p>: alpha</dd>
     <dd><samp>o</samp>: opacity</dd>

     <dd><samp>c</samp>: cyan value of CMYK color of pixel</dd>
     <dd><samp>y</samp>: yellow</dd>
     <dd><samp>m</samp>: magenta</dd>
     <dd><samp>k</samp>: black</dd>

     <dd><samp>all</samp>: all channels</dd>
     <dd><samp>this</samp>: this channel</dd>

     <dd><samp>intensity</samp>: pixel intensity</dd>

     <dd><samp>hue</samp>: pixel hue</dd>
     <dd><samp>saturation</samp>: pixel saturation</dd>
     <dd><samp>lightness</samp>: pixe