public void frustum(float left, float right, float bottom,
                      float top, float znear, float zfar) {

    leftScreen = left;
    rightScreen = right;
    bottomScreen = bottom;
    topScreen = top;
    nearPlane = znear;
    frustumMode = true;

    //System.out.println(projection);
    projection.set((2*znear)/(right-left), 0, (right+left)/(right-left), 0,
                   0, (2*znear)/(top-bottom), (top+bottom)/(top-bottom), 0,
                   0, 0, -(zfar+znear)/(zfar-znear),-(2*zfar*znear)/(zfar-znear),
                   0, 0, -1, 0);
  }