Help:Export QGraphicsView to Image File

[cometyang]

Hi, All.

I'm trying to export the GraphicsScene's content to Image File.
And I use the code example from the documentation.

QGraphicsScene scene;
 scene.addRect(QRectF(0, 0, 100, 200), QPen(Qt::black), QBrush(Qt::green));
 
 QPixmap pixmap;
 QPainter painter(&pixmap);
 painter.setRenderHint(QPainter::Antialiasing);
 scene.render(&painter);
 painter.end();
 
 pixmap.save("scene.png");

QGraphicsScene scene;
 scene.addRect(QRectF(0, 0, 100, 200), QPen(Qt::black), QBrush(Qt::green));

 QPixmap pixmap;
 QPainter painter(&pixmap);
 painter.setRenderHint(QPainter::Antialiasing);
 scene.render(&painter);
 painter.end();

 pixmap.save("scene.png");

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However, it failed.

The error message show that:
QPainter::begin:Cannot paint on a null pixmap
QPainter::setRenderHint: Painter must be active to set rendering hints
....

Anyone know what's wrong with the example code(I already correct some error in the given documentation)

I use Qt 4.3.2 on Linux platform

[marcel]

That's because, as the warning says, you're painting on a null pixmap. You first need to initialize the pixmap to the scene's size.
See QGraphicsScene::sceneRect()

[wysota]

The size doesn't have to be the size of the scene. This is perfectly correct:

QGraphicsScene scene;
//...
QImage img(1024,768,QImage::Format_ARGB32_Premultiplied);
QPainter p(&img);
scene.render(&p);
p.end();
img.save("XXX.png");

QGraphicsScene scene;
//...
QImage img(1024,768,QImage::Format_ARGB32_Premultiplied);
QPainter p(&img);
scene.render(&p);
p.end();
img.save("XXX.png");

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[marcel]

The size doesn't have to be the size of the scene. This is perfectly correct:

QGraphicsScene scene;
//...
QImage img(1024,768,QImage::Format_ARGB32_Premultiplied);
QPainter p(&img);
scene.render(&p);
p.end();
img.save("XXX.png");

QGraphicsScene scene;
//...
QImage img(1024,768,QImage::Format_ARGB32_Premultiplied);
QPainter p(&img);
scene.render(&p);
p.end();
img.save("XXX.png");

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I assumed he wanted to export the entire scene. Of course, the output image can be initialized to any size, as long as it is not (0,0).

[cometyang]

Thanks for your help.

The problem solved!

[wysota]

I assumed he wanted to export the entire scene.

If you don't pass the size to QGraphicsScene::render(), it will always draw the entire scene. The painter (image) size is only to determine the scale which is to be used to draw the scene.