title: OpenGL - Frustum not culling polygons beyond far plane
body: <p>I have implemented frustum culling and am checking the bounding box for its intersection with the frustum planes. I added the ability to pause frustum updates which lets me see if the frustum culling has been working correctly. When I turn around after I have paused it, nothing renders behind me and to the left and right side, they taper off as well just as you would expect. Beyond the clip distance (far plane), they still render and I am not sure whether it is a problem with my frustum updating or bounding box checking code or I am using the wrong matrix or what. As I put the distance in the projection matrix at 3000.0f, it still says that bounding boxes well past that are still in the frustum, which isn't the case.</p>
  
  <p>Here is where I create my modelview matrix:</p>
  
  <pre><code>projectionMatrix = glm::perspective(newFOV, 4.0f / 3.0f, 0.1f, 3000.0f);
  
  viewMatrix = glm::mat4(1.0);
  viewMatrix = glm::scale(viewMatrix, glm::vec3(1.0, 1.0, -1.0));
  viewMatrix = glm::rotate(viewMatrix, anglePitch, glm::vec3(1.0, 0.0, 0.0));
  viewMatrix = glm::rotate(viewMatrix, angleYaw, glm::vec3(0.0, 1.0, 0.0));
  viewMatrix = glm::translate(viewMatrix, glm::vec3(-x, -y, -z));
  
  modelViewProjectiomMatrix = projectionMatrix * viewMatrix;
  </code></pre>
  
  <p>The reason I scale it by -1 in the Z direction is because the levels were designed to be rendered with DirectX so I reverse the Z direction.</p>
  
  <p>Here is where I update my frustum:</p>
  
  <pre><code>void CFrustum::calculateFrustum()
  {
      glm::mat4 mat = camera.getModelViewProjectionMatrix();
  
      // Calculate the LEFT side
      m_Frustum[LEFT][A] = (mat[0][3]) + (mat[0][0]);
      m_Frustum[LEFT][B] = (mat[1][3]) + (mat[1][0]);
      m_Frustum[LEFT][C] = (mat[2][3]) + (mat[2][0]);
      m_Frustum[LEFT][D] = (mat[3][3]) + (mat[3][0]);
  
      // Calculate the RIGHT side
      m_Frustum[RIGHT][A] = (mat[0][3]) - (mat[0][0]);
      m_Frustum[RIGHT][B] = (mat[1][3]) - (mat[1][0]);
      m_Frustum[RIGHT][C] = (mat[2][3]) - (mat[2][0]);
      m_Frustum[RIGHT][D] = (mat[3][3]) - (mat[3][0]);
  
      // Calculate the TOP side
      m_Frustum[TOP][A] = (mat[0][3]) - (mat[0][1]);
      m_Frustum[TOP][B] = (mat[1][3]) - (mat[1][1]);
      m_Frustum[TOP][C] = (mat[2][3]) - (mat[2][1]);
      m_Frustum[TOP][D] = (mat[3][3]) - (mat[3][1]);
  
      // Calculate the BOTTOM side
      m_Frustum[BOTTOM][A] = (mat[0][3]) + (mat[0][1]);
      m_Frustum[BOTTOM][B] = (mat[1][3]) + (mat[1][1]);
      m_Frustum[BOTTOM][C] = (mat[2][3]) + (mat[2][1]);
      m_Frustum[BOTTOM][D] = (mat[3][3]) + (mat[3][1]);
  
      // Calculate the FRONT side
      m_Frustum[FRONT][A] = (mat[0][3]) + (mat[0][2]);
      m_Frustum[FRONT][B] = (mat[1][3]) + (mat[1][2]);
      m_Frustum[FRONT][C] = (mat[2][3]) + (mat[2][2]);
      m_Frustum[FRONT][D] = (mat[3][3]) + (mat[3][2]);
  
      // Calculate the BACK side
      m_Frustum[BACK][A] = (mat[0][3]) - (mat[0][2]);
      m_Frustum[BACK][B] = (mat[1][3]) - (mat[1][2]);
      m_Frustum[BACK][C] = (mat[2][3]) - (mat[2][2]);
      m_Frustum[BACK][D] = (mat[3][3]) - (mat[3][2]);
  
      // Normalize all the sides
      NormalizePlane(m_Frustum, LEFT);
      NormalizePlane(m_Frustum, RIGHT);
      NormalizePlane(m_Frustum, TOP);
      NormalizePlane(m_Frustum, BOTTOM);
      NormalizePlane(m_Frustum, FRONT);
      NormalizePlane(m_Frustum, BACK);
  }
  </code></pre>
  
  <p>And finally, where I check the bounding box:</p>
  
  <pre><code>bool CFrustum::BoxInFrustum( float x, float y, float z, float x2, float y2, float z2)
  {
      // Go through all of the corners of the box and check then again each plane
      // in the frustum.  If all of them are behind one of the planes, then it most
      // like is not in the frustum.
      for(int i = 0; i &lt; 6; i++ )
      {
          if(m_Frustum[i][A] * x  + m_Frustum[i][B] * y  + m_Frustum[i][C] * z  + m_Frustum[i][D] &gt; 0)  continue;
          if(m_Frustum[i][A] * x2 + m_Frustum[i][B] * y  + m_Frustum[i][C] * z  + m_Frustum[i][D] &gt; 0)  continue;
          if(m_Frustum[i][A] * x  + m_Frustum[i][B] * y2 + m_Frustum[i][C] * z  + m_Frustum[i][D] &gt; 0)  continue;
          if(m_Frustum[i][A] * x2 + m_Frustum[i][B] * y2 + m_Frustum[i][C] * z  + m_Frustum[i][D] &gt; 0)  continue;
          if(m_Frustum[i][A] * x  + m_Frustum[i][B] * y  + m_Frustum[i][C] * z2 + m_Frustum[i][D] &gt; 0)  continue;
          if(m_Frustum[i][A] * x2 + m_Frustum[i][B] * y  + m_Frustum[i][C] * z2 + m_Frustum[i][D] &gt; 0)  continue;
          if(m_Frustum[i][A] * x  + m_Frustum[i][B] * y2 + m_Frustum[i][C] * z2 + m_Frustum[i][D] &gt; 0)  continue;
          if(m_Frustum[i][A] * x2 + m_Frustum[i][B] * y2 + m_Frustum[i][C] * z2 + m_Frustum[i][D] &gt; 0)  continue;
  
          // If we get here, it isn't in the frustum
          return false;
      }
  
      // Return a true for the box being inside of the frustum
      return true;
  }
  </code></pre>