On Tue, Dec 17, 2019 at 07:30:49PM +0100, Hans Hagen wrote:

> you need to make a MWE to show what you mean
> 
> normally
> 
> \hbox to <somewidth>{\hss<come content>\hss}
> 
> is good enough for centering

Here is my MWE. Think of a photo album. To make it interesting the pages should
all be different. To this purpose layers are a good means. I'd like to centre
the layers on their page without doing so manually and without changig the distances
between them. Using luacode one can calculate the distance d between the leftmost
and the rightmost edge. Half the difference (\textwidth - d) is the amount to move
the whole layer ensemble.

My question is now: Does in context exist a possibility to move all layers jointly
without correcting all their x-values subsequently?
I append the pdf-file of my MWE.

Rudolf

---------------------------------------------------------- MWE ---------------------

\setuppapersize[A4,landscape]
\setupexternalfigures[location={default}]

\starttext

\definelayer[LAY]%

\setlayer[LAY][x=0pt,y=-8pt]%
   {%
   \vbox
	{%
	\hsize=.35\textwidth
	\externalfigure[cow][width=\hsize]%

        \vskip\lineheight

	\input tufte
	}
   }

\setlayer[LAY][x=210pt,y=0pt]%
   {%
   \definedfont[Serif at 82pt]
   \rotate[rotation=90,frame=on,offset=2pt]{\hbox{JOURNEY}}
   }

\setlayer[LAY][x=360pt,y=50pt]%
   {%
   \vbox
	{%
        \hsize=200pt
        \input ward
        \vskip\lineheight
	\externalfigure[hacker][height=.4\textheight]%
	}
   }

\flushlayer[LAY]
\page

\definelayer[LAY]%

\setlayer[LAY][x=0pt,y=0pt]%
   {\externalfigure[mill][height=.4\textheight]}

\setlayer[LAY][x=150pt,y=0pt]%
   {\externalfigure[hacker][height=.4\textheight]}

\setlayer[LAY][x=0pt,y=200pt]%
   {\externalfigure[cow][width=.5\textwidth]}

\setlayer[LAY][x=\dimexpr.65\textwidth+40pt\relax,y=0pt]%
   {%
   \vbox
	{%
        \hsize=.2\textwidth
        \startalignment[flushleft]
        \input ward
        \vskip\lineheight
        \input ward
        \stopalignment
	}
   }

\flushlayer[LAY]

\stoptext

------------------------------------------------------------------------------------