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From: Kenneth Galea on 3 Apr 2010 15:17
Hi everyone. I'm trying to plot a line perpendicular and in the middle of another line. I'm already half way through but I'm stuck when trying to plot from the middle:
check this link:
http://dl.dropbox.com/u/5800838/untitled.jpg

I did the following code:

x1 =GR_endpoints(plotnext,2) %start point of line
y1 =GR_endpoints(plotnext,1)
x2 =GR_endpoints(plotnext+1,2) %end point of line
y2 =GR_endpoints(plotnext+1,1)

P1 = [x1 y1]
P2 = [x2 y2]
d = 75

t = sqrt((x2-x1)^2+(y2-y1)^2);
x3 = x1-(y2-y1)/t*d
y3 = y1+(x2-x1)/t*d
line([x3 x1], [y3 y1] , 'Color','b', 'LineStyle','-', 'LineWidth', 0.5)

N.B. I did not use gradient since I saw one example in this forum but this can lead to a division by 0. Please help

Regards
Kenneth
From: Roger Stafford on 3 Apr 2010 17:44
"Kenneth Galea" <k.galea(a)hotmail.com> wrote in message <hp8483$igb$1(a)fred.mathworks.com>...
> Hi everyone. I'm trying to plot a line perpendicular and in the middle of another line. I'm already half way through but I'm stuck when trying to plot from the middle:
> ........
> t = sqrt((x2-x1)^2+(y2-y1)^2);
> x3 = x1-(y2-y1)/t*d
> y3 = y1+(x2-x1)/t*d
> line([x3 x1], [y3 y1] , 'Color','b', 'LineStyle','-', 'LineWidth', 0.5)
> .......

You are using the wrong base point. For the midpoint use this:

t = sqrt((x2-x1)^2+(y2-y1)^2);
x0 = (x1+x2)/2;
y0 = (y1+y2)/2;
x3 = x0-(y2-y1)/t*d;
y3 = y0+(x2-x1)/t*d;
line([x3 x0], [y3 y0] , .....

Roger Stafford
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