Two different methods of using QBrush with QPainter - why is only one working?

[EMKAH]

Hi everybody,

I just managed to complete a long untouched work on a widget I want to use in one of my projects. It is a "circular progress bar", simply drawn with QPainter::drawPie().

pie.png

During development, I have learned that I cannot assign a pointer to a QBrush to QPainter::setBrush() and then change some values of the brush to use different painting styles (well, actually I can - but in this case, all the area of the ellipse is painted black instead of only the pie itself). Instead, I have to create an all-new QBrush everytime I want to change the drawing style.

Here is some code to explain the problem:

//[... somewhere in the widget's header ...]
 
    QPen *_foreGround;
    QBrush *_backGround;
 
//[... somewhere in the widget's initialization process ...]
 
    _foreGround = new QPen();
    _foreGround->setCosmetic(true);
    _foreGround->setWidth(3);
    _backGround = new QBrush();
    _backGround->setColor(QColor(qRgba(0,0,0,0)));
 
//[...]
 
void QtProgressPie::paintEvent(QPaintEvent *e)
{
    // recalculate the value according to the conditions of QPainter::drawPie
    // step 1: subtract _minimum from _value (so the calculation base is always 0)
    // step 2: calculate the percentage of value (value/max * 100)
    // step 3: translate it to circular conditions (value/max * [100 * 3.6 * 16]) [...] = _FULLCIRCLE
    // step 4: take it times -1 (QPainter::drawPie works counter-clockwise with positive values)
 
    int _painterValue = ((_value - _minimum) / double(_maximum - _minimum) * _FULLCIRCLE) * -1;
 
    QPainter p;
    p.begin(this);  // Begin drawing on the widget
 
    // make sure that the widget appears always as a circle, not as an ellipse
    int _squaredSize = qMin(this->width(), this->height());
    p.setViewport((this->width() - _squaredSize) / 2,(this->height() - _squaredSize) / 2,
                  _squaredSize, _squaredSize);
    // subtract the width of the pen ("the border width") from the widget's rect() to get the drawable area
    int _borderWidth = _foreGround->width();
    QRect drawableArea(0 + _borderWidth, 0 + _borderWidth,
                       this->width() - _borderWidth * 2, this->height() - _borderWidth * 2);
 
 
    p.setRenderHint(QPainter::HighQualityAntialiasing, true);
    p.setPen(*_foreGround);
    //p.setBrush(*_backGround); <--- DOESN'T WORK
 
    // draw surrounding circle
    // _backGround->setStyle(Qt::NoBrush); <--- DOESN'T WORK
    p.setBrush(QBrush(QColor(qRgba(0,0,0,0)),Qt::NoBrush)); // <--- WORKS!
    p.drawEllipse(drawableArea);
 
 
    // draw pie if value < 0
    if (_painterValue < 0)
    {
        //_backGround->setStyle(Qt::SolidPattern); <--- DOESN'T WORK
        p.setBrush(QBrush(QColor(qRgba(0,0,0,0)),Qt::SolidPattern)); // <--- WORKS!
        p.drawPie(drawableArea, _OFFSET, _painterValue);
        // _OFFSET = 1/4 of _FULLCIRCLE (transition from 3 o'clock (drawPie's base) to 12 o'clock)
    }
 
    p.end();    // Drawing finished

//[... somewhere in the widget's header ...]

    QPen *_foreGround;
    QBrush *_backGround;

//[... somewhere in the widget's initialization process ...]

    _foreGround = new QPen();
    _foreGround->setCosmetic(true);
    _foreGround->setWidth(3);
    _backGround = new QBrush();
    _backGround->setColor(QColor(qRgba(0,0,0,0)));

//[...]

void QtProgressPie::paintEvent(QPaintEvent *e)
{
    // recalculate the value according to the conditions of QPainter::drawPie
    // step 1: subtract _minimum from _value (so the calculation base is always 0)
    // step 2: calculate the percentage of value (value/max * 100)
    // step 3: translate it to circular conditions (value/max * [100 * 3.6 * 16]) [...] = _FULLCIRCLE
    // step 4: take it times -1 (QPainter::drawPie works counter-clockwise with positive values)

    int _painterValue = ((_value - _minimum) / double(_maximum - _minimum) * _FULLCIRCLE) * -1;

    QPainter p;
    p.begin(this);  // Begin drawing on the widget

    // make sure that the widget appears always as a circle, not as an ellipse
    int _squaredSize = qMin(this->width(), this->height());
    p.setViewport((this->width() - _squaredSize) / 2,(this->height() - _squaredSize) / 2,
                  _squaredSize, _squaredSize);
    // subtract the width of the pen ("the border width") from the widget's rect() to get the drawable area
    int _borderWidth = _foreGround->width();
    QRect drawableArea(0 + _borderWidth, 0 + _borderWidth,
                       this->width() - _borderWidth * 2, this->height() - _borderWidth * 2);


    p.setRenderHint(QPainter::HighQualityAntialiasing, true);
    p.setPen(*_foreGround);
    //p.setBrush(*_backGround); <--- DOESN'T WORK

    // draw surrounding circle
    // _backGround->setStyle(Qt::NoBrush); <--- DOESN'T WORK
    p.setBrush(QBrush(QColor(qRgba(0,0,0,0)),Qt::NoBrush)); // <--- WORKS!
    p.drawEllipse(drawableArea);


    // draw pie if value < 0
    if (_painterValue < 0)
    {
        //_backGround->setStyle(Qt::SolidPattern); <--- DOESN'T WORK
        p.setBrush(QBrush(QColor(qRgba(0,0,0,0)),Qt::SolidPattern)); // <--- WORKS!
        p.drawPie(drawableArea, _OFFSET, _painterValue);
        // _OFFSET = 1/4 of _FULLCIRCLE (transition from 3 o'clock (drawPie's base) to 12 o'clock)
    }

    p.end();    // Drawing finished

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So, my question is: why can't I use the "more elegant" solution with the pointer variable?

Greetings,
Markus

EDIT: picture added, futher explanation of what goes wrong when using a pointer.