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    QGraphicsscene:: How to choose the recessive object ?

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    • X
      xmaze last edited by

      Hi, guys!
      I have a question. I use Qgrapchicscene to overlay an aircraft into a map and QLineF to paint the aircrafts route. The airplane is only a triangle, and the route is a QLineF.
      The problem is that the aircraft (triangle) is in the background.
      I don't like this and i want to ask how can i make the triangle recessive so the route do not overlays the triangle. For a better understood please take a look of the screenshot!

      [IMG]http://i58.tinypic.com/2mgqzw9.png[/IMG]

      And the second Question is how can i erase the route if the user changes the airplane?

      Thank you in advance!

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      • Chris Kawa
        Chris Kawa Moderators last edited by

        You can add line objects using addLine.
        The returned object inherits QGraphicsItem so you can set the Z-order of it using setZValue.

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        • X
          xmaze last edited by

          beautiful!! thank you!
          And do you know how can erase all the lines when i want to do that ?

          And do you have any suggestions, how can i change the color indepence of the speed like the flightradar24.com ?

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          • Chris Kawa
            Chris Kawa Moderators last edited by Chris Kawa

            For this a QGraphicsPathItem might be a better choice.

            A simple example

            //make a path consisting of some points
            QPainterPath path;
            path.moveTo(planePositionAt(0));
            path.lineTo(planePositionAt(1));
            ... //and so on (of course a loop would be better here)
            
            //create a gradient with wanted colors
            QLinearGradient gradient;
            gradient.setStart(path.pointAtPercent(0.0));
            gradient.setFinalStop(path.pointAtPercent(1.0));
            gradient.setColorAt(0.0, Qt::red);
            gradient.setColorAt(0.5, Qt::green);
            gradient.setColorAt(1.0, Qt::blue);
            
            //make a wide pen
            QPen pen;
            pen.setWidth(3);
            pen.setBrush(QBrush(gradient));
            
            //use the path and the pen to add a path item to the scene
            QGraphicsPathItem* pathItem = myScene->addPath(path, pen);
            
            //at some point remove the item from the scene and delete it
            myScene->removeItem(pathItem);
            delete pathItem;
            

            If the gradient is not good enough a solution you can just add individual lines with different pen colors, e.g.

            QList<QGraphicsLineItem*> lines;
            lines << myScene->addLine(myPlaneLineFragment(0), QPen(colorForSpeed(0)));
                  << myScene->addLine(myPlaneLineFragment(1), QPen(colorForSpeed(1)));
                  ... //and so on (of course a loop would be better here)
            

            and remove and delete them as in previous example.

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            • X
              xmaze last edited by

              i am little bit confused, so i post my way:

              this is the code in one class

              QVector<QLineF> aircraftsPath;
              QLineF temp;
              temp.setLine((qreal)pxPrevious,(qreal)pyPrevious,(qreal)px,(qreal)py);
              if(temp.length()<30) aircraftsPath.push_back(temp); //avoid the error lines from bad signals
              

              this is the code in mainwindow Class:

              if(pointOfAirplane->aircraftsPath.size()>0) {
                 for(int i=0;i<pointOfAirplane->aircraftsPath.size();i++) {
                     QLineF *temp= &pointOfAirplane->aircraftsPath[i];
                     QPen *pen = new QPen(Qt::red, 3, Qt::DashDotLine, Qt::RoundCap, Qt::RoundJoin);
                     pen->setColor(Qt::red); // Here i try to find a way to have different colors
              
                     scene->addLine(*temp,*pen)->setZValue(1);
                     pointOfAirplane->aircraftsPath.remove(i);
                 }
              
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              • Chris Kawa
                Chris Kawa Moderators last edited by

                You are leaking the QPen instance (you use new and never delete it). You don't need those pointers or the first if (since you loop anyway).

                So you could do something more or less like this:
                Assuming you've got some speed stored somewhere, e.g. when you add a path point you could add a speed color too:

                struct PathSegment {
                   QLineF line;
                   QColor speedColor;
                };
                QVector<PathSegment> aircraftsPath;
                

                Then it's just a matter of adding it to the scene

                QVector<QGraphincsLineIntem*> lineItems;
                
                foreach(const PathSegment& segment, pointOfAirplane->aircraftsPath) {
                   QPen pen(segment.speedColor, 3.0, Qt::DashDotLine, Qt::RoundCap, Qt::RoundJoin);
                   lineItems << scene->addLine(segment.line, pen);
                   lineItems.last()->setZValue(1.0);
                }
                

                When you need to clear the lines just iterate through the lineItems vector and delete them like I showed you in the previous post.

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