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.