text: #ifdef GL_ES
  precision highp float;
  #endif
  
  uniform int     frequency;
  uniform vec4    color0;
  uniform vec4    color1;
  
  varying vec2    v_st;
  
  void
  main()
  {
      float   s, t, delta;
      
      s = mod(floor(v_st.x * float(frequency * 2)), 2.0);
      t = mod(floor(v_st.y * float(frequency * 2)), 2.0);    
      delta = abs(s - t);
      
      gl_FragColor = mix(color1, color0, delta);
  }
  