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# The select example

Selection of scene objects using `select()` and an GL_SELECT render mode.

Use the `select()` callback function to implement your object selection function. This examples is based on a generic GL_SELECT implementation that can easily be cut and pasted in your applications.

Analytic intersection computations are also possible once the screen coordinates have be converted to a half line using `convertClickToLine()`. Make a selection and then move the camera to see a representation of the intersection line.

## select.h

    #include <QGLViewer/qglviewer.h>

    class Viewer : public QGLViewer
    {
    protected :
      virtual void draw();
      virtual void drawWithNames();
      virtual void postSelection(const QPoint& point);
      virtual void init();
      virtual QString helpString() const;

    private :
      qglviewer::Vec orig, dir, selectedPoint;
    };

## select.cpp

    #include "select.h"
    #include <qmessagebox.h>
    #include <math.h>

    using namespace std;

    static void drawSpiral(const bool specialColor = false)
    {
      const float nbSteps = 100.0;
      glBegin(GL_QUAD_STRIP);
      for (float i=0; i<nbSteps; ++i)
        {
          float ratio = i/nbSteps;
          float angle = 21.0*ratio;
          float c = cos(angle);
          float s = sin(angle);
          float r1 = 0.5 - 0.3*ratio;
          float r2 = 0.3 - 0.3*ratio;
          float alt = ratio - 0.5;
          const float nor = .5;
          const float up = sqrt(1.0-nor*nor);
          if (specialColor)
    	glColor3f(1.0-ratio, 0.8f , ratio/2.0);
          else
    	glColor3f(1.0-ratio, 0.2f, ratio);
          glNormal3f(nor*c, nor*s, up);
          glVertex3f(r2*c, r2*s, alt+0.05f);
          glVertex3f(r1*c, r1*s, alt);
        }
      glEnd();
    }

    void Viewer::drawWithNames()
    {
      // Draw spirals, pushing a name (id) for each of them
      const int nb = 10;
      for (int i=0; i<nb; ++i)
        {
          glPushMatrix();
          glTranslatef(cos(2.0*i*M_PI/nb), sin(2.0*i*M_PI/nb), 0.);

          glPushName(i);
          drawSpiral();
          glPopName();

          glPopMatrix();
        }
    }

    void Viewer::postSelection(const QPoint& point)
    {
      // Compute orig and dir, used to draw a representation of the intersecting line
      camera()->convertClickToLine(point, orig, dir);

      // Find the selectedPoint coordinates, using camera()->pointUnderPixel().
      bool found;
      selectedPoint = camera()->pointUnderPixel(point, found);
      selectedPoint -= 0.01f*dir; // Small offset to make point clearly visible.
      // Note that "found" is different from (selectedObjectId()>=0) because of the size of the select region.

      if (selectedName() == -1)
        QMessageBox::information(this, "No selection",
    			     "No object selected under pixel " + QString::number(point.x()) + "," + QString::number(point.y()));
      else
        QMessageBox::information(this, "Selection",
    			     "Spiral number " + QString::number(selectedName()) + " selected under pixel " +
    			     QString::number(point.x()) + "," + QString::number(point.y()));
    }

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

      glLineWidth(3.0);
      glPointSize(10.0);

      help();
    }

    void Viewer::draw()
    {
      // Draw ten spirals
      const int nb = 10;
      for (int i=0; i<nb; ++i)
        {
          glPushMatrix();
          glTranslatef(cos(2.0*i*M_PI/nb), sin(2.0*i*M_PI/nb), 0.0);
          drawSpiral(i == selectedName());
          glPopMatrix();
        }

      // Draw the intersection line
      glBegin(GL_LINES);
      glVertex3fv(orig);
      glVertex3fv(orig + 100.0*dir);
      glEnd();

      // Draw (approximated) intersection point on selected object
      if (selectedName() >= 0)
        {
          glColor3f(0.9f, 0.2f, 0.1f);
          glBegin(GL_POINTS);
          glVertex3fv(selectedPoint);
          glEnd();
        }
    }

    QString Viewer::helpString() const
    {
      QString text("<h2>S e l e c t</h2>");
      text += "Left click while pressing the <b>Shift</b> key to select an object of the scene.<br><br>";
      text += "A line is drawn between the selected point and the camera selection position. ";
      text += "using <i>convertClickToLine()</i>, a useful function for analytical intersections.<br><br>";
      text += "To add object selection in your viewer, all you need to do is to define the <i>drawWithNames</i> function. ";
      text += "It gives a name to each selectable object and selection is then performed using the OpenGL <i>GL_SELECT</i> render mode.<br><br>";
      text += "Feel free to cut and paste this implementation in your own applications.";
      return text;
    }

## main.cpp

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

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

      Viewer viewer;

      viewer.setWindowTitle("select");

      viewer.show();

      return application.exec();
    }

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Last modified on Thursday, January 22, 2015.