Marcus Chen
Painting outside the selected area in Qt can be challenging, as the default behaviour is to restrict painting within specific boundaries. While some workarounds exist, such as creating a QPixmap object or accessing the Hwnd of the window, the recommended approach is to utilise the provided tools and classes, like QPainter, QPaintDevice, and QPaintEngine, which offer a range of drawing functions and capabilities. Understanding the limitations and behaviours of these tools is essential for achieving the desired painting effects outside the selected area in Qt.
| Characteristics | Values |
|---|---|
| Qt version | Qt 4, Qt 5.12.6, Qt 6.9.1 |
| Classes | QPainter, QPaintDevice, QPaintEngine, QPixmap, QWidget, QPen, QPainterbegin(), QPaintersave(), QPainter::restore() |
| Functions | paintEvent(), drawSelectingArea(), drawRect(), drawPixmap(), drawImage(), drawCircle(), drawPoint(), drawPoints(), drawLine(), drawRoundedRect(), drawEllipse(), drawArc(), drawPie(), drawChord(), drawPolyline(), drawPolygon(), drawCubicBezier(), drawRects(), drawLines(), fillRect(), eraseRect() |
| Techniques | Clipping, WA_PaintOutsidePaintEvent flag, QPainterbegin(), QPaintersave(), QPainter::restore() |
| Limitations | Cannot paint outside paintEvent(), painting outside widget geometry may not work, pixmap method requires additional work |
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What You'll Learn
Using QPainter to paint outside paintEvent()
It seems that painting outside of paintEvent() is not possible in Qt. However, there are some workarounds and alternative methods that can be used to achieve similar results.
One method is to call the update() function, which will trigger paintEvent() to be called as soon as possible. This allows your code inside paintEvent() to handle the actual painting. This approach ensures that the painting occurs within the designated event, but it may not be suitable if you specifically need to paint outside of this event.
Another method is to create a QPixmap object with the same dimensions as your widget. You can then pass a pointer to this QPixmap to the constructor of your QPainter object and perform the painting on the QPixmap. Once you're done, calling update() will trigger paintEvent(), and you can use drawPixmap() to copy the pixels from the QPixmap to the widget's on-screen buffer. However, this method is less efficient as it requires additional copying of pixels and may cause the image to be drawn more often than necessary.
Some users have also suggested using QPainterPath to store reusable, scalable vector graphics, or storing "painter programs" inside a QPicture and replaying them within the paintEvent().
It's important to note that attempting to use QPainter outside of paintEvent() can lead to issues, such as the graphic being eliminated or errors like "QPainter::drawRects: Painter not active". Therefore, it's recommended to work within the paintEvent() or utilize the suggested alternatives.
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Creating a QPixmap object
When working with images in Qt applications, the QPixmap class is a useful tool. QPixmap is designed and optimised for displaying images on-screen, and it can be easily displayed using QLabel or one of QAbstractButton's subclasses, such as QPushButton and QToolButton.
To create a QPixmap object, you can start by loading an image into it. This can be done using the file path of the image. The following code snippet demonstrates this:
Python
Image_path, _ = QFileDialog.getOpenFileName(self, 'Open Image', '', "Image files (*.jpg *.png)")
If image_path:
Pixmap = QPixmap(image_path)
Here, `QFileDialog.getOpenFileName()` is used to obtain the file path of the image, and then the `QPixmap` class is initialised with this file path to create a QPixmap object.
Another approach is to create a QPixmap object from raw image data. This can be achieved by first creating a QImage from the raw data using `QImage::setPixel()`, and then using that QImage to generate the QPixmap.
Additionally, QPixmap objects can be converted to and from QImage using the `toImage()` and `fromImage()` functions, respectively. QPixmap also provides functions like `scaled()`, `scaledToWidth()`, and `copy()` for creating transformed versions of the original pixmap.
It's important to note that QPixmap objects can be passed around by value since the QPixmap class employs implicit data sharing. Furthermore, QPixmap objects can be streamed, and they can be drawn on directly using QPainter functions.
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Drawing a circle on a widget area
To draw a circle on a widget area in Qt, you can follow these steps:
- Create a custom widget subclass that inherits from QWidget, such as CircleWidget.
- In the constructor of your custom widget, set the initial properties, such as the background colour and size policy.
- Implement the paintEvent function, which will be called when the widget needs to be repainted, such as when it is resized or exposed after being hidden.
- Inside the paintEvent function, create a QPainter object and set its properties, such as the antialiasing flag.
- Translate the painter's coordinate system to ensure that the centre of the circle will be equivalent to the widget's centre.
- Use the QPainter::drawEllipse method to draw the circle, specifying the centre point and the radii.
- If you want to create animated concentric circles, you can use the nextAnimationFrame slot to control the animation. Increment the frame number and schedule a paint event to update the circles' colours and create the animation effect.
Cpp
#include
Class MyFrame: public QFrame {
Q_OBJECT
Public:
Explicit MyFrame(QWidget *p = 0): QFrame(p) {
SetFrameStyle(QFrame::Box);
}
Protected:
Void paintEvent(QPaintEvent *event) {
QFrame::paintEvent(event);
Int radius = qMin(event->rect().width(), event->rect().height()) / 2;
QPainter p(this);
P.drawEllipse(event->rect().center(), radius, radius);
}
};
Remember that Qt generates paint events on a widget when it needs to be repainted, so you don't need to manually create a QFrame paint event.
Additionally, if you need to paint outside of the paintEvent function, you can call the update function, which will cause paintEvent to be called as soon as possible. However, it is generally more efficient to perform painting inside the paintEvent function.
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Using the QPainter::begin() function
The QPainter class is used to perform low-level painting on widgets and other paint devices. It provides highly optimized functions for drawing simple lines, complex shapes, aligned text, and pixmaps. QPainter can operate on any object that inherits the QPaintDevice class.
The QPainter::begin() function, or the QPainter constructor, copies the attributes from the paint device. This function allows you to save the QPainter's state by calling the save() function, which stores all the available settings on an internal stack. You can then restore the settings by using the restore() function.
QPainter provides a wide range of drawing functions, including:
- DrawPoint()
- DrawPoints()
- DrawLine()
- DrawRect()
- DrawRoundedRect()
- DrawEllipse()
- DrawArc()
- DrawPie()
- DrawChord()
- DrawPolyline()
- DrawPolygon()
- DrawCubicBezier()
Additionally, QPainter offers convenience functions such as drawRects() and drawLines(), which allow you to draw multiple rectangles or lines using the current pen and brush settings.
It's important to note that the QPainter class is typically used inside a widget's paint event. After constructing and customizing the painter, it's important to remember to destroy the QPainter object once the drawing is complete.
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Drawing a rectangle with QPainter
To draw a rectangle with QPainter in Qt, you need to follow these steps:
- Create a QPainter Object: Start by creating an instance of the QPainter class, which provides a rich framework for graphical operations. This class allows you to draw lines, shapes, text, and pixmaps.
- Set Brush and Pen Properties: Next, you need to define the appearance of your rectangle. Set the brush style and colour to determine the fill of the rectangle. Additionally, create a QPen object to specify properties such as line style, width, colour, cap style, and join style. The QPen class works together with QBrush to define the overall look of your rectangle.
- Use the drawRect() Method: Now, you can utilise the drawRect() method provided by QPainter to draw the rectangle on a paint device, such as a widget. This method allows you to specify the coordinates and dimensions of the rectangle you want to draw.
- Implement the paintEvent() Method: The paintEvent() method is crucial for custom painting with QPainter. It is called periodically to redraw widgets and allows you to perform custom painting when the widget needs to be repainted. Make sure to include the QPainter header file in your main window header file and implement the paintEvent() method in the main window class.
- Consider Clipping: If needed, you can use clipping to restrict painting to specific areas. This ensures that you don't paint over other widgets unintentionally.
- Understand Composition Modes: QPainter offers CompositionMode enum, which defines rules for combining pixels from different images. The most common forms are Source and SourceOver. Source is used for opaque objects, replacing pixels in the destination image, while SourceOver makes the source object transparent, drawing it on top of the destination.
Remember that Qt provides a cross-platform application development framework, allowing you to create graphical user interfaces (GUIs) and other features using C++ programming language.
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Frequently asked questions
You can't paint outside the selected area in Qt as it does not work that way. If you want to repaint your widget, call update() instead. This will cause paintEvent() to be called ASAP, and your code inside paintEvent() can do the actual painting.
You can create a QPixmap object with the same width and height as your widget, and pass a pointer to a QPainter object. Then, paint into the QPixmap and call update() to cause paintEvent() to be called. Finally, in the paintEvent() call, you can call drawPixmap() to copy the pixels from the QPixmap to the widget’s on-screen buffer. Note that this approach is less efficient and requires additional copying of pixels.
You can use clipping to restrict the painting area. This can be achieved by sending the painter to a function in the X class and letting it draw within the desired boundaries.
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