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MathGroup Archive 1998

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Graphic bug in Mathematica

  • To: mathgroup at smc.vnet.net
  • Subject: [mg13355] Graphic bug in Mathematica
  • From: Andrzej Kozlowski <andrzej at tuins.ac.jp>
  • Date: Mon, 20 Jul 1998 02:50:10 -0400
  • Sender: owner-wri-mathgroup at wolfram.com

This message concerns a bug in Mathematica involving the graphic
directive Dashing[] and the primitive Point[]. This is a genuine bug,
acknowledged as such by wri's Technical Support. I learned about it
from Gianluca Gorni (<gorni at dimi.uniud.it>) who informed Technical
Support. However, since we found their reply and the remedy proposed
not entirely satisfactory we thought it would be a good idea to make it
more widely known, for reasons that will become apparent later in this
message.

First of all I would like to point out that the bug involves only screen
display: printing to a PostScript printer or exporting a graphic as an
EPS is not affected. The bug has been confirmed on Macintosh and
Windows platforms and as it involves the kernel rather than the Front
End it is almost certainly cross platform.

The bug is in fact two bugs. The first one is common to both Mathematica
2 and 3. The second one affects only Mathematica 3. The second one
seems to me to be the more serious.

The bug (bugs) can be summarized like this: Dashing[] before a Point[]
will kill the point when using Show or Display["stdout",graphic]. Here
is a simple example:

g1=Graphics[{Dashing[{0.01}],
Line[{{-1,1},{2,1}}],PointSize[0.1],Point[{0,0}]},PlotRange->All];Show[g1]

If you evaluate it in either Mathematica 2 or 3 you will see that the
point is not drawn. Putting the Point before Dashing makes the point
display properly.

In Mathematica 2 there is a better way to solve this problem. Just
isolate Dashing and Line by list brackets:

g2=Graphics[{{Dashing[{0.01}],
Line[{{-1,1},{2,1}}]},PointSize[0.1],Point[{0,0}]},PlotRange->All];Show[g2]

This works fine in Mathematica 2 but not in Mathematica 3. This is the
second bug I had in mind, and the more serious one. Why is it more
serious? Well, the only thing Technical Support could suggest is to be
careful about the order in which one enters primitives and directives.
That's perhaps O.K. when you are only using graphics entirely
constructed by yourself, but what about using graphic packages created
by others? Consider the most extreme case. Suppose you want to display
together two graphics, say graphic1 and graphic2 and suppose both of
them contain both Dashing and Point. Suppose that they come from two
different graphic packages.

graphic1=Graphics[{{PointSize[0.1],Point[{0,0}]},{Dashing[{0.01}],
 Line[{{-1,-1},{-1,2}}]}}]

graphic2=Graphics[{{PointSize[0.2],Point[{0.5,0.5}]},{Dashing[{0.1}],
 Line[{{-1,1},{2,-1}}]}}]

Note that they both have been written using "correct" ordering.
Nevertheless you can never display them correctly together. Neither
Show[{graphic1,graphic2}] nor Show[{graphic2,graphic1}] will produce
the correct picture. You would have to look at the code of the two
graphics and re-write it yourself.  Indeed, whenever you are displaying
together any graphics coming from different packages you potentially
face this problem.

I have found a very simple (I hope temporary, until the next release of
Mathematica) remedy. All you need to do is to redefine Point, e.g.:

NewPoint[{x_,y_}]:={Dashing[{}],Line[{{x,y},{x,y}}],Point[{x,y}]}

Replacing Point by NewPoint in the graphic1 and graphic2 solves the
above problem. I think this point is worth remembering for any one
writing a graphic package which makes use of the Point primitive.

Andrzej Kozlowski



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