    The <code class="varname">gl_ClipDistance</code> variable provides a forward compatible
            mechanism for controlling user clipping. The element <em class="parameter"><code>gl_ClipDistance</code></em>[<span class="emphasis"><em>i</em></span>]
            specifies a clip distance for each user clip plane <span class="emphasis"><em>i</em></span>. A distance of 0.0 means that
            the vertex is on the plane, a positive distance means that the vertex is insider the clip plane, and a negative