Get intermediate color from a gradient

Question

Say I have a liner gradient as shown:

QLinearGradient linearGrad(QPointF(0, 0), QPointF(0, 100));
linearGrad.setColorAt(1, Qt::red);
linearGrad.setColorAt(0.5, Qt::yellow);
linearGrad.setColorAt(0, Qt::green);

How to get the color of the point QPointF(0, 28.5) in this gradient?

Indeed I want to have this kind of color distribution to be able to choose intermediate colors. I don't care if it is done by using QLinearGradient or something else.

Answer 1

I store the colors of gradient in one QList and then compute with color interpolation.

QColor ColorGradient::getColor(double value)
{
  qDebug()<< "ColorGradient::getColor:";
    //Asume mGradientColors.count()>1 and value=[0,1]
    double stepbase = 1.0/(mGradientColors.count()-1);
    int interval=mGradientColors.count()-1; //to fix 1<=0.99999999;

      for (int i=1; i<mGradientColors.count();i++)//remove begin and end
        {
            if(value<=i*stepbase ){interval=i;break;}
        }
       double percentage = (value-stepbase*(interval-1))/stepbase;
       QColor color(interpolate(mGradientColors[interval],mGradientColors[interval-1],percentage));        
       return color;
}
QColor ColorGradient::interpolate(QColor start,QColor end,double ratio)
{
    int r = (int)(ratio*start.red() + (1-ratio)*end.red());
    int g = (int)(ratio*start.green() + (1-ratio)*end.green());
    int b = (int)(ratio*start.blue() + (1-ratio)*end.blue());
    return QColor::fromRgb(r,g,b);
}

Answer 2

Mason Zhang answer does work, and very well ! Let controlPoints() return a QMap<qreal,QColor>, with a key between 0.0 and 1.0. Here is how i did (thanks to Mason Zhang)

QColor getColor(qreal key) const
{
    // key must belong to [0,1]
    key = Clip(key, 0.0, 1.0) ;

    // directly get color if known
    if(controlPoints().contains(key))
    {
        return controlPoints().value(key) ;
    }

    // else, emulate a linear gradient
    QPropertyAnimation interpolator ;
    const qreal granularite = 100.0 ;
    interpolator.setEasingCurve(QEasingCurve::Linear) ;
    interpolator.setDuration(granularite) ;
    foreach( qreal key, controlPoints().keys() )
    {
        interpolator.setKeyValueAt(key, controlPoints().value(key)) ;
    }
    interpolator.setCurrentTime(key*granularite) ;
    return interpolator.currentValue().value<QColor>() ;
}