   // Source code of vertex shader
   uniform mat4 mvpMatrix;
   attribute vec4 vPosition;
   uniform vec4 eyePosition;
   uniform vec4 lightPosition;
   varying vec3 N; //normal direction
   varying vec3 L; //light source direction
   varying vec3 V; //view vector
   void main() { 
      gl_Position = mvpMatrix * vPosition; 
      N = vPosition.xyz;   //normal of  a point on sphere
      L = lightPosition.xyz - vPosition.xyz;
      V = eyePosition.xyz - vPosition.xyz;
   }
