center_integer</code>. By
            default, <code class="varname">gl_FragCoord</code> assumes a lower-left origin for window coordinates
            and assumes pixel centers are located at half-pixel centers. For example, the (x, y) location
            (0.5, 0.5) is returned for the lower-left-most pixel in a window. The origin of
            <code class="varname">gl_FragCoord</code> may be changed by redeclaring <code class="varname">gl_FragCoord</code>
            with the <code class="function">orig