an class="lineno">   83</span>&#160;<span class="keyword">template</span> &lt;<span class="keyword">typename</span> GrayChannel, <span class="keyword">typename</span> RedChannel, <span class="keyword">typename</span> GreenChannel, <span class="keyword">typename</span> BlueChannel&gt;</div><div class="line"><a name="l00084"></a><span class="lineno">   84</span>&#160;<span class="keyword">typename</span> channel_traits&lt;GrayChannel&gt;::value_type rgb_to_luminance(<span class="keyword">const</span> RedChann