Multithreading in QGraphicsItem painting

[wysota]

And to add to that -- painting everything to a pixmap that is created and destroyed in the paint routine and then painting that pixmap to the target device is slower then just painting to that device directly. Using clipping yields an additional performance hit as every paint operation has to check against the clipping rect/region.

[karankumar1609]

Thanks for all the precious comments., I have converted my paint from painting in pixmap to native painting.
The difference is sharpness.
when i use QPixmap it looks pretty sharp but still the time to paint on the screen is same for both native painting or pixmap painting.

well i have to do that mannually coz size of data and points will be changed at the run time according to slider position.
I have to do something smarter.,

[wysota]

well i have to do that mannually coz size of data and points will be changed at the run time according to slider position.

When the number of point changes, just create or destroy items representing those points.

[karankumar1609]

What if my points are in the range of 1000 - 6000 and my QGraphicsItem bounding rect is 300x200, how does it scale itself accordingly.
i cant scale my whole QGraphicsView to do that., just because if i scale my view i have to scale everything in the QGraphicsScene.

And ya i have use QGraphicsGridLayout to set plotter item.
There is something which i messed, just because i have jumped to my start point from where to start calculation like we have 100000 points and we have to start calculation from 500 to 550 now my pointer jumps to 500 directly and do calculation till 550 .
But doing this not return me a good result.

i am trying to find out the way to optimize my calculation to make it sharper.

[wysota]

What if my points are in the range of 1000 - 6000 and my QGraphicsItem bounding rect is 300x200, how does it scale itself accordingly.

Bounding rect of your item should be the size of the item (thus the size of the point) and should not be related to item position.

Here is a code sample that handles 100k items in the scene quite efficiently (use the mouse wheel to zoom).

#include <QApplication>
#include <QGraphicsView>
#include <QGraphicsScene>
#include <QGraphicsEllipseItem>
#include <QWheelEvent>
#include <QStyleOptionGraphicsItem>
 
class PointItem : public QGraphicsEllipseItem {
public:
    PointItem(QGraphicsItem *parent = 0) : QGraphicsEllipseItem(parent) {
        setPointSize(2);
        setBrush(Qt::red);
        setPen(Qt::NoPen);
        setFlag(QGraphicsItem::ItemIgnoresTransformations);
        setCacheMode(QGraphicsItem::DeviceCoordinateCache);
    }
    void setPointSize(qreal s) {
        setRect(-s, -s, 2*s, 2*s);
    }
};
 
class GraphicsView : public QGraphicsView {
public:
    GraphicsView(QWidget *parent = 0) : QGraphicsView(parent) {}
protected:
    void wheelEvent(QWheelEvent *event) {
        if(event->delta() > 0) {
            scale(1.25, 1.25);
        } else {
            scale( 0.8, 0.8);
        }
    }
};
 
int main(int argc, char *argv[])
{
    QApplication a(argc, argv);
    GraphicsView view;
    QGraphicsScene scene(0, 0, 10000, 5000);
    for(int i=0;i<100000;++i) {
        PointItem *item = new PointItem;
        item->setPos(qrand() % 10000, qrand() % 5000);
        scene.addItem(item);
    }
    view.setScene(&scene);
    view.setRenderHint(QPainter::Antialiasing);
    view.show();
    return a.exec();
}

#include <QApplication>
#include <QGraphicsView>
#include <QGraphicsScene>
#include <QGraphicsEllipseItem>
#include <QWheelEvent>
#include <QStyleOptionGraphicsItem>

class PointItem : public QGraphicsEllipseItem {
public:
    PointItem(QGraphicsItem *parent = 0) : QGraphicsEllipseItem(parent) {
        setPointSize(2);
        setBrush(Qt::red);
        setPen(Qt::NoPen);
        setFlag(QGraphicsItem::ItemIgnoresTransformations);
        setCacheMode(QGraphicsItem::DeviceCoordinateCache);
    }
    void setPointSize(qreal s) {
        setRect(-s, -s, 2*s, 2*s);
    }
};

class GraphicsView : public QGraphicsView {
public:
    GraphicsView(QWidget *parent = 0) : QGraphicsView(parent) {}
protected:
    void wheelEvent(QWheelEvent *event) {
        if(event->delta() > 0) {
            scale(1.25, 1.25);
        } else {
            scale( 0.8, 0.8);
        }
    }
};

int main(int argc, char *argv[])
{
    QApplication a(argc, argv);
    GraphicsView view;
    QGraphicsScene scene(0, 0, 10000, 5000);
    for(int i=0;i<100000;++i) {
        PointItem *item = new PointItem;
        item->setPos(qrand() % 10000, qrand() % 5000);
        scene.addItem(item);
    }
    view.setScene(&scene);
    view.setRenderHint(QPainter::Antialiasing);
    view.show();
    return a.exec();
}

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[karankumar1609]

Hello wysota,

your example is the pretty simple one.
But i am working it in a different way.

What your example does:
1. Create a View and Scene of a size
2. plot points(Items) on the scene
3. Scale the scene accordingly on whele event

but something i am working on is different
1. Create a view and scene of a size
2. create axis and plot area and set them in the layout (Layout for multiple graph stacked in a scene)
3. read all the data points and initialize to the plot item
4. On every update plot item draw each graph in it

The different is i can't use scale for zoomIn and ZoomOut purpose coz i have an item of fixed size., only the drawing will vary
means item will virtually zoom in and zoom out horizontally via drawing not via scale., coz if i scale my scene each graph and axis will scale instead of the graph item
and also i have to do something custom with the graph so i did my own painting.

well when i complete zoomOut your code nt remain smooth., anyway thanks for your code.
I think there is some issue with the pixmap painting or the calculation.
may be it should need optimization.

[wysota]

Then why are you using QGraphicsView at all?

[karankumar1609]

i cant scale my whole QGraphicsView to do that., just because if i scale my view i have to scale everything in the QGraphicsScene.

sorry for missunderstanding.,
but i said that i dont want to scale my view.
the problem with the scale is that i have to scale my whole scene .

[wysota]

Please answer my question.

[karankumar1609]

well graphicsView is the best API i have seen for drawing purpose.
It automatically update its child items which i dont want to handle.
QWidget need to update manually on each small changes.. that will effect the performance.

On Widget i have to mannually handle and draw each element.
like if i have to add a dynamic line element on graph i have to draw it and handle each and every moment.
In QGraphicsScene its easy.

[wysota]

well graphicsView is the best API i have seen for drawing purpose.
It automatically update its child items which i dont want to handle.

You only have one item currently.

QWidget need to update manually on each small changes.. that will effect the performance.

Based on what you have now, changing location of one point in the plot requires all 100k points to be redrawn. That sounds to me like "each small changes requires manual update of the whole plot".

On Widget i have to mannually handle and draw each element.

That's what you do now too. Both QWidget and QGraphicsView use QPainter API for painting. Each time a piece of your data changes, you redraw the whole canvas which is exactly the situation with widgets. With graphics view, modifying one point should only redraw that one point (and possibly others that intersect with it).

like if i have to add a dynamic line element on graph i have to draw it and handle each and every moment.

That's what you do now.

In QGraphicsScene its easy.

Yes, I know. Unfortunately you're using QGraphicsView like it were a plain widget.

[karankumar1609]

ya i did the same with QGraphicsItem like with QWidget.

Firstly i started my project with different idea, which suits with QGraphicsView.

nyway thanks for your help. i will do it in QWidget later.
well is QWidget is faster than QGraphicsView ?