images in list</dd>

     <dd><samp>i</samp>: column offset</dd>
     <dd><samp>j</samp>: row offset</dd>
     <dd><samp>p</samp>: pixel to use (absolute or relative to current pixel)</dd>

     <dd><samp>w</samp>: width of this image</dd>
     <dd><samp>h</samp>: height of this image</dd>
     <dd><samp>z</samp>: channel depth</dd>

     <dd><samp>r</samp>: red value (from RGBA), of a specific or current pixel</dd>
     <dd><samp>g</samp>: green</dd>
     <dd><samp>b</samp>: blue</dd>
     <dd><samp>a</sam