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Re: Perpendicular lines do not appear perpendicular

  • To: mathgroup at smc.vnet.net
  • Subject: [mg100310] Re: Perpendicular lines do not appear perpendicular
  • From: Szabolcs Horvát <szhorvat at gmail.com>
  • Date: Mon, 1 Jun 2009 07:04:17 -0400 (EDT)
  • References: <gvtmhv$gh8$1@smc.vnet.net>

Bill wrote:
> Perpendicular lines do not appear perpendicular
> 
> Hi:
> 
> I have taken the line equation 2*x+4*y-6==0, and rotated it 90 degrees counterclockwise about {0,0}, 
> using a 3x3 rotation matrix.
> Here's the Mathematica 6.0.1 code that I used:
> 
> rMat = {{Cos[90 Degree], - Sin[90 Degree], 0}, {Sin[90 Degree], 
> Cos[90 Degree], 0}, {0, 0, 1}}
> 
> l={2, 4, -6}
> 
> rMat.l
> 
> Output: {-4,2,-6}
> 
> Placing these numbers into the general form of the equation of the line Ax+By+c=0, I get -4*x + 2*y - 6 == 0. 
> Placing the 2 perpendicular line equations into ContourPlot:
> 
> ContourPlot[{2*x + 4*y - 6 == 0, -4*x + 2*y - 6 == 0}, {x, -4, 
>   4}, {y, -4, 4}, AspectRatio -> 1]
>   
> and plotting, I notice that on my pc screen that the lines do not look perpendicular, as does the plot frame. 
> Perhaps this is just my computer screen, a HP f2105. I can hold up a square cut piece of paper to the lines 
> on the screen, and notice the out-of-square condition. I used AspectRatio->1, but still no joy.
> 
> Question: Can someone suggest a remedy for my problem, if one exists?
> 

You need AspectRatio -> Automatic, not AspectRatio -> 1 (the latter just 
makes the figure a square), but for this particular plot this does not 
matter (because the x and y ranges are the same).  The lines are 
perpendicular on my screen.  Do check if your screen is operating in its 
native resolution.


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