class="lineno">   63</span>&#160;<span class="comment">// The default implementation of to_luminance uses float0..1 as the intermediate channel type</span></div><div class="line"><a name="l00064"></a><span class="lineno">   64</span>&#160;<span class="keyword">template</span> &lt;<span class="keyword">typename</span> RedChannel, <span class="keyword">typename</span> GreenChannel, <span class="keyword">typename</span> BlueChannel, <span class="keyword">typename</span> GrayChannelValue&gt;</div><div class="li