Extract planes from projection matrix

ExtractPlanes.cpp

struct Matrix4x4
{
 // The elements of the 4x4 matrix are stored in
 // column-major order (see "OpenGL Programming Guide",
 // 3rd edition, pp 106, glLoadMatrix).
 float _11, _21, _31, _41;
 float _12, _22, _32, _42;
 float _13, _23, _33, _43;
 float _14, _24, _34, _44;
};
void ExtractPlanesGL(
 Plane * p_planes,
 const Matrix4x4 & comboMatrix,
 bool normalize)
{
 // Left clipping plane
 p_planes[0].a = comboMatrix._41 + comboMatrix._11;
 p_planes[0].b = comboMatrix._42 + comboMatrix._12;
 p_planes[0].c = comboMatrix._43 + comboMatrix._13;
 p_planes[0].d = comboMatrix._44 + comboMatrix._14;
 // Right clipping plane
 p_planes[1].a = comboMatrix._41 - comboMatrix._11;
 p_planes[1].b = comboMatrix._42 - comboMatrix._12;
 p_planes[1].c = comboMatrix._43 - comboMatrix._13;
 p_planes[1].d = comboMatrix._44 - comboMatrix._14;
 // Top clipping plane
 p_planes[2].a = comboMatrix._41 - comboMatrix._21;
 p_planes[2].b = comboMatrix._42 - comboMatrix._22;
 p_planes[2].c = comboMatrix._43 - comboMatrix._23;
 p_planes[2].d = comboMatrix._44 - comboMatrix._24;
 // Bottom clipping plane
 p_planes[3].a = comboMatrix._41 + comboMatrix._21;
 p_planes[3].b = comboMatrix._42 + comboMatrix._22;
 p_planes[3].c = comboMatrix._43 + comboMatrix._23;
 p_planes[3].d = comboMatrix._44 + comboMatrix._24;
 // Near clipping plane
 p_planes[4].a = comboMatrix._41 + comboMatrix._31;
 p_planes[4].b = comboMatrix._42 + comboMatrix._32;
 p_planes[4].c = comboMatrix._43 + comboMatrix._33;
 p_planes[4].d = comboMatrix._44 + comboMatrix._34;
 // Far clipping plane
 p_planes[5].a = comboMatrix._41 - comboMatrix._31;
 p_planes[5].b = comboMatrix._42 - comboMatrix._32;
 p_planes[5].c = comboMatrix._43 - comboMatrix._33;
 p_planes[5].d = comboMatrix._44 - comboMatrix._34;
 // Normalize the plane equations, if requested
 if (normalize == true)
 {
 NormalizePlane(p_planes[0]);
 NormalizePlane(p_planes[1]);
 NormalizePlane(p_planes[2]);
 NormalizePlane(p_planes[3]);
 NormalizePlane(p_planes[4]);
 NormalizePlane(p_planes[5]);
 }
}