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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 <math.h>
    #include <qmessagebox.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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