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# The overpainting example

Mixing OpenGL (3D) and QPainter (2D) drawing in the same viewer.

A semi-transparent eounded square is painted in 2D using a QPainter on top of a classical 3D OpenGL rendering. Useful to add 2D objects (annotations, menus, head-up display) to your viewers.

Inspired from the Qt's overpainting example. Note that this functionnality is only available with Qt 4.

## viewer.h

    #include <QGLViewer/qglviewer.h>

    class QPaintEvent;
    class QPainter;

    class Viewer : public QGLViewer
    {
    public :
      Viewer(QWidget* parent = 0);

    protected :
      virtual void draw();
      virtual void init();
      void drawOverpaint(QPainter *painter);

      virtual void paintGL() { update(); };
      virtual void paintEvent(QPaintEvent *event);
      // Could be overloaded to defer final initializations
      //virtual void showEvent(QShowEvent *event);

      virtual QString helpString() const;
    };

## main.cpp

    #include "viewer.h"
    #include <qapplication.h>

    int main(int argc, char** argv)
    {
      QApplication application(argc,argv);
      Viewer viewer;

      viewer.setWindowTitle("overpainting");

      viewer.show();
      return application.exec();
    }

## viewer.cpp

    #include "viewer.h"

    #include <QPainter>

    using namespace std;

    #ifndef GL_MULTISAMPLE
    #define GL_MULTISAMPLE  0x809D
    #endif

    Viewer::Viewer(QWidget* parent)
    : QGLViewer(QGLFormat(QGL::SampleBuffers), parent) {
      setAttribute(Qt::WA_NoSystemBackground);
    }

    void Viewer::drawOverpaint(QPainter *painter) {
    	painter->save();
        painter->translate(width()/2, height()/2);
    	QRadialGradient radialGrad(QPointF(-40, -40), 100);
    	radialGrad.setColorAt(0, QColor(255, 255, 255, 100));
    	radialGrad.setColorAt(1, QColor(200, 200, 0, 100));
        painter->setBrush(QBrush(radialGrad));
    	painter->drawRoundRect(-100, -100, 200, 200);
        painter->restore();
    }

    // Draws a spiral
    void Viewer::draw()
    {
      const float nbSteps = 200.0;

      glBegin(GL_QUAD_STRIP);
      for (int i=0; i<nbSteps; ++i)
        {
          const float ratio = i/nbSteps;
          const float angle = 21.0*ratio;
          const float c = cos(angle);
          const float s = sin(angle);
          const float r1 = 1.0 - 0.8f*ratio;
          const float r2 = 0.8f - 0.8f*ratio;
          const float alt = ratio - 0.5f;
          const float nor = 0.5f;
          const float up = sqrt(1.0-nor*nor);
          glColor3f(1.0-ratio, 0.2f , ratio);
          glNormal3f(nor*c, up, nor*s);
          glVertex3f(r1*c, alt, r1*s);
          glVertex3f(r2*c, alt+0.05f, r2*s);
        }
      glEnd();
    }

    void Viewer::init()
    {
      restoreStateFromFile();
      help();
    }

    void Viewer::paintEvent(QPaintEvent *event)
    {
        Q_UNUSED(event)
        QPainter painter;
        painter.begin(this);
        painter.setRenderHint(QPainter::Antialiasing);

        // Save current OpenGL state
        glPushAttrib(GL_ALL_ATTRIB_BITS);
        glMatrixMode(GL_PROJECTION);
        glPushMatrix();
        glMatrixMode(GL_MODELVIEW);
        glPushMatrix();

        // Reset OpenGL parameters
        glShadeModel(GL_SMOOTH);
        glEnable(GL_DEPTH_TEST);
        glEnable(GL_CULL_FACE);
        glEnable(GL_LIGHTING);
        glEnable(GL_LIGHT0);
        glEnable(GL_MULTISAMPLE);
        static GLfloat lightPosition[4] = { 1.0, 5.0, 5.0, 1.0 };
        glLightfv(GL_LIGHT0, GL_POSITION, lightPosition);
        qglClearColor(backgroundColor());

        // Classical 3D drawing, usually performed by paintGL().
        preDraw();
        draw();
        postDraw();
        // Restore OpenGL state
        glMatrixMode(GL_MODELVIEW);
        glPopMatrix();
        glMatrixMode(GL_PROJECTION);
        glPopMatrix();
        glPopAttrib();

        drawOverpaint(&painter);

        painter.end();
    }

    QString Viewer::helpString() const
    {
      QString text("<h2>O v e r p a i n t</h2>");
      text += "This example shows how to mix the 2D QPainter drawing with regular 3D in an OpenGL QGLViewer.<br>";
      text += "The <code>paintEvent</code> method is overloaded to interleave the two drawing codes.";
      return text;
    }

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Last modified on Wednesday, June 17, 2015.