overlay at given position: help needed

["Peter Münster" <pmlists@free.fr>]

Hello,

Below is a test file, to show a bit what I'm looking for: a
background-image behind a text-area at a given position relative to the
center of this area. I'm not yet satisfied with the result.

Here my questions:

1.) Is \startframedtext a good choice for placing the background, or is
    there something better?
2.) Is \framed[loffset=..., toffset=...]{} a good choice for specifying the
    position, or is there something better?
3.) When "left=1", why is the image completely outside the frame and not
    only 50%?
4.) How do I make a step between text and background according to
    http://wiki.contextgarden.net/Presentation_effects ?
    I would like to show first the text, and after a mouse-click the
	background image.

Thanks in advance for any help!
Cheers, Peter


The test-file:


% Begins a text-area with a background-image:
\def\startBGimg[#1]{%
  \getparameters[BGimg][top=0, left=0, width=2cm, #1]%
  \start
    \newdimen\TopOffset \newdimen\LeftOffset
    \setupframedtexts[before=, after=]
    \defineoverlay[myBG][{%
        \TopOffset=\overlayheight      \LeftOffset=\overlaywidth
        \TopOffset=\BGimgtop\TopOffset \LeftOffset=\BGimgleft\LeftOffset
        \framed[frame=on, width=\overlaywidth, height=\overlayheight,
          offset=overlay, toffset=\TopOffset, loffset=\LeftOffset]{%
          \externalfigure[\BGimgimg][width=\BGimgwidth]}}]
    \startframedtext[background=myBG, frame=off, offset=overlay,
      width=\textwidth]}

% Stops the text-area with a background-image:
\def\stopBGimg{\stopframedtext\stop}

\starttext
\startBGimg[img=hacker.jpg, top=-1, left=-1]
\input tufte
\stopBGimg

\startBGimg[img=hacker.jpg, top=-1, left=1]
\input tufte
\stopBGimg

\startBGimg[img=hacker.jpg, top=1, left=-1]
\input tufte
\stopBGimg

\startBGimg[img=hacker.jpg, top=1, left=1]
\input tufte
\stopBGimg
\stoptext

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