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How to draw normal line at given points ?

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Hello everyone,
I have a red curve that fit 4 bleu points. I want to draw normal at these points.
Here attached my curve.
and the x, y coordinates of the bleu points are :
x=[142;127;181;234];
y=[251;251;261;255];
So, please, how to get the normal line ?
I will be very grateful if anyone could help me.
  2 Commenti
Ameer Hamza
Ameer Hamza il 27 Mar 2020
How did you create this curved line fitting through these points?
MMSAAH
MMSAAH il 30 Mar 2020
Hello Ameer,
Here is my code:
x=[142;127;181;234]
y=[251;251;261;255]
p = polyfit(x, y, 1);
v = polyval(p, x);
xint = linspace(127,300,50)';
spl = spline(x,y);
figure
t=plot(y,x,'.',ppval(spl,xint),xint,'r-')
xlim([200 300])
ylim([50 234])

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Ameer Hamza
Ameer Hamza il 31 Mar 2020
Try this:
x=[142;127;181;234];
y=[251;251;261;255];
[x, idx] = sort(x);
y = y(idx);
points = [x y];
xint = linspace(127,234,50)';
spl = spline(x,y);
tangent_vector = zeros(4,2);
for i=1:4
if i==4 % last polynomial need to evalauted at endpoint
deri_coef = polyder(spl.coefs(i-1,:));
tangent_vector(i, :) = [1 polyval(deri_coef, x(end)-x(end-1))];
else
deri_coef = polyder(spl.coefs(i,:));
tangent_vector(i, :) = [1 polyval(deri_coef, 0)];
end
end
normal_vec = [-tangent_vector(:,2) tangent_vector(:,1)];
start_points = points;
end_points = 10*normal_vec + points;
fig = figure;
ax = axes();
hold(ax);
t = plot(y,x,'.',ppval(spl,xint),xint,'r-');
for i=1:4
p1 = start_points(i,:);
p2 = end_points(i,:);
plot([p1(2) p2(2)], [p1(1) p2(1)], 'r', 'LineWidth', 2);
end
daspect([1 1 1]);
xlim([180 340])
ylim([100 250])
  2 Commenti
MMSAAH
MMSAAH il 1 Apr 2020
Thank you very much for the answer.It worked perfectly.
However, could you explain me more the for loop ? and why did you evaluated the polynome at endpoint ?
Thank you though!
Ameer Hamza
Ameer Hamza il 1 Apr 2020
For loop is just calculating the tangent vector at each point, since we can calculate the normal vectors from tangent vectors. The spline function output 3 polynomials for 4 data points. So to compute tangent at 4 data points, we need to do it like this
datapoint # 1 -> polynomial # 1 (start point)
datapoint # 2 -> polynomial # 2 (start point) or polynomial # 1 (end point)
datapoint # 3 -> polynomial # 3 (start point) or polynomial # 2 (end point)
datapoint # 4 -> polynomial # 3 (end point)
Which shows where I evaluated the derivative of each polynomial

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Più risposte (1)

michael scheinfeild
michael scheinfeild il 28 Ago 2021
https://michaelsheinfeild.medium.com/unit-normal-vector-to-curve-d63ef0124acd

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