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MathGroup Archive 2000

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Display Bug

  • To: mathgroup at smc.vnet.net
  • Subject: [mg22383] Display Bug
  • From: me at talmanl1.mscd.edu
  • Date: Sat, 26 Feb 2000 22:05:12 -0500 (EST)
  • Sender: owner-wri-mathgroup at wolfram.com

Readers of this list may be interested in the fact that Mathematica's v3/4
Display operator is buggy.  When the operator is used to write a graphics
object to non-vector graphics formats (GIF in particular) it does not
treat the Thickness directive correctly.  This can be seen by first
using SetDirectory to define a convenient file path and then executing

  Show[Graphics[{Thickness[0.03], Map[Line, Table[{{0,0},{Cos[t],Sin[t]}},
     {t, 0, 15 Pi/8, Pi/8}]]}], AspectRatio->Automatic,
     DisplayFunction -> ((Display["MyFile.GIF", #, "GIF"]; #)&)]
     
  Import["Myfile.GIF"]

  Show[%]

The appearance of the resulting "spoked wheel" seems to suffer from a
peculiar kind of astigmatism.

That the defect is in the Display operator is evident from two circumstances:

  1.)  The same distortion is evident in the GIF if we view it using some
       piece of software other than Mathematica
       
  2.)  Version 4.0's Export operator does not exhibit this behavior.     

When I informed WRI of this anomaly, their response was that, indeed, 
"Export" is the preferred manner of exporting graphics.  

It should be noted that Export is considerably slower than Display, and
that Display does not seem to malfunction when applied to a graphic whose
construction does not involve the Thickness directive.

I discovered this behavior while preparing some of the QuickTime animations
that now reside at

     http://clem.mscd.edu/~talmanl/MathAnim.html
     
and which may be of interest to some of the readers of this list.     

--Lou Talman
  Department of Mathematical & Computer Sciences
  Metropolitan State College of Denver
  Campus Box 38
  PO Box 173362
  Denver CO 80217-3362
  
  http://clem.mscd.edu/~talmanl  


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