int s = 10, d=10;
            for ( int i=0; i<=s; i++ )
            {
                 DrawLine3D(
                    new Vector3( -s*0.5f * d, 0, (i-s*0.5f) *  d ),
                    new Vector3( +s*0.5f * d, 0, (i-s*0.5f) * d ),
                    Color.Gray );

                DrawLine3D(
                        new Vector3( (i-s*0.5f)*  d, 0, -s*0.5f * d ),
                        new Vector3( (i-s*0.5f) * d, 0, +s*0.5f * d ),
                        Color.Gray );
            }