painter.drawPixmap(....); // this pixmap is 10*10
pixmapList[10]=pixmap;
or
QPainter painter(&pixmapList[10]);
painter.drawPixmap(....); // this pixmap is 10*10
but they both gave me "QPainter::begin: Paint device returned engine == 0, type: 2". May I ask the right way to paint on pixmaps in the pixmapList? Thanks very much!

Answer 1

Your code is OK, except that the pixmaps are too big (they occupy ~400MBytes each).
After making the pixmaps smaller, it works fine (shown for Qt 5):
#include <QGuiApplication>
#include <QPixmap>
#include <QPainter>
int main(int argc, char *argv[])
{
QGuiApplication a(argc, argv);
QList<QPixmap> pixmapList;
for (int i=0;i<50;++i){
QPixmap pixmap = QPixmap(1000,1000);
pixmap.fill(Qt::transparent);
pixmapList<<pixmap;
}
QPainter painter(&pixmapList[10]);
painter.drawLine(0, 0, 100, 100);
return 0;
}