# How do I scale every line by the same amount?

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Mariam Shahab
on 24 Dec 2022

Commented: Walter Roberson
on 27 Dec 2022

Hi all,

I have a code that allows the user to pick 5 vertices of a polygon. An example of what the 5 vertices may look like, I have attached an image.

This is the code:

xlim([-50 50]);

ylim([-50 50]);

hold on

% code for selecting 5 vertices

n=5;

a=zeros(n,1);

b=zeros(n,1);

for j=1:5

[x,y] = ginput(1);

h1 = text(x,y,int2str(j), ...

'HorizontalAlignment','center', ...

'Color', [1 0 0], ...

'FontSize',8);

a(j)=x;

b(j)=y;

end

%plotting a line between all 5 vertices

plot([a ; a(1)],[b ; b(1)], 'b-');

Q1= [a(1),b(1)];

Q2= [a(2),b(2)];

Q3= [a(3),b(3)];

Q4= [a(4),b(4)];

Q5= [a(5),b(5)];

%% Center of polygon

polyin = polyshape({a},{b});

[x,y] = centroid(polyin);

plot(polyin)

hold on

plot(x,y,'r*')

P=[x,y];

% extention 1

sf=2;

K1= Q1-P;

J1= P + K1*sf;

plot([P(1),Q1(1)],[P(2),Q1(2)],'bo',[P(1),J1(1)],[P(2),J1(2)],'r-');

plot(J1(1), J1(2),'r+');

% extension 2

K2= Q2-P;

J2= P + K2*sf;

plot([P(1),Q2(1)],[P(2),Q2(2)],'bo',[P(1),J2(1)],[P(2),J2(2)],'r-');

plot(J2(1), J2(2),'r+');

% extension 3

K3= Q3-P;

J3= P + K3*sf;

plot([P(1),Q3(1)],[P(2),Q3(2)],'bo',[P(1),J3(1)],[P(2),J3(2)],'r-');

plot(J3(1), J3(2),'r+');

% extension 4

K4= Q4-P;

J4= P + K4*sf;

plot([P(1),Q4(1)],[P(2),Q4(2)],'bo',[P(1),J4(1)],[P(2),J4(2)],'r-');

plot(J4(1), J4(2),'r+');

% extension 5

K5= Q5-P;

J5= P + K5*sf;

plot([P(1),Q5(1)],[P(2),Q5(2)],'bo',[P(1),J5(1)],[P(2),J5(2)],'r-');

plot(J5(1), J5(2),'r+');

The problem i am encountering is that not all the extensions (i.e. lines on the plot) are increased in length by the same size. I want all five lines to be scaled up by the same amount. I will apprecaite any advice on this.

This is an example of what my final plot looks like. As you can see, every red line is enlarged by a diffrent amount.

##### 2 Comments

Walter Roberson
on 24 Dec 2022

### Accepted Answer

Walter Roberson
on 25 Dec 2022

If the task is to draw them larger, then what you do is create a hgtransform object, and parent the drawing of the unscaled shape to the hgtransform object. Then you set the Matrix property of the hgtransform to do any scaling / translation / rotation you would like. MATLAB will process the transformation and draw according to the transformed coordinates.

Creating the proper Matrix is made much easier by using makehgtform . You can pass multiple transforms in one call. Just remember that the rightmost transform is the one that is processed first.

If you were doing a rotation, the usual sequence would be to use a transform matrix to subtract the center of rotation from the coordinates, then do the rotation, and then add back the center of rotation. That would be like

makehgtform('translate', xc, yc, zc, 'xrotate', RADIANS, 'translate', -xc, -yc, -zc)

Remember processing is right-most first so that is first subtract off centers, then rotate by RANDIANS, then add back centers (leftmost)

In your case it might be more like

makehgtform('translate', xc, yc, zc, 'scale', SCALE, 'translate', -xc, -yc, -zc)

to scale distances relative to (xc, yc, zc)

##### 3 Comments

Walter Roberson
on 27 Dec 2022

Walter Roberson
on 27 Dec 2022

In particular plot() and plot3() both create line() objects, which are 3d objects.

### More Answers (1)

Sulaymon Eshkabilov
on 24 Dec 2022

The scaling up what you are creating can be done at the start of your code by introducing a scaling factor:

clearvars; close all

SF = 1.5; % Scaling Factor

xlim([-50*SF 50*SF]);

ylim([-50*SF 50*SF]);

hold on

% code for selecting 5 vertices

n=5;

a=zeros(n,1);

b=zeros(n,1);

for j=1:5

[x,y] = ginput(1);

x=x*SF; y=y*SF;

h1 = text(x,y,int2str(j), ...

'HorizontalAlignment','center', ...

'Color', [1 0 0], ...

'FontSize',8);

a(j)=x;

b(j)=y;

end

%plotting a line between all 5 vertices

plot([a ; a(1)],[b ; b(1)], 'b-');

...

##### 0 Comments

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