[NTG-context] placing layers

[Pablo Rodriguez]

Dear list,

I have the following sample:

    \setupexternalfigures[location=default]
    \starttext

    \definelayer[abc][hoffset=1cm, voffset=5cm]
    \setlayer[abc]{\externalfigure[cow.pdf][frame=on]}

    \definelayer[xyz][hoffset=12cm, voffset=5cm]
    \setlayer[xyz]{\externalfigure[spider.eps]}

    \setupbackgrounds[page][background={abc, xyz}]

    \null
    \stoptext

Is therea way to set the voffset value of the xyz layer to the value of
voffset of the abc layer plus the half of the height of that layer?

I mean, something like: voffset = ( abc.voffset() + (abc.height()/2) )

Many thanks for your help,

Pablo
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