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# The clippingPlane example

A clipping plane is manipulated using a ManipulatedFrame

The standard OpenGL GL_CLIP_PLANE feature is used to add an additionnal clipping plane in the scene, which position and orientation are set by a ManipulatedFrame.

Hold the Control key pressed down while using the mouse to modify the plane orientation (left button) and position (right button) and to interactively see the clipped result.

Since the plane equation is defined with respect to the current modelView matrix, a constant equation (normal along the Z axis) can be used since we transformed the coordinates system using the matrix() method.

## clippingPlane.h

    #include <QGLViewer/qglviewer.h>

    class Viewer : public QGLViewer {
    protected:
      virtual void draw();
      virtual void init();
      virtual QString helpString() const;
    };

## clippingPlane.cpp

    #include "clippingPlane.h"
    #include "QGLViewer/manipulatedFrame.h"

    using namespace std;
    using namespace qglviewer;

    // 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();

      glPushMatrix();
      glMultMatrixd(manipulatedFrame()->matrix());
      // Since the Clipping Plane equation is multiplied by the current modelView,
      // we can define a constant equation (plane normal along Z and passing by the
      // origin) since we are here in the manipulatedFrame coordinates system (we
      // glMultMatrixd() with the manipulatedFrame matrix()).
      static const GLdouble equation[] = {0.0, 0.0, 1.0, 0.0};
      glClipPlane(GL_CLIP_PLANE0, equation);

      // Draw a plane representation: Its normal...
      glColor3f(0.8f, 0.8f, 0.8f);
      drawArrow(0.4f, 0.015f);
      // ...and a quad (with a slightly shifted z so that it is not clipped).
      glBegin(GL_QUADS);
      glVertex3f(-1.0, -1.0, 0.001f);
      glVertex3f(-1.0, 1.0, 0.001f);
      glVertex3f(1.0, 1.0, 0.001f);
      glVertex3f(1.0, -1.0, 0.001f);
      glEnd();
      glPopMatrix();
    }

    void Viewer::init() {
      // Restore previous viewer state.
      restoreStateFromFile();

      // Opens help window
      help();

      // The ManipulatedFrame will be used to position the clipping plane
      setManipulatedFrame(new ManipulatedFrame());

      // Enable plane clipping
      glEnable(GL_CLIP_PLANE0);
    }

    QString Viewer::helpString() const {
      QString text("<h2>C l i p p i n g P l a n e</h2>");
      text += "The standard OpenGL <i>GL_CLIP_PLANE</i> feature is used to add an "
              "additionnal clipping ";
      text += "plane in the scene, which position and orientation are set by a "
              "<b>ManipulatedFrame</b>.<br><br>";
      text += "Hold the <b>Control</b> key pressed down while using the mouse to "
              "modify the plane orientation (left button) ";
      text += "and position (right button) and to interactively see the clipped "
              "result.<br><br>";
      text += "Since the plane equation is defined with respect to the current "
              "modelView matrix, a constant equation (normal";
      text += "along the Z axis) can be used since we transformed the coordinates "
              "system using the <b>matrix()</b> method.";

      return text;
    }

## main.cpp

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

    int main(int argc, char **argv) {
      // Read command lines arguments.
      QApplication application(argc, argv);

      // Instantiate the viewer.
      Viewer viewer;

      viewer.setWindowTitle("clippingPlane");

      // Make the viewer window visible on screen.
      viewer.show();

      // Run main loop.
      return application.exec();
    }

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