add negative values to the progress bar - qt

Question

I have a realtime value from a sensor:
ui->progressBar_retraction->setValue(aX*100); //aX is realtime value that can take positive or negative values over time
In the above expression, the progress bar shown the instantaneous positive values of aX.
How do I show negative values in a different progress bar ?

Answer 1

◘ Well, if you want to use another QProgressBar, you obviously need another one. Then simply check the sign of aX and update one of the two accordingly :
if(aX < 0){
progress_bar_neg->setValue(qAbs(aX)*100);
progress_bar_pos->setValue(0);
}else{
progress_bar_pos->setValue(aX*100);
progress_bar_neg->setValue(0);
}
Note that if you want one of your QProgressbar to fill from right to left, you can use void setInvertedAppearance(bool invert) to set its invertedAppearance to true.
◘ Another alternative using the same QProgressBar could look like :
ui->progressBar_retraction->setValue(qAbs(aX)*100);
if(aX < 0){/*Set to a colour*/}
else{/*Set to another colour*}
To edit the colour :
QPalette p;
p.setColor(QPalette::Highlight, Qt::red); //Change background colour, here red
ui->progressBar_retraction->setPalette(p);
p.setColor(QPalette::HighlightedText , Qt::black); //Change text colour, here black
ui->progressBar_retraction->setPalette(p);
◘ If you want a bi-directional QProgessBar, it is unfortunately not possible by only calling a method as far as I know (correct me if I am wrong).
What I did was an override of void QProgressBar::paintEvent(QPaintEvent *) to start the drawing of the rectangle from the middle. You then simply fix the range of the QProgressBar with void QProgressBar::setRange(int minimum, int maximum). You may even update the colour to make it a bit simpler to see.