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Re: Anticlip

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  Re: Anticlip jdaw1 <jdawiseman@gmail.com> - 2015-08-31 15:55 -0700
    Re: Anticlip jdaw1 <jdawiseman@gmail.com> - 2015-08-31 16:27 -0700
    Re: Anticlip Carlos <angus@quovadis.com.ar> - 2015-09-01 22:38 +0200

#2331 — Re: Anticlip

Because I still want it, please allow me to re-ask this 10¼ years later. 

The Problem

• There is a background that cannot be re-painted. Just assume that it cannot be re-painted.

• There a currentpath, easily regeneratable. Or, equivalently, there is accessible code that generates the currentpath.

• What is wanted is to do the partial stroke of this path that is ≤6pt from the path, but >5pt.

• To make matters worse, the path is typically generated by charpath, so might be complicated, and its strokepath is sure to be nastily self-intersecting.

◊ If the background were certain to be empty white (but it isn’t), this could be done with 
gsave 12 setlinewidth 0 setgray stroke grestore 10 setlinewidth 1 setgray stroke
which is how it is currently done, quite unsatisfactorily. 

◊ If there were an anticlip operator (but there isn’t), it could be done with
gsave 10 setlinewidth strokepath anticlip regeneratepath 12 setlinewidth 0 setgray stroke grestore newpath 

••• But, neither of these cases applies. Please, have you any ideas as to how to achieve this over a non-repaintable background?

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#2333

Example of the problem with white-over-background: 
http://www.jdawiseman.com/2015/20150831_anticlip_need.pdf
http://www.jdawiseman.com/2015/20150831_anticlip_need.png

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#2334

[jdaw1 <jdawiseman@gmail.com>, 2015-08-31 15:55]
> Because I still want it, please allow me to re-ask this 10¼ years
> later. 
> 
> The Problem
> 
> • There is a background that cannot be re-painted. Just assume that
> it cannot be re-painted.
> 
> • There a currentpath, easily regeneratable. Or, equivalently, there
> is accessible code that generates the currentpath.
> 
> • What is wanted is to do the partial stroke of this path that is
> ≤6pt from the path, but >5pt.
> 
> • To make matters worse, the path is typically generated by charpath,
> so might be complicated, and its strokepath is sure to be nastily
> self-intersecting.
> 
> ◊ If the background were certain to be empty white (but it isn’t),
> this could be done with gsave 12 setlinewidth 0 setgray stroke
> grestore 10 setlinewidth 1 setgray stroke which is how it is
> currently done, quite unsatisfactorily. 
> 
> ◊ If there were an anticlip operator (but there isn’t), it could be
> done with gsave 10 setlinewidth strokepath anticlip regeneratepath 12
> setlinewidth 0 setgray stroke grestore newpath 
> 
> ••• But, neither of these cases applies. Please, have you any ideas
> as to how to achieve this over a non-repaintable background?

Ghostscript supports PDF transparency. Here is an example; it requires
the background to be painted after the foreground.

HTH

%!

/W 200 def
/H 150 def

<< /PageSize [ W H ] >> setpagedevice

/draw-bg {
    4 20 H {
        10 20 W {
            2 copy  mul  W H mul  div  0.5 0.6 sethsbcolor
            1 index
            8 0 360 arc
            fill
        } for
    } for
} def

/draw-fg {
    30 30 moveto  W 30 sub  H 30 sub  lineto
    W  H 2 div   W  0   W 2 div  H 2 div  curveto
    0  H   0  H 2 div   30  30  curveto
    closepath
} def

% no transparency
gsave
    draw-bg
    draw-fg
    gsave 12 setlinewidth  0 setgray  stroke  grestore
    10 setlinewidth 1 setgray stroke
grestore

showpage

% transparency
gsave
    draw-fg
    gsave 12 setlinewidth  0 setgray  stroke  grestore
    10 setlinewidth 1 setgray stroke
    /Darken .setblendmode
    draw-bg
grestore

showpage


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