I'm trying to write a piece of software that allows one to click on a video frame and mark the x,y coordinates of a location in the frame. To design this, I've been wanting to use a QGraphicsView subclass and, on the mouse click event, instantiate a QLabel with a PNG image "target" on where the click occurred.
So far I've gotten everything to work except getting the QLabel to be transparent. All of the info I've found online doesn't seem to work with the latest Qt. Should I totally rethink my design and utilize some sort of integration with painting in Qt? Or is there a way to salvage the QLabel PNG implementation and indeed make the label transparent?
Thanks,
--Dany.
QLabel inside QGraphicsView is not a good idea indeed. QGraphicsView was designed to host QGraphicsItems, to display an image you should use QGraphicsPixmapItem.
Embedding QWidget into QGraphicsView has some overhead and was really designed for complex widgets that can't be easily reimplemented in terms of QGraphicItems.
I think a QGraphicsItem with ItemIgnoresTransformations flag is just what you want.
Related
I am looking for some example code that I can use to save an image of my Qt application. The application is a QMainWindow that contains a QGLWidget. I want to be able to screen capture all the GUIs (including the QGLWidget rendering) but have not been able to find any example code for this.
What I've tried:
QPixmap pixmap = QPixMap::grabWidget(QApplication::activeWindow);
pixmap.save(QString("test.png"));
This only save the GUIs but leaves a black background around the QGLWidget.
Anyone know of a technique for doing this in Qt 4.8.4?
Thanks,
Yes, you are right. We have QListView already, it is prefect when we are trying to display simple list with Model/View.
But, QListView has lots of problem when we need to display complex list with rich text and widgets. Just think about a timeline-listview for Facebook or Twitter.
Sure, we can implement our own delegate for rich text or images, but ListView can print static item only. So, there isn't a way to show clickable hyperlink (you can calculate position of the mouse and hyperlink, but it is a really drity work) or load asynchronous images.
Well, QListWidget seems our solution. We can put widgets into it. But. we will lost our Model/View/Delegate architecture, that is terrible!
Now, my solution is writing my listview in QML. Other widget are still native Qt widget. (I don't like a non-native pure QML user interface.)
QML is really flexiable when doing that kind of work. Then export my model, finally put a viewer into my QMainWindow. But coding in two programming languages and trying to communicate with other native widget is really difficult.
So, is there a way to use Qt's Model/View architecture with QListWidget? Or I have to implement them by myself?
QListWidget does use Qt's MVC, as it derives from QListView and...
QListWidget uses an internal model to manage each QListWidgetItem in
the list.
Just use QListWidget::model () const to access the model.
Anyone have any idea how I can implement this? I'd like to have a function basically exactly like impoly in matlab or the "polygon sections" tool in imageJ, where you click to form a polygonal section and then each node can be adjusted, etc. I'd also like to have access to this function from Qt since I'm trying to make a gui for a small program I wrote.
Also, I'd like to avoid making calls to the matlab function because it's part of the image processing toolbox which isnt free. Thanks.
I think the best way to implement this is using the Qt Graphics View framework. Create a scene with an Item displaying your image in the back and add draggable Items on top representing the corners of your polygon.
Your selection tool should probably be a subclass of QGraphicsObject hosting the polygon corners as child items and a QGraphicsPolygonItem below the corners being updated whenever the user readjusts the selection. As QGraphicsObject inherits QObject, you can emit signals with a QPolygonF or QPolygon argument whenever the selection changes, informing other parts of your application
This demo should be a good example of the corner-adjust functionality you need.
Qt Pathstroke Demo
(uh well, the example implements the drawing and dragging of the control points from scratch.. I'm sure you can do it by using QGraphicsEllipseItem instead and react on their position changes)
I think you would need to code this yourself. There is an excellent example in the C++ GUI Programming with Qt 4 book (there's a PDF copy floating around online; I think it's legal) where they show you how to create a diagram with nodes and links. The chapter is called "Item-based rendering with Graphics View".
The basic idea is that you have some draggable nodes, which are QGraphicsItems with the ItemIsMovable flag set to true, and then some links that connect them, which are QGraphicsLineItems. All of these would go into a composite QGraphicsItem representing the ROI, and all of those would go into a QGraphicsScene, which would be displayed by a QGraphicsView.
Bottom line: there isn't a built-in copy of the MATLAB function, but all the tools are there for you.
I am trying to implement a custom OpenGL scene (wrapped in a QGLWidget), but still use QT controls (such as slide bars and buttons). I tried to make my main widget a QGLWidget, which has QWidgets as children. The child widgets seem to work (when I click a pulldown menu, its options appear), but the widget fails to draw (I see a white square). I changed the base class of my children widgets from QWidget to QGLWidget, and now QPainter calls work, but QT GUI stuff is still not displaying...
Some claim that a QGLWidget can not have QWidgets as children... I am not sure about that and I'm not willing to give up. By the way, I am using Visualization Library to draw the OpenGL scene, but it is just an OpenGL wrapper...
Any clues where the problem might be? Also, key events stop being processed for some reason when I add subwidgets to the GQLWidget.
Update:
I tried various combinations of widgets and layouts. It seems that the QGLWidget just gets drawn on top of anything. I even tried with raise() to arrange Z-depth of the widgets, to no avail. Is overlayGL() the only way to draw on top of an OpenGL widget?
Update 2
After months of trying, I figured out it is something related to QT. Whenever a QGLWidget is drawn on top of another QGLWidget, the first's background() function is not called. So a button is there and can be clicked, but it's not drawn. My workaround was to draw everything myself using QPainter - that works. What I found curious is, one has to use QPainter::startNativePainting() in order to draw with it. One would expect that they would overload start() in QPainter to call startNativePainting() whenever a QGLWidget is the painting device...
Another (maybe useful) fact is that QPainter calls CHANGE the Opengl context. So if you have another tool in cooperation with QPainter (in my case Visualization Library), chances are that one of them will crash (in my case VL which checks if the context state has been cleared before each frame).
There is a labs example of rendering Qt widgets using opengl it's a little old and I haven't tried it with the improved qglwidget in 4.8
There is also an issue with drawing over the top of a fullscreen QGLwidget - it was supposedly a bug in NVidia's openGL driver. It doesn't seem to have been resolved
https://bugreports.qt-project.org/browse/QTBUG-7556
If I were you I would use the QGLWidget as a viewport for a QGraphicsScene, then you can draw your widgets using a QGraphicsProxyWidget. That way you get a proper OpenGL viewport with the ability to put widgets in and manipulate them easily.
Did u try to call update() explicitly? I find that sometimes QGLWidget update the display stuff kind of slow.
This is an old question, but I recently had quite a bit of trouble with this, and this is what helped me -
Use QML/QT Quick. QML is a javascript-like language that allows you to layout your widget elements on a window and do basic processing on events like mouse clicks. There are a few other cool features, but the one that is most relevant to this question is that you can import qt widgets from C++ code.
So you can make a custom QGLWidget, import it into the QML window, and set the size/position/stick widgets on top of it in whatever you want, very easily. QT's Scenegraph example does a great job explaining this. Scenegraph's code is also included as an example in at least Qt Creator 3.2.1 Open Source (that's the version I have anyway)
I want to reimplement the Qt fingerpaint example using my QGraphicsScene rather than a QWidget. However I cannot find an example of this so am unsure if I should capture the events at the view and pass them down to the scene or something else. What is the advice here?
How about the the pinch zoom example? It uses a QGraphicsView as the touch area.