Image Placement

If you’re using ConTeXt for complex layout tasks, you need to calculate with image sizes. Often it makes sense to use Lua functions.

Beware, TeX calculates internally in "scaled points" (sp), 1 sp = 1/65536 pt.

Measures in pure ConTeXt

You can get at a picture’s dimensions with one line of TeX code:

\setupexternalfigures[location=default] % necessary to find pictures in tree
\getfiguredimensions[hacker.jpg]

This defines a bunch of variables:

Measures in Lua

There is a way to take them in Lua:

\setupexternalfigures[location=default]
\setupbodyfont[sans, 20pt]
\starttext
\startTEXpage[offset=1em]
\startluacode
  local f = figures.getinfo("cow.pdf")
  context(f.used.width)
  context.par()
  context(f.used.height)
  context.getfiguredimensions({ "cow.pdf" })
  context.par()
  context(context.noffigurepages)
  context.par()
  context(context.figurewidth)
  context.par()
  context(context.figureheight)
\stopluacode
\stopTEXpage
\stoptext

The figures.getinfo gives just numbers and it has less options. context.getfiguredimensions gives the same outputs than it TeX counterpart.

Calculations in Lua

TEXpt = 65536 -- sp per pt
TEXptpi = 72.27 -- pt per inch

function sp2mm(n)
	-- convert sp into mm
	return math.floor(n * 35.28 / TEXpt)/100
end

function glue2num(glue)
	-- convert TeX glue (e.g. skips) into dimensions(? or numbers)
	return glue.width + (glue.stretch * glue.stretch_order) - (glue.shrink * glue.shrink_order)
end

function ImgSize(resolution)
	local filename = figures.current().status.fullname -- current image
	local pic = img.scan{filename = filename}
	local picH = pic.ysize * TEXptpi * TEXpt / resolution  -- picture height in sp
	local picW = pic.xsize * TEXptpi * TEXpt / resolution  -- picture width in sp
	context("height=" .. picH .. ",width=" .. picW)
end

Postponing

If you want to place full page images or similar content on distinct pages, try the postponing mechanism:

\definemakeup[fullpage][
  page=no,
  doublesided=no,
  % headerstate=empty,
  % footerstate=empty,
  pagestate=start,
]

\startpostponing[pagenumber]
\startfullpagemakeup
% use \setlayer & friends
\stopfullpagemakeup
\stoppostponing

The pagenumber argument can be absolute or relative, e.g. 23 or +2. Values like +0 or -1 might fit your usecase.

((WORK IN PROGRESS))