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MathGroup Archive 1999

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Re: Clippling of polygons within a defined region

  • To: mathgroup at smc.vnet.net
  • Subject: [mg18991] Re: Clippling of polygons within a defined region
  • From: "Allan Hayes" <hay at haystack.demon.co.uk>
  • Date: Tue, 3 Aug 1999 13:44:36 -0400
  • References: <7nrf6u$ie2@smc.vnet.net>
  • Sender: owner-wri-mathgroup at wolfram.com

John,

The same problem arises with big points, thick lines and with text.
Here is an first attempt at dealing with it.

Make an example for treatment

gr = Show[Graphics[{Polygon[{{-1, 0}, {2, -2}, {2, 1}}],
        PointSize[.5], Point[{1, 1}]}],
    Frame -> True, FrameTicks -> True,
    PlotRange -> {{0, 1}, {0, 1}}
    ]

Defint the masking function

Masked[gr_Graphics, st___] :=
  Module[{fat, pr, mask, e, x1, x2, y1, y2},
    (*get the explicit graphics for frame, and frameticks*)
    fat = Delete[FullGraphics[gr], {1, 1}];
   (*get the explicit plotrange for gt*)
   {{x1, x2}, {y1, y2}} = pr =  PlotRange[gr];
    (*calculate a suitable mask with style st*)
    e = Max[x2 - x1, y2 - y1];
    mask =
      Graphics[{st,
          Polygon[{{x1, y1}, {x2, y1}, {x2, y2}, {x1, y2}, {x1, y1},
              {x1 - e, y1 - e}, {x1 - e, y2 + e}, {x2 + e, y2 + e}, {x2 + e,
                y1 - e}, {x1 - e, y1 - e}}]}];
    (*show the graphics in the right order*)
    Show[gr, mask, fat, PlotRange -> pr]]

One undesirable feature of this is that we are relying the hidden frame and
ticks in gr to make room for the versions in fat.

Masked[gr, Hue[.2, .2, 1]]

Allan
---------------------
Allan Hayes
Mathematica Training and Consulting
Leicester UK
www.haystack.demon.co.uk
hay at haystack.demon.co.uk
Voice: +44 (0)116 271 4198
Fax: +44 (0)870 164 0565


John Tanner <john at janacek.demon.co.uk> wrote in message
news:7nrf6u$ie2 at smc.vnet.net...
> With most Graphics functions, "PlotRange" clips plots to within defined
> (rectangular) areas, and when the option Frame is used, the clipping
> occurs at the frame boundary.
>
> Unfortunately, while this clipping applies correctly to Line, Point
> etc., Polygons are not clipped at all in the presence of the Frame
> option: these appear within the whole visible area (obscuring the axes
> etc.) and not just within the PlotRange.
> It is possible to force the clipping by brute force by adding an extra
> Polygon masking all but the wanted region, with colour of White: however
> this now also masks the axes of the Frame!
>
> (1) Is there a better way of clipping Polygons, since PlotRange is
> ineffective?
>
> (2) Is there a way of forcing the Frame and Axes to be plotted last
> instead of first, thus plotting on top of a White Polygon?
> --
>   from -   John Tanner                 home -  john at janacek.demon.co.uk
>   mantra - curse Microsoft, curse...   work -  john.tanner at gecm.com
> I hate this 'orrible computer,  I really ought to sell it:
> It never does what I want,      but only what I tell it.
>
>



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