QGraphicsItem::shape

Jupiter

hi,

I want to set a QPainterPath containing an ellipse as shape of a Pixmap Item.

@QPainterPath p;
p.addEllipse(0, 0, 50, 50);
return p;@ is the code of my PainterPath.

it is also clipped because i set QGraphicsItem::ItemClipsToShape .

However, it looks very strange !http://www.image-load.net/users/public/images/MTIhXoLlBA.png(Screenshot)!

Why does it have such a jagged border?

any suggestions to fix it?

I already tried @setRenderHint(QPainter::Antialiasing);@

Jupiter

i forgot to mention, Im using raster paint engine on MacOS Lion

sigrid

Are you handling the item in the same coordinate space? I.e. are you doing clipping based on item coordinates or on scene coordinates? If you are mixing the two it could cause the problem you are having.

If mixing of the two is not the problem, then I would need more information on what exactly you are doing. Can you provide a small, complete example that shows the problem? That would make it easier to investigate what the cause of the problem can be.

Jupiter

i just set the shape in the item, and set the flag ItemClipsToShape in the same item. so AFAIK its both in the item coordinate system

Jupiter

ok when i set the same path as clipPath to the painter in the paint method, its not a problem. it looks like it should. but that doesn't solve my problem, since i don't have collision detection on the shape then

sigrid

I am unable to reproduce this problem with the example below. Does my example reproduce the problem for you? If not, can you modify it so that it does? Which version of Qt are you using and on which platform?

@
#include <QtGui>

class GraphicsItem : public QGraphicsPixmapItem
{

public:
GraphicsItem()
{
QPixmap pix("logo.png");
setFlags(flags() | QGraphicsItem::ItemClipsToShape);
setPixmap(pix);
}
QPainterPath shape () const
{
QPainterPath p;
p.addEllipse(0, 0, 50, 50);
return p;
}};
class GraphicsView : public QGraphicsView
{
Q_OBJECT
public:
GraphicsView()
{
QGraphicsScene *myScene = new QGraphicsScene(this);
setScene(myScene);
GraphicsItem *myItem = new GraphicsItem();
myScene->addItem(myItem);
setRenderHint(QPainter::Antialiasing);

}

};

#include "main.moc"

int main(int argc, char** argv)
{
QApplication app(argc, argv);
GraphicsView window;
window.show();
return app.exec();

}
@

Jupiter

ok i can't reproduce it either with your code, i have to find the difference.

Jupiter

sigrid what exactly do you mean with handling the item in the same coordinate space? that is always given, when i implement the shape method isn't it?

sigrid

It is usually a given if you work with only the item functions, but you could be taking coordinates from the scene and not translating them to the item for example and then that could cause problems.

Jupiter

no i don't do that.

also if i use the path in the paint method, and clip manually to it using (QPainter::fillPath) it works fine. but why is it a problem using the shape method?

sigrid

I need to see an example in order to see what goes wrong in that case for you. Can you modify the example I sent previously so that it reproduces the issue you are having?

Jupiter

i tried, but didn't work :)

Jupiter

hi sigrid.

i finally managed to change your code to show the same problem.

@#include <QtGui>

class GraphicsItemParent :public QGraphicsItem {
public:
GraphicsItemParent() {
setFlag(ItemClipsChildrenToShape);
}

QRectF boundingRect() const {
    return QRectF(0, 0, 200, 200);
}

void paint(QPainter *painter, const QStyleOptionGraphicsItem *option, QWidget *widget) {
    painter->setPen(Qt::black);
    painter->drawRect(boundingRect());
}

};

class GraphicsItem : public QGraphicsItem
{

public:
GraphicsItem(QColor c, QGraphicsItem* parent = 0):QGraphicsItem(parent)
{
setFlags(flags() | QGraphicsItem::ItemClipsToShape);
setFlag(ItemIsMovable);
color = c;
}

QRectF boundingRect() const {
return QRectF(0,0, 100, 100);
}
void paint(QPainter *painter, const QStyleOptionGraphicsItem *option, QWidget *widget) {
painter->setRenderHint(QPainter::Antialiasing);
painter->fillPath(shape(), color);
}

QPainterPath shape () const
{
QPainterPath p;
p.addEllipse(50, 50, 50, 50);
return p;
}
QColor color;
};
class GraphicsView : public QGraphicsView
{
Q_OBJECT
public:
GraphicsView()
{
QGraphicsScene myScene = new QGraphicsScene(this);
setScene(myScene);
GraphicsItemParent parent = new GraphicsItemParent();
myScene->addItem(parent);
GraphicsItem *myItem1 = new GraphicsItem(Qt::red, parent);
GraphicsItem *myItem2 = new GraphicsItem(Qt::green, parent);
myItem1->setPos(50, 50);
myItem2->setRotation(15);
setSceneRect(0, 0, 100, 100);
scale(3, 3);
setRenderHint(QPainter::Antialiasing);

}

};

#include "main.moc"

int main(int argc, char** argv)
{
QApplication app(argc, argv);
GraphicsView window;
window.show();
return app.exec();

}@

as you can see, i only changed to draw a colored circle instead of an image (same behavior with image). as well as i added a parent item.

now the both circles are jagged, as well as the green circle is clipped to the parents shape with jags because of its rotation.

What is the best work around to get it painted probably?

thanks

Nazgul

Jupiter

oh i forgot to say, that I'm also scaling the view. but imo that shouldn't make the difference.

sigrid

I do not see that problem here when running your example on Windows using Qt 4.7.3. Which Qt version are you using? Do you only see this problem on Mac? Can you send a screenshot of what you see when running the example you sent?

Jupiter

I'm using Qt 4.7.4 on Mac. I currently don't have a windows machine to test it there.

Here is the screen:

!http://www.image-load.net/users/public/images/IzBG3bbCdy.png(screenshot)!

Jupiter

as you can see, it becomes also jagged, when it clips to the rectangle

Jupiter

ok, i now tried with Qt 4.7.3 on Mac and got the same results as with qt 4.7.4.

then i tried on a windows machine (Windows Vista) also with qt 4.7.3 .

On windows there was a regular clipping like intended. no jags at all, like you experienced too.

is it a Qt Bug?

sigrid

This sounds like a Qt bug, I suggest you report it in "Jira":https://bugreports.qt.nokia.com//secure/Dashboard.jspa and include the example that reproduces the problem and the screenshots as well.

Jupiter

ok i created one.