el&amp; red, <span class="keyword">const</span> GreenChannel&amp; green, <span class="keyword">const</span> BlueChannel&amp; blue) {</div><div class="line"><a name="l00085"></a><span class="lineno">   85</span>&#160;    <span class="keywordflow">return</span> rgb_to_luminance_fn&lt;RedChannel,GreenChannel,BlueChannel,</div><div class="line"><a name="l00086"></a><span class="lineno">   86</span>&#160;                               <span class="keyword">typename</span> channel_traits&lt;GrayChannel&gt;::value