text: #ifndef SURFACE_FRAG_GLSL_HXX
  #define SURFACE_FRAG_GLSL_HXX
  
  static const std::string SURFACE_FRAG_GLSL = R"LITERAL(
  #version 330
  
  in vec3 vfPosition;
  in vec3 vfDirection;
  out vec4 fo_FragColor;
  
  vec3 colormap(vec3 direction);
  bool is_visible(vec3 position, vec3 direction);
  
  void main(void) {
    if (is_visible(vfPosition, vfDirection)) {
      vec3 color = colormap(normalize(vfDirection));
      fo_FragColor = vec4(color, 1.0);
    } else {
      discard;
    }
  }
  )LITERAL";
  
  #endif
  
  