st the input channel.  Use <samp>mean.all</samp> to set the overall mean of the input channels.</p>

<h4>Apply an Expression to Select Image Channels</h4>

<p>Use the <a href="command-line-options.html#channel">-channel</a> setting to specify the output channel of the result. If no output channel is given, the result is set over all channels except the opacity channel. For example, to replace the red channel of <samp>alpha.png</samp> with the average of the green channels from the images <samp>alpha.png</sa