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<h1>The frustumCulling example</h1>

<center>
  <img src="../images/frustumCulling.jpg" width="330" height="228" alt="frustumCulling"/>
</center>

<p>
 Frustum culling using <code>getFrustumPlanesCoefficients</code>.
</p>
<p>
 A hierarchical octree structure is clipped against a camera's frustum clipping planes, obtained
 using <code>getFrustumPlanesCoefficients</code>. A second viewer displays an external view of the
 scene that exhibits the clipping (using <code>drawCamera()</code> to display the frustum).
</p>
<p>
 This frustum culling implementation is quite naive. Many optimisation techniques are available in
 the litterature.
</p>
<h2>frustumCulling.h</h2>
<pre>
#include &lt;QGLViewer/qglviewer.h&gt;

class CullingCamera;

class Viewer : public QGLViewer {
public:
  void setCullingCamera(const CullingCamera *const cc) { cullingCamera = cc; }

protected:
  virtual void draw();
  virtual void init();
  virtual QString helpString() const;

private:
  const CullingCamera *cullingCamera;
};
</pre>


<h2>frustumCulling.cpp</h2>
<pre>
#include "frustumCulling.h"
#include "box.h"
#include "cullingCamera.h"

using namespace qglviewer;

void Viewer::draw() {
  Box::Root-&gt;drawIfAllChildrenAreVisible(cullingCamera);

  if (cullingCamera == camera())
    // Main viewer computes its plane equation
    cullingCamera-&gt;computeFrustumPlanesEquations();
  else {
    // Observer viewer draws cullingCamera
    glLineWidth(4.0);
    glColor4f(1.0, 1.0, 1.0, 0.5);
    cullingCamera-&gt;draw();
  }
}

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

  if (cullingCamera != camera()) {
    // Observer viewer configuration
    glEnable(GL_BLEND);
    glBlendFunc(GL_SRC_ALPHA, GL_ONE_MINUS_SRC_ALPHA);
    help();
  }

  glDisable(GL_LIGHTING);
}

QString Viewer::helpString() const {
  QString text("&lt;h2&gt;F r u s t u m C u l l i n g&lt;/h2&gt;");

  text += "A hierarchical octree structure is clipped against the camera's "
          "frustum clipping planes, obtained ";
  text += "using &lt;code&gt;getFrustumPlanesCoefficients&lt;/code&gt;. A second viewer "
          "uses &lt;code&gt;drawCamera()&lt;/code&gt; to ";
  text += "display an external view of the first viewer's camera.&lt;br&gt;&lt;br&gt;";

  text += "This frustum culling implementation is quite naive. Many "
          "optimisation techniques are available in ";
  text += "the litterature.";

  return text;
}
</pre>


<h2>box.h</h2>
<pre>
#include &lt;QGLViewer/camera.h&gt;

class CullingCamera;

// An Axis Aligned Bounding Box octree hierarchy element.
class Box {
public:
  Box(const qglviewer::Vec &amp;P1, const qglviewer::Vec &amp;P2) : p1(P1), p2(P2){};

  void draw() const;
  void drawIfAllChildrenAreVisible(const CullingCamera *camera) const;
  void buildBoxHierarchy(int l);

  // Lazy static member, so that it is shared by viewers
  static Box *Root;

private:
  qglviewer::Vec p1, p2;
  Box *child[8];
  int level;
};
</pre>


<h2>cullingCamera.h</h2>
<pre>
#include &lt;QGLViewer/camera.h&gt;

class CullingCamera : public qglviewer::Camera {
public:
  void computeFrustumPlanesEquations() const {
    getFrustumPlanesCoefficients(planeCoefficients);
  }

  float distanceToFrustumPlane(int index, const qglviewer::Vec &amp;pos) const;
  bool sphereIsVisible(const qglviewer::Vec &amp;center, float radius) const;
  bool aaBoxIsVisible(const qglviewer::Vec &amp;p1, const qglviewer::Vec &amp;p2,
                      bool *entirely = NULL) const;

private:
  // F r u s t u m   p l a n e s
  mutable GLdouble planeCoefficients[6][4];
};
</pre>


<h2>box.cpp</h2>
<pre>
#include "box.h"
#include "cullingCamera.h"

using namespace std;
using namespace qglviewer;

// Static member initialization
Box *Box::Root;

void Box::draw() const {
  glColor3f(0.3 * level, 0.2f, 1.0 - 0.3 * level);
  glLineWidth(level + 1);

  glBegin(GL_LINE_STRIP);
  glVertex3fv(Vec(p1.x, p1.y, p1.z));
  glVertex3fv(Vec(p1.x, p2.y, p1.z));
  glVertex3fv(Vec(p2.x, p2.y, p1.z));
  glVertex3fv(Vec(p2.x, p1.y, p1.z));
  glVertex3fv(Vec(p1.x, p1.y, p1.z));
  glVertex3fv(Vec(p1.x, p1.y, p2.z));
  glVertex3fv(Vec(p1.x, p2.y, p2.z));
  glVertex3fv(Vec(p2.x, p2.y, p2.z));
  glVertex3fv(Vec(p2.x, p1.y, p2.z));
  glVertex3fv(Vec(p1.x, p1.y, p2.z));
  glEnd();

  glBegin(GL_LINES);
  glVertex3fv(Vec(p1.x, p2.y, p1.z));
  glVertex3fv(Vec(p1.x, p2.y, p2.z));
  glVertex3fv(Vec(p2.x, p2.y, p1.z));
  glVertex3fv(Vec(p2.x, p2.y, p2.z));
  glVertex3fv(Vec(p2.x, p1.y, p1.z));
  glVertex3fv(Vec(p2.x, p1.y, p2.z));
  glEnd();
}

void Box::buildBoxHierarchy(int l) {
  level = l;
  const Vec middle = (p1 + p2) / 2.0;
  for (unsigned int i = 0; i &lt; 8; ++i) {
    // point in one of the 8 box corners
    const Vec point((i &amp; 4) ? p1.x : p2.x, (i &amp; 2) ? p1.y : p2.y,
                    (i &amp; 1) ? p1.z : p2.z);
    if (level &gt; 0) {
      child[i] = new Box(point, middle);
      child[i]-&gt;buildBoxHierarchy(level - 1);
    } else
      child[i] = NULL;
  }
}

void Box::drawIfAllChildrenAreVisible(const CullingCamera *camera) const {
  static bool *entirely = new bool;

  if (camera-&gt;aaBoxIsVisible(p1, p2, entirely)) {
    if (*entirely)
      draw();
    else if (child[0])
      for (int i = 0; i &lt; 8; ++i)
        child[i]-&gt;drawIfAllChildrenAreVisible(camera);
    else
      draw();
  }
}
</pre>


<h2>cullingCamera.cpp</h2>
<pre>
#include "cullingCamera.h"

using namespace qglviewer;

float CullingCamera::distanceToFrustumPlane(int index, const Vec &amp;pos) const {
  return pos * Vec(planeCoefficients[index]) - planeCoefficients[index][3];
}

bool CullingCamera::sphereIsVisible(const Vec &amp;center, float radius) const {
  for (int i = 0; i &lt; 6; ++i)
    if (distanceToFrustumPlane(i, center) &gt; radius)
      return false;
  return true;
}

bool CullingCamera::aaBoxIsVisible(const Vec &amp;p1, const Vec &amp;p2,
                                   bool *entirely) const {
  bool allInForAllPlanes = true;
  for (int i = 0; i &lt; 6; ++i) {
    bool allOut = true;
    for (unsigned int c = 0; c &lt; 8; ++c) {
      const Vec pos((c &amp; 4) ? p1.x : p2.x, (c &amp; 2) ? p1.y : p2.y,
                    (c &amp; 1) ? p1.z : p2.z);
      if (distanceToFrustumPlane(i, pos) &gt; 0.0)
        allInForAllPlanes = false;
      else
        allOut = false;
    }

    // The eight points are on the outside side of this plane
    if (allOut)
      return false;
  }

  if (entirely)
    // Entirely visible : the eight points are on the inside side of the 6
    // planes
    *entirely = allInForAllPlanes;

  // Too conservative, but tangent cases are too expensive to detect
  return true;
}
</pre>


<h2>main.cpp</h2>
<pre>
#include "box.h"
#include "cullingCamera.h"
#include "frustumCulling.h"

#include &lt;QApplication&gt;
#include &lt;QGLViewer/manipulatedCameraFrame.h&gt;

using namespace qglviewer;

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

  // Create octree AABBox hierarchy
  const qglviewer::Vec p(1.0, 0.7f, 1.3f);
  Box::Root = new Box(-p, p);
  Box::Root-&gt;buildBoxHierarchy(4);

  // Instantiate the two viewers.
  Viewer viewer, observer;

  // Give v a cullingCamera;
  Camera *c = viewer.camera();
  CullingCamera *cc = new CullingCamera();
  viewer.setCamera(cc);
  delete c;

  // Both viewers share the culling camera
  viewer.setCullingCamera(cc);
  observer.setCullingCamera(cc);

  // Place observer
  observer.setSceneRadius(10.0);
  observer.camera()-&gt;setViewDirection(qglviewer::Vec(0.0, -1.0, 0.0));
  observer.showEntireScene();

  // Make sure every culling Camera movement updates the outer viewer
  QObject::connect(viewer.camera()-&gt;frame(), SIGNAL(manipulated()), &amp;observer,
                   SLOT(update()));
  QObject::connect(viewer.camera()-&gt;frame(), SIGNAL(spun()), &amp;observer,
                   SLOT(update()));

  viewer.setWindowTitle("frustumCulling");
  observer.setWindowTitle("scene observer");

  // Show the viewers' windows.
  viewer.show();
  observer.show();

  return application.exec();
}
</pre>



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