overthinking overprinting

overthinking overprinting

[Henning Hraban Ramm via ntg-context]

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A question on TeX-D-L reminded me to look into this subject.

At the moment, there is \setupcolors[intent=overprint|knockout] with 
\startoverprint|knockout, as defined in colo-ext.mkxl|mkiv.

I’m not sure what that does as a general setting – usually you need 
overprinting for black text and lines in CMYK documents.

There are special cases where you want other (spot) colors to overprint, 
but that’s quite rare, and I can use transparency features to achieve 
the same (a=multiply) in an even more flexible way – as long as I don’t 
need a format like PDF/X-3 that doesn’t allow transparencies.
If I declare such a format, ConTeXt just switches off transparencies and 
doesn’t try to emulate (“flatten”) them as e.g. InDesign does.

So it would be nice if I could e.g.
\definecolor[OPred][m=1,y=1,intent=overprint]

OTOH there are cases where I want a black area or text to knockout the 
background, these might work with \startknockout etc.


When I try my example:

\useMPlibrary[dum]
\setupcolors[
    rgb=no,cmyk=yes,
    intent=overprint,% or knockout
]
\setupbodyfont[modern,ss,18pt]

\starttext

\definecollector[Test]
\setcollector [Test] {\externalfigure[none][width=8cm,height=6cm,frame=on]}
\setcollector [Test] [corner=middle,location=middle] {{\bf Toast}}
\setcollector [Test]
                [corner={left,top},location={right,bottom}]
                {\framed{LT}}
\setcollector [Test]
                [corner={left,bottom},location={right,top}]
                {\framed{LB}}

\composedcollector{Test}

\stoptext

... then the text is “knocked out” regardless how set the intent.
It also doesn’t help to use \startoverprint for the Text.
The attached screenshot shows the color separation preview from Acrobat 
Pro 9.

Is this broken or intended to be used differently?

Hraban

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