title: Occlusion shader turns whole scene black
body: <p>I'm trying cull away every face, if it's normal has an angle of: 90 &lt; x &lt; 270 (degrees) with the vector from cameraPosition to vertexPosition.</p>
  
  <p>Everything I see is the screen turning black. I just started with GLSL so I don't know what the reason could be... Or how to debug.</p>
  
  <p>Vertex Shader:</p>
  
  <pre><code>uniform vec3 camera_Position;
  
  void main(void)
  {
   vec4 vertex_Position = gl_ModelViewProjectionMatrix * gl_Vertex;
  
   vec3 N = gl_Normal.xyz; 
  
   vec3 V = camera_Position - vertex_Position;
  
   float angle = degrees(acos(dot(N,V)));
   if(angle &gt;= 90) {
    return;
   }
   if(angle &lt;= 270) {
    return;
   }
  
   gl_Position = vertex_Position;
  }
  </code></pre>
  
  <p>Fragment Shader:</p>
  
  <pre><code>void main()
  {
   gl_FragColor = gl_Color;
  }
  </code></pre>