How to change Qlabel text color (QPalette not working, Stylesheet too slow)

[kyotishi]

Hi all,

I am working on a QT GUI application, I receive data from a microcontroller through serial at 5Hz and I am updating the GUI accordingaly

I use QLabel to display received values.


everything works fine, except that I want to change the text color of the QLabel when a value exceeds a certain limit, I have one hundered value to updated and using
stylesheet makes the GUI VERY SLOOOW, and if I use QPalette then there is no effect at all (the color is not changed)


here is my two version of the code

//using Stylesheets:
 
void GUI::checkLimits(double max, float value, QLabel * label){
 
    if ((value > max)
    {
             label->setStyleSheet(QString::fromUtf8("color: rgb(255, 0, 0);\n"
                                                    ""));
 
      }else{
            label->setStyleSheet(QString::fromUtf8("color: rgb(0, 255, 255);\n"
                                                ""));
       }
    }
 
 
//using QPalette:
 
void GUI::checkLimits(double max, float value, QLabel * label){
 
    QPalette pal = label->palette ();
    if ((value > max)
    {
            pal->setColor(QPalette::WindowText, Qt::red);
 
      }else{
            pal->setColor(QPalette::WindowText, Qt::white);
       }
 
    label->setPalette (pal);
    label->setAutoFillBackground(true);
 
    }
 
 
//then call the function for each Qlabel:
 
//update the measurements as read from the PMU
void Display::updateData()
{
 
    double max1 = 3.3; 
    double max2 = 5.5;
    double max3 = 7.7;
 
    //value 1
    //get the received value
    value1 = (receivedData->rec1);
    //check if the received value exceeds the max allowed value
    checkLimits(max1 ,value1, iuGUI->Qlabel1);
     //change the text of the label
    iuGUI->Qlabel1->setText(QString::number(value));
 
    //value 2
    value2 = (receivedData->rec2);
    checkLimits(max2 ,value2, iuGUI->Qlabel2);
    iuGUI->Qlabel2->setText(QString::number(value));
 
    //value 3
    value3 = (receivedData->rec3);
    checkLimits(max3 ,value3, iuGUI->Qlabel3);
    iuGUI->Qlabel3->setText(QString::number(value));
 
    //etc....repeat the same for all the values...100!!
 
}

//using Stylesheets:

void GUI::checkLimits(double max, float value, QLabel * label){

    if ((value > max)
    {
             label->setStyleSheet(QString::fromUtf8("color: rgb(255, 0, 0);\n"
                                                    ""));
         
      }else{
            label->setStyleSheet(QString::fromUtf8("color: rgb(0, 255, 255);\n"
                                                ""));
       }
    }


//using QPalette:

void GUI::checkLimits(double max, float value, QLabel * label){

    QPalette pal = label->palette ();
    if ((value > max)
    {
            pal->setColor(QPalette::WindowText, Qt::red);
         
      }else{
            pal->setColor(QPalette::WindowText, Qt::white);
       }

    label->setPalette (pal);
    label->setAutoFillBackground(true);

    }


//then call the function for each Qlabel:

//update the measurements as read from the PMU
void Display::updateData()
{

    double max1 = 3.3; 
    double max2 = 5.5;
    double max3 = 7.7;

    //value 1
    //get the received value
    value1 = (receivedData->rec1);
    //check if the received value exceeds the max allowed value
    checkLimits(max1 ,value1, iuGUI->Qlabel1);
     //change the text of the label
    iuGUI->Qlabel1->setText(QString::number(value));

    //value 2
    value2 = (receivedData->rec2);
    checkLimits(max2 ,value2, iuGUI->Qlabel2);
    iuGUI->Qlabel2->setText(QString::number(value));

    //value 3
    value3 = (receivedData->rec3);
    checkLimits(max3 ,value3, iuGUI->Qlabel3);
    iuGUI->Qlabel3->setText(QString::number(value));

    //etc....repeat the same for all the values...100!!

}

To copy to clipboard, switch view to plain text mode 

I am using QT 4.7 and running on Windows 7

[pradeepreddyg95]

use tr function to display text colors.
lineEdit->setText(tr("<font color = red >"+your text+QString("\n")+QString(" </font size = 11 >")));

[Santosh Reddy]

As such this is the only problem I see

void Display::updateData()
{
 
    double max1 = 3.3; 
    double max2 = 5.5;
    double max3 = 7.7;
 
    //value 1
    //get the received value
    value1 = (receivedData->rec1);
    //check if the received value exceeds the max allowed value
    checkLimits(max1 ,value1, iuGUI->Qlabel1);
     //change the text of the label
    iuGUI->Qlabel1->setText(QString::number(value)); // should this be value1?
 
    //value 2
    value2 = (receivedData->rec2);
    checkLimits(max2 ,value2, iuGUI->Qlabel2);
    iuGUI->Qlabel2->setText(QString::number(value));// should this be value2?
 
    //value 3
    value3 = (receivedData->rec3);
    checkLimits(max3 ,value3, iuGUI->Qlabel3);
    iuGUI->Qlabel3->setText(QString::number(value));// should this be value3?
 
    //etc....repeat the same for all the values...100!!
 
}

void Display::updateData()
{

    double max1 = 3.3; 
    double max2 = 5.5;
    double max3 = 7.7;

    //value 1
    //get the received value
    value1 = (receivedData->rec1);
    //check if the received value exceeds the max allowed value
    checkLimits(max1 ,value1, iuGUI->Qlabel1);
     //change the text of the label
    iuGUI->Qlabel1->setText(QString::number(value)); // should this be value1?

    //value 2
    value2 = (receivedData->rec2);
    checkLimits(max2 ,value2, iuGUI->Qlabel2);
    iuGUI->Qlabel2->setText(QString::number(value));// should this be value2?

    //value 3
    value3 = (receivedData->rec3);
    checkLimits(max3 ,value3, iuGUI->Qlabel3);
    iuGUI->Qlabel3->setText(QString::number(value));// should this be value3?

    //etc....repeat the same for all the values...100!!

}

To copy to clipboard, switch view to plain text mode 

How often you call updateData(), and from where? (5Hz will be 200 ms), do you call once every 200 ms?

[d_stranz]

How often you call updateData(), and from where? (5Hz will be 200 ms), do you call once every 200 ms?

What Santosh is talking about is this: if you are calling updateData() from within a loop that is monitoring your signal, and your never get out of that loop to permit the Qt event loop to process pending events, then none of the changes to your labels will appear. Once every 200 ms is plenty of time to update the text and color of a label if you allow the event loop to run.