Coordinate System▲

The coordinate system is controlled by the QPainter class. Together with the QPaintDevice and QPaintEngine classes, QPainter form the basis of Qt's painting system, Arthur. QPainter is used to perform drawing operations, QPaintDevice is an abstraction of a two-dimensional space that can be painted on using a QPainter, and QPaintEngine provides the interface that the painter uses to draw onto different types of devices.

The QPaintDevice class is the base class of objects that can be painted: Its drawing capabilities are inherited by the QWidget, QImage, QPixmap, QPicture, and QOpenGLPaintDevice classes. The default coordinate system of a paint device has its origin at the top-left corner. The x values increase to the right and the y values increase downwards. The default unit is one pixel on pixel-based devices and one point (1/72 of an inch) on printers.

The mapping of the logical QPainter coordinates to the physical QPaintDevice coordinates are handled by QPainter's transformation matrix, viewport and "window". The logical and physical coordinate systems coincide by default. QPainter also supports coordinate transformations (e.g. rotation and scaling).

Rendering▲

 

Logical Representation▲

The size (width and height) of a graphics primitive always correspond to its mathematical model, ignoring the width of the pen it is rendered with:

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QRect(QPoint(1, 2), QPoint(7, 6))

QLine(QPoint(2, 7), QPoint(6, 1))
 

QLine(2, 7, 6, 1)

QRect(QPoint(1, 2), QSize(6, 4))

QRect(1, 2, 6, 4)

Aliased Painting▲

When drawing, the pixel rendering is controlled by the QPainter::Antialiasing render hint.

The RenderHint enum is used to specify flags to QPainter that may or may not be respected by any given engine. The QPainter::Antialiasing value indicates that the engine should antialias edges of primitives if possible, i.e. smoothing the edges by using different color intensities.

But by default the painter is aliased and other rules apply: When rendering with a one pixel wide pen the pixels will be rendered to the right and below the mathematically defined points. For example:

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Sélectionnez
QPainter painter(this);

painter.setPen(Qt::darkGreen);
// Using the (x  y  w  h) overload
painter.drawRect(1, 2, 6, 4);
 
Sélectionnez
QPainter painter(this);

painter.setPen(Qt::darkGreen);
painter.drawLine(2, 7, 6, 1);

When rendering with a pen with an even number of pixels, the pixels will be rendered symetrically around the mathematical defined points, while rendering with a pen with an odd number of pixels, the spare pixel will be rendered to the right and below the mathematical point as in the one pixel case. See the QRectF diagrams below for concrete examples.

QRectF

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Logical representation

One pixel wide pen

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