Closed-curve point offset

Hi, I was wondering if anyone would know how to offset a closed curve where I know the x and y coordinates of the points (black curve) and also know the distance at which I want to create the blue curve and the distance at which I want the red curve.

In short, what I want to get are the coordinates of the red and blue curve points.

I tried with Offset Curve but I couldn't get it to work properly.

Answer 1

[KSSV]

Try something like this. S is a Affine transformation matrix for scaling.

% circle of radius 1 for demo 
th = linspace(0,2*pi) ; 
x = cos(th) ; 
y = sin(th) ; 
o = ones(size(x)) ; 
C = [x ; y ;o]' ; 
% up scaling by 1.5 
cx = 1.5 ; cy = 1.5 ; 
S = [cx 0 0 ; 0 cy 0 ; 0 0 1] ; 
C0 = C*S ; 
x0 = C0(:,1) ; y0 = C0(:,2) ; 
% downscaling by 0.5
cx = 0.5 ; cy = 0.5 ; 
S = [cx 0 0 ; 0 cy 0 ; 0 0 1] ; 
C1 = C*S ;
x1 = C1(:,1) ; y1 = C1(:,2) ; 

figure
hold on
plot(x,y,'k')
plot(x0,y0,'b')
plot(x1,y1,'r')

[Alejandro Fernández]

I tried it with my data but it's not focused on 0.0 and it doesn't work either, can you think of anything else?

I don't know the coordinates of the center since it's not a circle.

[Alejandro Fernández]

These are my points (I narrowed it down a bit so as not to have to pass you the total)

20071345	18508878
22889302	22151369
29597266	17896695
37285389	20069946
37867359	6969222
25615370	10795368
17865987	6357039
13822830	16733547
21878513	13550193
18876776	17070247
20071345	18508878

[KSSV]

points = [20071345	18508878
22889302	22151369
29597266	17896695
37285389	20069946
37867359	6969222
25615370	10795368
17865987	6357039
13822830	16733547
21878513	13550193
18876776	17070247
20071345	18508878]; 

 
x = points(:,1) ;  
y = points(:,2) ; 
o = ones(size(x)) ; 
C = [x y o] ; 
center = [mean(x) mean(y) 1] ; 
C = C-center ; 

cx = 1.5 ; cy = 1.5 ; 
S = [cx 0 0 ; 0 cy 0 ; 0 0 1] ; 
C0 = C*S+center ; 
x0 = C0(:,1) ; y0 = C0(:,2) ; 

cx = 0.5 ; cy = 0.5 ; 
S = [cx 0 0 ; 0 cy 0 ; 0 0 1] ; 
C1 = C*S+[mean(x) mean(y) 1] ;
x1 = C1(:,1) ; y1 = C1(:,2) ; 

figure
hold on
plot(x,y,'k')
plot(x0,y0,'b')
plot(x1,y1,'r')