How to find the center to contour angle in matlab

Hi, I am completely new to matlab.

Please guide me how can I find center to contour angle (in degree) for each contour pixel?

Suppose I have an input image like bell-12.gif. Now, I know the centeroid of it. Say suppose[110,99]. I also know the boundary pixel array of bell-12.gif. Now I want to calculate the angle between centroid to each boundary pixel and store it into an array Angle. How can I find the Angle array?

I tried the following code by fixing one angle:

for i=1:lob-1

x = [cents1(1) cents1(2)]; % common point

y = [boundary(1,1),boundary(1,2)];

a = [cents1(1) cents1(2)]; % common point

b = [boundary(i,1) boundary(i,2)];

CCDANG(i)=(atan((y(2)-x(2))/(y(1)-x(1))) - atan((b(2)-a(2))/(b(1)-a(1)))) * 180/pi;

This gives me negative value for angle. I am not sure what exactly is the way to solve it.

[darova]

Can you attach something?

[Atreyee Mondal]

@darova I modified my question and attached the relevant documents.

[darova]

Where is the angle?

[Atreyee Mondal]

kindly consider this diagram .

Answer 1

[darova]

• Use edge to detect contour
• find (x,y) coordinates (remember x - columns, y - rows)
• coordinates need to be sorted (hint: divide them into 2 groups - x < x0 and x >= x0 then sort by y)
• use interparc to interpolate data if needed
• calculate angles

[Atreyee Mondal]

Thank you. I tried to find the angle by fixing a boundary point but is gives negative angular value . Here I am attaching my code:

for i=1:lob-1

x = [cents1(1) cents1(2)]; % common point

y = [boundary(1,1),boundary(1,2)];

a = [cents1(1) cents1(2)]; % common point

b = [boundary(i,1) boundary(i,2)];

CCDANG(i)=(atan((y(2)-x(2))/(y(1)-x(1))) - atan((b(2)-a(2))/(b(1)-a(1)))) * 180/pi;

[darova]

I thought you want angle between tangents

Can't you just change the sign? Is the value correct? Only sign is wrong?