We have big QT project where painting procedure often doesn't follow the rule that it should be done in overridden paintEvent method. As result we have warnings about it: Painter not active etc... But all work fine and at first glance I don't see any problems. Could you explain should I worry about it or not? What is the price of the incorrect use of this functionality?
Paint event is sended to the window when it is should be updated, eg when it is shown or something else. For example if widget is covered by another window, and this windows is moved away, then widget should be updated. Common way is to paint on the pixmap and draw this pixmap on the widget in the paint event handler. Or you can update/repaint each time you need to repaint it.
You can use QPainter to draw on pixmap, printer and so on whenever you want, but to draw on windget it must be done in paintEvent.
I found mistake - it is happened when invalid pixmap(I have created pixmap with size 0x0) is used. I have add check on it and now all are okey.
Related
My paintEvent has access to a pointer whose value changes from time to time and what gets painted depends on these values. With basic debugging I'm sure that this function is being called but the window does not get updated. The new stuff only appears on the window when it loses focus to another application.
If this is about performance I can set a static variable in the paintEvent to check if the pointer has been updated or not to avoid unnecessary repaints. It would be nice if Qt would just paint when I told it to.
I was hoping someone could help me out or point me in the direction of the right documentation. Thanks.
you must call update() method to repaint your widget.
I am having what appears to be the same problem as asked in this (unanswered) question: Qt resizeGL problem
I am testing a new QGLWidget for a larger application. The resizeGL method is wired up to change the glViewport and repaint the OpenGL view. My QGLWidget is not part of a layout and is simply being created displayed as follows:
boost::shared_ptr<StandardCustomWidgetBuilder>
builder(new StandardCustomWidgetBuilder());
WaterfallDirector<StandardCustomWidgetBuilder, DataSource> director(builder);
director.construct();
std::unique_ptr<CustomWidget> widget = builder->getWidget();
widget->show();
On my computer, this defaults to creating a 640x480 window and calls resizeGL upon initialization. Whenever I resize the window, resizeGL is never called.
In my attempts to remedy this I have attempted creating a separate QWidget that has a QVBoxLayout containing only the CustomWidget. This created a very small window, so I fixed my sizeHint and sizePolicy for CustomWidget, though this still had no affect on having resizeGL called. At this point I'm not sure precisely how to proceed.
I resolved my problem with some help from my co-worker. As it turns out, I had implemented the event method and forgot to call the QGLWidget::event method inside it. The widget now correctly resizes.
If you haven't done so already, I would suggest checking the size hints and size policies of all widgets concerned.
For example, the following will make sure your widget uses available space when the window grows:
widget->setSizePolcy( QSizePolicy::MinimumExpanding,
QSizePolicy::MinimumExpanding );
I don't think that the default size policy for QGLWidget makes it want to expand, so I'm thinking it's just possible that this needs changing.
I have a main window with some widgets on it, each needs its own graphic. I would like to use QPainter to draw shapes, lines, etc. on them, but only after a specific event, like the press of a button.
The problem is, if I just create a QPainter in any function, it won't work:
QPainter::setPen: Painter not active
The QPainter methods can only be called inside a paintEvent(QPaintEvent *) function! This raises the following problems:
I have to derive my custom classes for all the widgets I would like to paint on, so I can't use the Designer to place my widgets. This can get frustrating with a large number of widgets.
The widgets redraw themselves after each paint event of the window, like moving it around, or moving other windows in front of it. I do a lot of drawing in those widgets, so they will visibly blink in these cases.
Is there a better and simpler way to solve this? I started to think about just displaying images, and re-manufacturing those images only when the specific buttons are pressed. I doubt that it's the most elegant solution...
You can use custom widgets in the designer: Creating Custom Widgets for Qt Designer.
Qt Designer's plugin-based architecture allows user-defined and third party custom widgets to be edited just like you do with standard Qt widgets.
For your second question, one of the approaches is to create a QPixmap for each of your widgets. When your widget's appearance needs to be changed, you draw in that pixmap (using QPainter's constructor that takes a QPaintDevice - QPixmap is a QPaintDevice).
In your widget's paintEvent function, you simply fill your widget with that "cache" pixmap. This way, you only do the (potentially expensive) painting when it's actually necessary.
I want to render the video from my webcam into QWidget. I've set QWidget, as a parent to IVideoWindow. Here is the code:
m_iVideoWindow->put_Owner((OAHWND)widget_->winId());
m_iVideoWindow->put_WindowStyle(WS_CHILD | WS_CLIPCHILDREN);
m_iVideoWindow->put_Left(0);
m_iVideoWindow->put_Top(0);
widget_->setChild(m_iVideoWindow);
Also i've reimplemented QWidget's resize event, and so when it's resizing it also resizes the IVideoWindow. Everything works good, when the widget is not set to the QLayout. When is - it becomes blank. That is the problem.
I think when Qt draws its widgets, it doesn't redraw them unless it knows they need to be updated. I'm also unsure as to whether it passes on whatever redraw stuff windows requires, to the window you've assigned as a child of the widget.
When I embed my own windows in a Qt widget, I use http://qt.nokia.com/products/appdev/add-on-products/catalog/4/Windows/qtwinmigrate/, but that may not solve your problem, as I still have to use a QTimer to force an update of my window.
I am trying to create an app in Qt/C++ with Qt4.5 and want the any active windows to change opacity on a mouseover event...
As i understand it, there is no explicit mouseover event in Qt.
However, I got rudimentary functioning by reimplementing QWidget's mousemoveevent() in the class that declares my mainwindow. But the mainwindow's mousemoveevent is not called whenever the mouse travels over any of the group boxes i have created in there (understandbly since QGroupbox has its own reimplementation of mousemoveevent).
So as a cheap work around, I am still using the mousemoveevent of my mainwindow but a query the global mouse position and based on the (x,y) position of the mainwindow (obtained through ->pos()) and the window size (-> size -> rHeight and rWidth), I check if the mouse is within the bounds of the area of the mainwindow and change the opacity thus.
This has had very limited success. The right border works fine, the the left changes opacity 4 pixels early. The top does not work (presumably because the mouse goes through the menubar and title bar) and the bottom changes way too early.
I thought of creating an empty container QWidget class and then place all the rest in there, but i felt that it would still not solve the big issue of the base widget not receiving the mousemoveevent if it has already been implemented in a child widget.
Please suggest any corrections/errors I have made in my method or any alternate methods to achieve this.
p.s. I doubt this matters, but I am working Qt Creator IDE, not Qt integration into VS2008 (it's the same classes anyways - different compiler though, mingw)
Installing event filters for each of your child widgets might do the trick. This will allow your main window to receive child events such as the ones from you group boxes. You can find example code here.
You may be interested in Event filters. QObject proves a way to intercept all events zipping around your application.
http://doc.trolltech.com/4.5/eventsandfilters.html#event-filters
If I understand what you are attempting to do, I would reimplement the widget's enterEvent() and leaveEvent(). The mouse enter event would trigger the fade-in and the leaveEvent would trigger the fade-out.
EDIT: After re-reading several times, I'm still not sure what you are trying to accomplish. Sorry if my suggestion doesn't help. :-)