CODEWITHSUNDEEP
c++imageqt
Thalia
Thalia

Reputation: 14615

can I reuse the QPainter?

Rendering to bitmap, I have to create a QPainter. If I have to render multiple areas to multiple bitmaps, how do I reset the QPainter ?

QImage img1(scene1.sceneRect().size().toSize(), QImage::Format_ARGB32_Premultiplied);
img1.fill(Qt::color0);
QPainter painter1(&img1);
painter1.setRenderHint(QPainter::Antialiasing);
scene1.render(&painter1);
painter1.end();

QImage img2(scene2.sceneRect().size().toSize(), QImage::Format_ARGB32_Premultiplied);
img2.fill(Qt::color0);
QPainter painter2(&img2);
painter2.setRenderHint(QPainter::Antialiasing);
scene2.render(&painter2);
painter2.end();

How can I reuse the painter ? am I getting performance hits/higher memory usage by creating a new QPainter for each scene / image ?

Trying

QImage img(scene1.sceneRect().size().toSize(), QImage::Format_ARGB32_Premultiplied);
img.fill(Qt::color0);
QPainter painter(&img);
painter.setRenderHint(QPainter::Antialiasing);
scene1.render(&painter);
painter.end();
img.save("img.png");

img = QImage(scene2.sceneRect().size().toSize(), QImage::Format_ARGB32_Premultiplied);
img.fill(Qt::color0);
painter = QPainter(&img);
painter.setRenderHint(QPainter::Antialiasing);
scene2.render(&painter);
painter.end();

I get errors of the type

error: 'QPainter& QPainter::operator=(const QPainter&)' is private

Upvotes: 1

Views: 1058

Answers (4)

p.i.g.
p.i.g.

Reputation: 2985

You can reuse your QPainter with the following syntax:

QPainter painter;

painter.begin( &img1 );
...
painter.begin( &img2 );
...

But you can only use one QPainter for one QPaintDevice (in this case an image).

Read this.

Upvotes: 3

p.i.g.
p.i.g.

Reputation: 2985

I think you do not need to worry about the reusing of the QImage. You can instantiate a new instance and use the being( ... ) function of the QPainter class.

Upvotes: 0

p.i.g.
p.i.g.

Reputation: 2985

You do NOT need to write this:

QPainter* painter = new QPainter( &img );

you can write this instead:

QPainter painter( &img );

with this you don't have to think about the deleting of the obejct.

Yes, all the QObject derived classes have private copy constructors. You can read about the reasons here.

Upvotes: 0

Thalia
Thalia

Reputation: 14615

It appears that QObject and Classes derived from it, as well as QPainter and maybe other classes, have private constructors (??? why)

So...

QImage img(scene1.sceneRect().size().toSize(), QImage::Format_ARGB32_Premultiplied);
img.fill(Qt::color0);
QPainter* painter = new QPainter(&img);
painter->setRenderHint(QPainter::Antialiasing);
scene1.render(painter);
painter->end();
img.save("img.png");

img = QImage(scene2.sceneRect().size().toSize(), QImage::Format_ARGB32_Premultiplied);
img.fill(Qt::color0);
painter = new QPainter(&img);
painter->setRenderHint(QPainter::Antialiasing);
scene2.render(painter);
painter->end();

delete painter;

I was writing this as vizhanyolajos was posting his answer, and I think I prefer that answer.

Upvotes: 0

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