Alternating backgrounds and repeating layers

I was in need of adding a watermark to the background of a book. Left and right pages needed a different watermark. I also needed the ability to position the watermark anywhere on the page. Through a nice discussion on mailing list Hans was able to make clear how to go about this.

In my final usage of the code below, I changed the preset to bottomleft/bottomright and added hoffset/voffset values to position the text where I wanted it. The solution below would also allow for placing images in the background instead of text.

\setuppagenumbering
 [alternative=doublesided]

\setupcolors
 [state=start]

\setupbackgrounds
 [leftpage]
 [background=Watermark-Left]

\setupbackgrounds
 [rightpage]
 [background=Watermark-Right]

\definelayer
 [Watermark-Left]
 [repeat=yes,
  width=\paperwidth,
  height=\paperheight]

\definelayer
 [Watermark-Right]
 [repeat=yes,
  width=\paperwidth,
  height=\paperheight]

\setlayerframed
 [Watermark-Left]
 [preset=middle]
 [foregroundcolor=gray]
 {\scale[sx=12,sy=12]{\rotate[rotation=20.0]{left page watermark}}}

\setlayerframed
 [Watermark-Right]
 [preset=middle]
 [foregroundcolor=gray]
 {\scale[sx=12,sy=12]{\rotate[rotation=-20.0]{right page watermark}}}

\starttext
   \chapter{first}  \dorecurse{30}{\input tufte  }
\stoptext

For a somewhat different example, see Draft Watermark