const char *volume_vs =
  "#version 400\n"
  "precision highp float;\n"
  "\n"
  "in vec3 vertexPosition;\n"
  "\n"
  "void main()\n"
  "{\n"
  "    gl_Position = vec4(vertexPosition, 1.0);\n"
  "}\n"
  "\n";
