Here is a trick (due to Hans, of course) to produce a watermark image. You can play with the idea in order to produce your own effect.
\definecolor [percent] [s=1,a=1,t=.9] \externalfigure [cow] [background={foreground,color},backgroundcolor=percent]
Alan
On Wed, 18 May 2016 11:55:11 +0000 "Meer, Hans van der" H.vanderMeer@uva.nl wrote:
This code from three years back ia a reply to a question of placing an overlay background. Because the cow is very black I am curious if the \externalfigure can have a paramter to change the alpha of the cow, making it more transparent. By the way, it is not out of curiosity only. If possible it would forego the need to make separate pictures of different transparency beforehand.
On 15 Jul 2013, at 11:22, Hans Hagen pragma@wxs.nl wrote:
\definetextbackground [mybackgroundOverlay] [mp=mpos:region:whatever]
\startuseMPgraphic{mpos:region:whatever} for i=1 upto nofmultipars : draw textext("\externalfigure[cow.pdf]") rotated 10 shifted center multipars[i] ; endfor ; \stopuseMPgraphic
\starttext
\startmybackgroundOverlay \dorecurse{10}{\input knuth \par} \stopmybackgroundOverlay
\stoptext
Hans van der Meer