QGradient Class

详细描述

The QGradient class is used in combination with QBrush to specify gradient fills.

Qt currently supports three types of gradient fills:

• Linear gradients interpolate colors between start and end points.
• Simple radial gradients interpolate colors between a focal point and end points on a circle surrounding it.
• Extended radial gradients interpolate colors between a center and a focal circle.
• Conical gradients interpolate colors around a center point.

A gradient's type can be retrieved using the type() function. Each of the types is represented by a subclass of QGradient:

QLinearGradient	QRadialGradient	QConicalGradient

The colors in a gradient are defined using stop points of the QGradientStop type; i.e., a position and a color. Use the setColorAt() function to define a single stop point. Alternatively, use the setStops() function to define several stop points in one go. Note that the latter function replaces the current set of stop points.

It is the gradient's complete set of stop points (accessible through the stops() function) that describes how the gradient area should be filled. If no stop points have been specified, a gradient of black at 0 to white at 1 is used.

A diagonal linear gradient from black at (100, 100) to white at (200, 200) could be specified like this:

      QLinearGradient linearGrad(QPointF(100, 100), QPointF(200, 200));
      linearGrad.setColorAt(0, Qt::black);
      linearGrad.setColorAt(1, Qt::white);

A gradient can have an arbitrary number of stop points. The following would create a radial gradient starting with red in the center, blue and then green on the edges:

      QRadialGradient radialGrad(QPointF(100, 100), 100);
      radialGrad.setColorAt(0, Qt::red);
      radialGrad.setColorAt(0.5, Qt::blue);
      radialGrad.setColorAt(1, Qt::green);

It is possible to repeat or reflect the gradient outside its area by specifiying the spread method using the setSpread() function. The default is to pad the outside area with the color at the closest stop point. The currently set spread method can be retrieved using the spread() function. The QGradient::Spread enum defines three different methods:

PadSpread	RepeatSpread	ReflectSpread

Note that the setSpread() function only has effect for linear and radial gradients. The reason is that the conical gradient is closed by definition, i.e. the conical gradient fills the entire circle from 0 - 360 degrees, while the boundary of a radial or a linear gradient can be specified through its radius or final stop points, respectively.

The gradient coordinates can be specified in logical coordinates, relative to device coordinates, or relative to object bounding box coordinates. The coordinate mode can be set using the setCoordinateMode() function. The default is LogicalMode, where the gradient coordinates are specified in the same way as the object coordinates. To retrieve the currently set coordinate mode use coordinateMode().