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MathGroup Archive 1996

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Re: A Bug in Parametric Plot ?

  • To: mathgroup at smc.vnet.net
  • Subject: [mg5376] Re: [mg5324] A Bug in Parametric Plot ?
  • From: Andrei Constantinescu <constant at athena.polytechnique.fr>
  • Date: Thu, 5 Dec 1996 14:50:18 -0500
  • Sender: owner-wri-mathgroup at wolfram.com

 I might say, there is no bug in ParametricPlot !

 Surely, :-) , you might say there is a bug, because you
 get the nice figures as you say, and not the circle as we might
 expect.

 But lets look a little deeper how ParametricPlot functions:

   - you give {x[t], y[t]} and {t, a, b}

   - Mma samples the interval {t, a , b } and computes for each t_i 

       {x[t_i], y[t_i]}

   - then the whole list comes in a Line object:

       Line[ list_of_points={....{x[t_i], y[t_i]} , .....} ]

   - and draws the line with Show & Graphics commands.

 What happens when you choose a large interval ?

   - the time samples are at large distances (the t_i) , and the 
   geometric points (the {x[t_i], y[t_i]}) are also espaced !
   So that a line between two of them is more a diameter than a little
   cord. 

 This gives the nice plot. So for special K , and special number of sampling points
 you can get just a triangle !


 In order to change this, try to change the PlotPoints options in ParametricPlot !

In[4]:= ParametricPlot[{Cos[t], Sin[t]} , {t, -1280 Pi, 1280 Pi}, 
        PlotPoints -> 100 ]

Out[4]= -Graphics-

In[5]:= Warning: Cannot convert string "9x15B" to type FontStruct
 ParametricPlot[{Cos[t], Sin[t]} , {t, -1280 Pi, 1280 Pi}, 
        PlotPoints -> 300 ]

Out[5]= -Graphics-


   a + andrei



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