[QtEmbedded] Translucent QGraphicsView with animated child widgets

Hi,

I'm working with Qt 4.6 and I've a QGraphicsView with some animated QGraphicsItems (I'm using QML). I want to make the window background transparent, so I can see the desktop under it.

I've tried this piece of code:

Code:

---

MainWindow::MainWindow(QWidget* parent)
    :   QmlView(parent)
{
    this->setWindowFlags(Qt::FramelessWindowHint);
    this->setAttribute(Qt::WA_NoSystemBackground);
    this->viewport()->setAutoFillBackground(false);
}

---

It works, but only for static widgets: the animated widget paints a trail on the window surface.

So, how can I erase this trail?
For example, how does KDE work with desktop semi-transparent animated widgets?

This is not how it works. WA_NoSystemBackground only means the background will not be initialized by the system when the widget is redrawn (hence the garbage left). Use WA_TranslucentBackground instead.

I know, but the result is the same in this case (Wa_TranslucentBackground set the WA_NoSystemBackground attribute too).

Does this work?

Code:

---

#include <QtGui>
#include <QPropertyAnimation>
 
class RectItem : public QObject, public QGraphicsRectItem {
  Q_OBJECT
  Q_PROPERTY(QPointF pos READ pos WRITE setPos)
public:
  RectItem(const QRectF &rect, const QPen &pen, const QBrush &brush) : QObject(), QGraphicsRectItem(rect){
    setPen(pen);
    setBrush(brush);
  }
};
 
#include "main.moc"
 
int main(int argc, char **argv){
  QApplication app(argc, argv);
  QGraphicsView view;
  view.setAttribute(Qt::WA_TranslucentBackground);
  view.viewport()->setAttribute(Qt::WA_TranslucentBackground);
  QGraphicsScene scene(QRectF(0,0,1000,800));
  view.setScene(&scene);
  RectItem *rect = new RectItem(QRectF(0,0,200,100), QPen(Qt::red), Qt::blue);
  scene.addItem(rect);
  QPropertyAnimation *anim = new QPropertyAnimation(rect, "pos");
  anim->setDuration(10000);
  anim->setStartValue(scene.sceneRect().topLeft());
  anim->setEndValue(scene.sceneRect().bottomRight());
  anim->start();
  view.show();
  return app.exec();
}

---

No, the behaviour is the same.

Please, note that I'm working on Qt Embedded for Linux and not on Qt x11.

In that case I'd assume your device doesn't allow you to use translucency.

Please post your problem in the right section of the forum next time.

I'm sorry.

Well, currently the test is running on QVFB and not on the final device.

I've tried to run it with Qt x11 and the result is a window with a black background (the rectangle doesn't paint the trail but the background is fully opaque).

Quote:

---

Originally Posted by zuck

I've tried to run it with Qt x11 and the result is a window with a black background (the rectangle doesn't paint the trail but the background is fully opaque).

---

It probably means you have composition extension disabled in your X config. Run xdpyinfo and check if Composite is reported as one of the extensions.

Yes, I have it:

Code:

---

number of extensions:    30
    BIG-REQUESTS
    Composite <----------
    DAMAGE
    DOUBLE-BUFFER
    DPMS
    DRI2
    GLX
    Generic Event Extension
    MIT-SCREEN-SAVER
    MIT-SHM
    NV-CONTROL
    NV-GLX
    RANDR
    RECORD
    RENDER
    SECURITY
    SHAPE
    SYNC
    X-Resource
    XC-MISC
    XFIXES
    XFree86-DGA
    XFree86-VidModeExtension
    XINERAMA
    XINERAMA
    XInputExtension
    XKEYBOARD
    XTEST
    XVideo
    XVideo-MotionCompensation

---

The code I gave you works on my system (Linux 2.6 + KDE4 + Qt 4.6). So if it doesn't work on yours, this has to be a hardware or software configuration issue on your end.

Ok, thanks!

It now works on X11 with xcompmgr activated, but it still remains bugged on Qt Embedded :(

Because you have no compositing support there. It simply won't work. You can use QPixmap::grabWidget() to grab the image of the content beneath and render it as background of your widget above.

Yeah! This is the final working code (Qt 4.6):

Code:

---

#include <QtGui/QDesktopWidget>
#include <QtGui/QApplication>
 
MainWindow::MainWindow(QWidget* parent)
    :   QmlView(parent) // or QGraphicsView...
{
    this->setWindowFlags(Qt::FramelessWindowHint);
    this->setCacheMode(QGraphicsView::CacheBackground);
}
 
void MainWindow::showEvent(QShowEvent* event)
{ 
    WId wid = QApplication::desktop()->winId();
 
    int x = this->x();
    int y = this->y();
    int w = this->width();
    int h = this->height();
 
    this->setBackgroundBrush(QPixmap::grabWindow(wid, x, y, w, h));
 
    QmlView::showEvent(event);
}

---

No "WA_TranslucentBackground" or "autoFillBackground(false)" are needed! :)