Brushes and brushes

We mentioned in the previous chapter ,Qt The drawing system defines two key attributes used in drawing ： Brushes and brushes . The former uses QBrush describe , Mostly used for filling ; The latter uses QPen describe , It is mostly used to draw contour lines .

QBrush Defined QPainter Fill patterns for , With style 、 Color 、 Gradient, texture and other attributes .

Paintbrush style() Defines the fill pattern , Use Qt::BrushStyle enumeration , The default value is Qt::NoBrush, That is, no filling . We can see the differences of various fill patterns from the following illustration ：

Paintbrush color() Defines the color of the fill pattern . This color can be Qt Predefined color constants , That is to say Qt::GlobalColor, It can also be any QColor object .

Paintbrush gradient() Defines gradient fill . This attribute is only available when the style is Qt::LinearGradientPattern、Qt::RadialGradientPattern perhaps Qt::ConicalGradientPattern It is effective only after one . Gradients can be made by QGradient Objects represent .Qt Three gradients are provided ：QLinearGradient、QConicalGradient and QRadialGradient, They are all QGradient Subclasses of . We can use the following code snippet to define a gradient brush ：

QRadialGradient gradient(50, 50, 50, 50, 50);
gradient.setColorAt(0, QColor::fromRgbF(0, 1, 0, 1));
gradient.setColorAt(1, QColor::fromRgbF(0, 0, 0, 0));
 
QBrush brush(gradient);

When the brush style is  Qt::TexturePattern when ,texture() Defines the texture used for filling . Be careful , Even if you don't set the style to Qt::TexturePattern, When you call setTexture() Function time ,QBrush Will automatically style() Set to Qt::TexturePattern.

QPen Defined for QPainter How to draw a line or outline . Brushes have styles 、 Width 、 A brush 、 Attributes such as cap style and connection style . The style of the brush style() Defines the style of the line . A brush brush() Used to fill the lines drawn by the brush . Cap style capStyle() Use defined QPainter The end of the drawn line ; Connection style joinStyle() Defines how the two lines are connected . The width of the brush width() or widthF() Defines the width of the brush . Be careful , Does not exist with a width of 0 Line . Suppose you set width by 0,QPainter A line will still be drawn , And the width of this line is 1 Pixels . in other words , The brush width is usually at least 1 Pixels .

So many parameters can be specified at construction time , You can also use set The function specifies , It all depends on your habits , for example ：

QPainter painter(this);
QPen pen(Qt::green, 3, Qt::DashDotLine, Qt::RoundCap, Qt::RoundJoin);
painter.setPen(pen);

Equivalent to

QPainter painter(this);
QPen pen;  // creates a default pen
 
pen.setStyle(Qt::DashDotLine);
pen.setWidth(3);
pen.setBrush(Qt::green);
pen.setCapStyle(Qt::RoundCap);
pen.setJoinStyle(Qt::RoundJoin);
 
painter.setPen(pen);

The advantage of using constructors is that the code is shorter , But the meaning of parameters is not clear ; Use set Function is just the opposite .

The default brush attribute is pure black ,0 Pixels , Square cap （Qt::SquareCap）, Inclined connection （Qt::BevelJoin）.

You can also use setDashPattern() Function custom style , For example, the following code snippet ：

 QPen pen;
 QVector<qreal> dashes;
 qreal space = 4;
 
 dashes << 1 << space << 3 << space << 9 << space
        << 27 << space << 9 << space;
 
 pen.setDashPattern(dashes);

The cap defines the style of the end of the brush , for example ：

The difference between them is ,Qt::SquareCap It is a square endpoint containing the last point , Use half the lineweight to cover ;Qt::FlatCap The last point is not included ;Qt::RoundCap Is the end of a circle containing the last point . For details, please refer to the following example （ come from 《C++ GUI Programming with Qt 4, 2nd Edition》）：

Again , You can refer to the following illustration to understand the details of these connection styles （ come from 《C++ GUI Programming with Qt 4, 2nd Edition》）：

Be careful , We said before ,QPainter It's also a state machine , All the properties we mentioned here are in this state machine , therefore , We should remember whether to save it or rebuild it .