typedef struct SurfacePointType
{
    float height;
    float vel;
    float acc;
} SurfacePointType;

#define RESOLUTION 20  /* to define a RES by RES grid */

class SurfaceClass
{
  private:
    float Phase;
    SurfacePointType MySurf [RESOLUTION] [RESOLUTION];

    void Init (void)
    {
      int i,j;
      for (i=0; i<RESOLUTION; i++)
        for (j=0; j<RESOLUTION; j++)
        {  
          MySurf[i][j].height = sin(360.0*i/RESOLUTION)*sin(360.0*j/RESOLUTION);
        } // of for
    } // of Init   

  public:

    SurfaceClass (void)
    {
      Phase = 0.0;
      Init();
    } // of Constructor   

    void Animate (void)
    {
      int i,j;
      for (i=0; i<RESOLUTION; i++)
        for (j=0; j<RESOLUTION; j++)
        {  
          MySurf[i][j].height = sin(Phase+360.0*i/RESOLUTION)*sin(Phase+360.0*j/RESOLUTION);
        } // of for
      Phase += 0.2;
    } // of Animate

    void  Draw (void)
    {
      glBegin (GL_QUADS);
      int i,j;
      for (i = 0; i < RESOLUTION-2; i++)
        for (j = 0; j < RESOLUTION-2; j++)
          {
            glColor3f(0.0,1.0,1.0);
            float x = (float) i / (float) RESOLUTION;
            float y = (float) j / (float) RESOLUTION;
            float step = 1.0 / (float) RESOLUTION;
            float dx = MySurf[i][j+1].height-MySurf[i][j].height;

            float dy = MySurf[i+1][j].height-MySurf[i][j].height;
            float len = sqrt(dx*dx+dy*dy+1); 
            glNormal3f ( dx/len, dy/len, -1.0/len);
            glVertex3f (x,MySurf[i][j].height,y);   


            dx = MySurf[i][j+2].height-MySurf[i][j+1].height;
            dy = MySurf[i+1][j+1].height-MySurf[i][j+1].height;
            glNormal3f ( dx/len, dy/len, -1.0/len);
            glVertex3f (x,MySurf[i][j+1].height,y+step);   

            dx = MySurf[i+1][j+2].height-MySurf[i+1][j+1].height;
            dy = MySurf[i+2][j+1].height-MySurf[i+1][j+1].height;
            glNormal3f ( dx/len, dy/len, -1.0/len);
            glVertex3f (x+step,MySurf[i+1][j+1].height,y+step);   

            dx = MySurf[i+1][j+1].height-MySurf[i+1][j].height;
            dy = MySurf[i+2][j].height-MySurf[i+1][j].height;
            glNormal3f ( dx/len, dy/len, -1.0/len);
            glVertex3f (x+step,MySurf[i+1][j].height,y);   


      } // of for
  glEnd ();
} // of Draw
}; // of SurfaceClass