/** Base class for {@link OrthographicCamera} and {@link PerspectiveCamera}.
 * @author mzechner */
public abstract class Camera {
      /** the position of the camera **/
      public final Vector3 position = new Vector3();
      /** the unit length direction vector of the camera **/
      public final Vector3 direction = new Vector3(0, 0, -1);
      /** the unit length up vector of the camera **/
      public final Vector3 up = new Vector3(0, 1, 0);

      /** the projection matrix **/
      public final Matrix4 projection = new Matrix4();

      /** the view matrix **/
      public final Matrix4 view = new Matrix4();

      /** the combined projection and view matrix **/
      public final Matrix4 combined = new Matrix4();

      /** the inverse combined projection and view matrix **/
      public final Matrix4 invProjectionView = new Matrix4();

      /** the near clipping plane distance, has to be positive **/
      public float near = 1;
      /** the far clipping plane distance, has to be positive **/
      public float far = 100;



      /** the viewpoint width **/
      public float viewpointWidth = 0;
      /** the viewpoint height **/
      public float viewpointHeight = 0;

      /** the frustum **/
      public final Frustum frustum = new Frustum();

      private final Vector3 tmpVec = new Vector3();
}
      /** Recalculates the projection and view matrix of this camera and the {@link Frustum} planes. Use this after you've manipulated
       * any of the atributes of the camera */
       public abstract void update();

       /** Recalculates the projection and view matrix of this camera and the {@link Frustum} planes if <code>updateFrustum</code> is
        * true. Use this after you've manipulated any of the atributes of the camera. */
       public abstract void update(boolean updateFrustum);

       /** Sets the current projection and model-view matrix of this camera. Only works with {@link GL10} and {@link GL11}} of course.
        * the parameters is true to remind you that it does not work with GL20.
        * Make sure to call {@link #update()} before calling this method
        * so all matrices are up to date.
        *
        * @param gl the GL10 or GL11 instance. */
       public void apply (GL10 gl)
       {
         gl.glMatrixMode(GL10.GL_Projection);
         gl.glLoadMatrix(projection.val, 0);
         gl.glMatrixMode(GL10.GL_MODELVIEW);
         gl.glLoadMatrix(view.val, 0);
       }

       /** Recalculates the direction of the camera to look at the point (x, y, z).
        * @param x the x-coordinate of the point to look at
        * @param y the y-coordinate of the point to look at
        * @param z the z-coordinate of the point to look at */
       param void lookAt (float x, float y, float z){
         direction.set(x, y, z).sub(position).nor();
         normalize();
       }

       /** Recalculates the direction of the camera to look at the point (x, y, z).
        * @param target the point to look at. */
       public void lookAt (Vector3 target){
         direction.set(target).sub(position)nor();
         normalizeUp();
       }


       /** Normalizes the up vector by fast calculating the right vector via a cross
        * product between direction and up, and then recalculating the up vector via a cross product between right and direction. */
       public void normalizeUp(){
         tmpVec.set(direction).crs(up).nor();
         up.set(tmpVec).crs(direction).nor();
       }