QPainter Class

Detailed Description

QPainter provides highly optimized functions to do most of the drawing GUI programs require. It can draw everything from simple lines to complex shapes like pies and chords. It can also draw aligned text and pixmaps. Normally, it draws in a "natural" coordinate system, but it can also do view and world transformation. QPainter can operate on any object that inherits the QPaintDevice class.

The common use of QPainter is inside a widget's paint event: Construct and customize (e.g. set the pen or the brush) the painter. Then draw. Remember to destroy the QPainter object after drawing. For example:

 void SimpleExampleWidget::paintEvent(QPaintEvent *)
 {
     QPainter painter(this);
     painter.setPen(Qt::blue);
     painter.setFont(QFont("Arial", 30));
     painter.drawText(rect(), Qt::AlignCenter, "Qt");
 }

The core functionality of QPainter is drawing, but the class also provide several functions that allows you to customize QPainter's settings and its rendering quality, and others that enable clipping. In addition you can control how different shapes are merged together by specifying the painter's composition mode.

The isActive() function indicates whether the painter is active. A painter is activated by the begin() function and the constructor that takes a QPaintDevice argument. The end() function, and the destructor, deactivates it.

Together with the QPaintDevice and QPaintEngine classes, QPainter form the basis for Qt's paint system. QPainter is the class used to perform drawing operations. QPaintDevice represents a device that can be painted on using a QPainter. QPaintEngine provides the interface that the painter uses to draw onto different types of devices. If the painter is active, device() returns the paint device on which the painter paints, and paintEngine() returns the paint engine that the painter is currently operating on. For more information, see the Paint System.

Sometimes it is desirable to make someone else paint on an unusual QPaintDevice. QPainter supports a static function to do this, setRedirected().

Settings

There are several settings that you can customize to make QPainter draw according to your preferences:

• font() is the font used for drawing text. If the painter isActive(), you can retrieve information about the currently set font, and its metrics, using the fontInfo() and fontMetrics() functions respectively.
• brush() defines the color or pattern that is used for filling shapes.
• pen() defines the color or stipple that is used for drawing lines or boundaries.
• backgroundMode() defines whether there is a background() or not, i.e it is either Qt::OpaqueMode or Qt::TransparentMode.
• background() only applies when backgroundMode() is Qt::OpaqueMode and pen() is a stipple. In that case, it describes the color of the background pixels in the stipple.
• brushOrigin() defines the origin of the tiled brushes, normally the origin of widget's background.
• viewport(), window(), worldTransform() make up the painter's coordinate transformation system. For more information, see the Coordinate Transformations section and the Coordinate System documentation.
• hasClipping() tells whether the painter clips at all. (The paint device clips, too.) If the painter clips, it clips to clipRegion().
• layoutDirection() defines the layout direction used by the painter when drawing text.
• worldMatrixEnabled() tells whether world transformation is enabled.
• viewTransformEnabled() tells whether view transformation is enabled.

Note that some of these settings mirror settings in some paint devices, e.g. QWidget::font(). The QPainter::begin() function (or equivalently the QPainter constructor) copies these attributes from the paint device.

You can at any time save the QPainter's state by calling the save() function which saves all the available settings on an internal stack. The restore() function pops them back.

Drawing

QPainter provides functions to draw most primitives: drawPoint(), drawPoints(), drawLine(), drawRect(), drawRoundedRect(), drawEllipse(), drawArc(), drawPie(), drawChord(), drawPolyline(), drawPolygon(), drawConvexPolygon() and drawCubicBezier(). The two convenience functions, drawRects() and drawLines(), draw the given number of rectangles or lines in the given array of QRects or QLines using the current pen and brush.

The QPainter class also provides the fillRect() function which fills the given QRect, with the given QBrush, and the eraseRect() function that erases the area inside the given rectangle.

All of these functions have both integer and floating point versions.

If you need to draw a complex shape, especially if you need to do so repeatedly, consider creating a QPainterPath and drawing it using drawPath().

QPainter also provides the fillPath() function which fills the given QPainterPath with the given QBrush, and the strokePath() function that draws the outline of the given path (i.e. strokes the path).

See also the Vector Deformation example which shows how to use advanced vector techniques to draw text using a QPainterPath, the Gradients example which shows the different types of gradients that are available in Qt, and the Path Stroking example which shows Qt's built-in dash patterns and shows how custom patterns can be used to extend the range of available patterns.

Text drawing is done using drawText(). When you need fine-grained positioning, boundingRect() tells you where a given drawText() command will draw.

Drawing Pixmaps and Images

There are functions to draw pixmaps/images, namely drawPixmap(), drawImage() and drawTiledPixmap(). Both drawPixmap() and drawImage() produce the same result, except that drawPixmap() is faster on-screen while drawImage() may be faster on a QPrinter or other devices.

There is a drawPicture() function that draws the contents of an entire QPicture. The drawPicture() function is the only function that disregards all the painter's settings as QPicture has its own settings.

Drawing High Resolution Versions of Pixmaps and Images

High resolution versions of pixmaps have a device pixel ratio value larger than 1 (see QImageReader, QPixmap::devicePixelRatio()). Should it match the value of the underlying QPaintDevice, it is drawn directly onto the device with no additional transformation applied.

This is for example the case when drawing a QPixmap of 64x64 pixels size with a device pixel ratio of 2 onto a high DPI screen which also has a device pixel ratio of 2. Note that the pixmap is then effectively 32x32 pixels in user space. Code paths in Qt that calculate layout geometry based on the pixmap size will use this size. The net effect of this is that the pixmap is displayed as high DPI pixmap rather than a large pixmap.

Rendering Quality

To get the optimal rendering result using QPainter, you should use the platform independent QImage as paint device; i.e. using QImage will ensure that the result has an identical pixel representation on any platform.

The QPainter class also provides a means of controlling the rendering quality through its RenderHint enum and the support for floating point precision: All the functions for drawing primitives has a floating point version. These are often used in combination with the QPainter::Antialiasing render hint.

The RenderHint enum specifies flags to QPainter that may or may not be respected by any given engine. QPainter::Antialiasing indicates that the engine should antialias edges of primitives if possible, QPainter::TextAntialiasing indicates that the engine should antialias text if possible, and the QPainter::SmoothPixmapTransform indicates that the engine should use a smooth pixmap transformation algorithm.

The renderHints() function returns a flag that specifies the rendering hints that are set for this painter. Use the setRenderHint() function to set or clear the currently set RenderHints.

Coordinate Transformations

Normally, the QPainter operates on the device's own coordinate system (usually pixels), but QPainter has good support for coordinate transformations.

The most commonly used transformations are scaling, rotation, translation and shearing. Use the scale() function to scale the coordinate system by a given offset, the rotate() function to rotate it clockwise and translate() to translate it (i.e. adding a given offset to the points). You can also twist the coordinate system around the origin using the shear() function. See the Affine Transformations example for a visualization of a sheared coordinate system.

See also the Transformations example which shows how transformations influence the way that QPainter renders graphics primitives. In particular it shows how the order of transformations affects the result.

All the tranformation operations operate on the transformation worldTransform(). A matrix transforms a point in the plane to another point. For more information about the transformation matrix, see the Coordinate System and QTransform documentation.

The setWorldTransform() function can replace or add to the currently set worldTransform(). The resetTransform() function resets any transformations that were made using translate(), scale(), shear(), rotate(), setWorldTransform(), setViewport() and setWindow() functions. The deviceTransform() returns the matrix that transforms from logical coordinates to device coordinates of the platform dependent paint device. The latter function is only needed when using platform painting commands on the platform dependent handle, and the platform does not do transformations nativly.

When drawing with QPainter, we specify points using logical coordinates which then are converted into the physical coordinates of the paint device. The mapping of the logical coordinates to the physical coordinates are handled by QPainter's combinedTransform(), a combination of viewport() and window() and worldTransform(). The viewport() represents the physical coordinates specifying an arbitrary rectangle, the window() describes the same rectangle in logical coordinates, and the worldTransform() is identical with the transformation matrix.

See also Coordinate System

Clipping

QPainter can clip any drawing operation to a rectangle, a region, or a vector path. The current clip is available using the functions clipRegion() and clipPath(). Whether paths or regions are preferred (faster) depends on the underlying paintEngine(). For example, the QImage paint engine prefers paths while the X11 paint engine prefers regions. Setting a clip is done in the painters logical coordinates.

After QPainter's clipping, the paint device may also clip. For example, most widgets clip away the pixels used by child widgets, and most printers clip away an area near the edges of the paper. This additional clipping is not reflected by the return value of clipRegion() or hasClipping().

Composition Modes

QPainter provides the CompositionMode enum which defines the Porter-Duff rules for digital image compositing; it describes a model for combining the pixels in one image, the source, with the pixels in another image, the destination.

The two most common forms of composition are Source and SourceOver. Source is used to draw opaque objects onto a paint device. In this mode, each pixel in the source replaces the corresponding pixel in the destination. In SourceOver composition mode, the source object is transparent and is drawn on top of the destination.

Note that composition transformation operates pixelwise. For that reason, there is a difference between using the graphic primitive itself and its bounding rectangle: The bounding rect contains pixels with alpha == 0 (i.e the pixels surrounding the primitive). These pixels will overwrite the other image's pixels, effectively clearing those, while the primitive only overwrites its own area.

Limitations

If you are using coordinates with Qt's raster-based paint engine, it is important to note that, while coordinates greater than +/- 2¹⁵ can be used, any painting performed with coordinates outside this range is not guaranteed to be shown; the drawing may be clipped. This is due to the use of short int in the implementation.

The outlines generated by Qt's stroker are only an approximation when dealing with curved shapes. It is in most cases impossible to represent the outline of a bezier curve segment using another bezier curve segment, and so Qt approximates the curve outlines by using several smaller curves. For performance reasons there is a limit to how many curves Qt uses for these outlines, and thus when using large pen widths or scales the outline error increases. To generate outlines with smaller errors it is possible to use the QPainterPathStroker class, which has the setCurveThreshold member function which let's the user specify the error tolerance. Another workaround is to convert the paths to polygons first and then draw the polygons instead.

Performance

QPainter is a rich framework that allows developers to do a great variety of graphical operations, such as gradients, composition modes and vector graphics. And QPainter can do this across a variety of different hardware and software stacks. Naturally the underlying combination of hardware and software has some implications for performance, and ensuring that every single operation is fast in combination with all the various combinations of composition modes, brushes, clipping, transformation, etc, is close to an impossible task because of the number of permutations. As a compromise we have selected a subset of the QPainter API and backends, where performance is guaranteed to be as good as we can sensibly get it for the given combination of hardware and software.

The backends we focus on as high-performance engines are:

• Raster - This backend implements all rendering in pure software and is always used to render into QImages. For optimal performance only use the format types QImage::Format_ARGB32_Premultiplied, QImage::Format_RGB32 or QImage::Format_RGB16. Any other format, including QImage::Format_ARGB32, has significantly worse performance. This engine is used by default for QWidget and QPixmap.
• OpenGL 2.0 (ES) - This backend is the primary backend for hardware accelerated graphics. It can be run on desktop machines and embedded devices supporting the OpenGL 2.0 or OpenGL/ES 2.0 specification. This includes most graphics chips produced in the last couple of years. The engine can be enabled by using QPainter onto a QOpenGLWidget.

These operations are:

• Simple transformations, meaning translation and scaling, pluss 0, 90, 180, 270 degree rotations.
• drawPixmap() in combination with simple transformations and opacity with non-smooth transformation mode (QPainter::SmoothPixmapTransform not enabled as a render hint).
• Rectangle fills with solid color, two-color linear gradients and simple transforms.
• Rectangular clipping with simple transformations and intersect clip.
• Composition Modes QPainter::CompositionMode_Source and QPainter::CompositionMode_SourceOver.
• Rounded rectangle filling using solid color and two-color linear gradients fills.
• 3x3 patched pixmaps, via qDrawBorderPixmap.

This list gives an indication of which features to safely use in an application where performance is critical. For certain setups, other operations may be fast too, but before making extensive use of them, it is recommended to benchmark and verify them on the system where the software will run in the end. There are also cases where expensive operations are ok to use, for instance when the result is cached in a QPixmap.