Error with bwtraceboundary function

Hi all,

I have a function where I have a set of x,y coordinates for points to a contour as input. I calculate its derivatives using spline interpolation. When I take ellipse or circle, i can manually calculate 8 points on contour and use this. But When I draw an arbitrary shape like this:

i need to get a point set first. It seems to me that bwtraceboundary is the function that i need to use, here is the code that i write; i check the beginning points with pixelinfo

I = imread('example3.png');
figure(1)
imshow(I)
BW = im2bw(I);
figure(2)
imshow(BW)
impixelinfo 
contour = bwtraceboundary(BW,[r c],'W',8,Inf,'counterclockwise');
figure(3)
imshow(I)
hold on;
plot(contour(:,2),contour(:,1),'g','LineWidth',3);

it gives me an empty contour.

Actually as I will always have a two color picture and one boundary there might be an easier way.

Any suggestion?

[ttopal]

I am checking the pixel values of binary image, where i have the contour it shows 0 and empty area as 1. Cant i change it to be otherwise? Then

[rows, columns] = find(BW ~= 0)

should be all I need. Am I wrong?

Answer 1

[Walter Roberson]

The [r, c] you supply needs to be a point that is on the boundary. You do not show how you set [r, c]

You can use find() to locate a pixel.

[ttopal]

Thanks Walter,

I use impixelinfo to locate a point on the contour. For example [498 263] is one of them. But i get an error:

Index exceeds matrix dimensions.
Error in graphtest2 (line 12)
plot(contour(:,2),contour(:,1),'g','LineWidth',3);

When I try your answer to another question where;

[r, c] = find(I, 1, 'first')

it draws a contour which is the frame of the picture. I noticed that in my binary image the contour has pixel value 0 and the empty areas has pixel value 1. Maybe thats the reason why i get this errors? Another thing is that Matlab suggests to use imbinarize instead of im2bw but when i do that it gives error the image should be two dimensional.

[Walter Roberson]

Yes you need to reverse your mask so that the boundary is 1 and the background is 0

[ttopal]

Thanks Walter, now I have the contour correct with this code :

I = imread('example3.png');
figure(1)
imshow(I)
IM = im2bw(I);
IM2 = imcomplement(IM);
[r, c] = find(IM2, 1, 'first');
mycontour = bwtraceboundary(IM2, [r c] ,'E',8,Inf,'counterclockwise') ;
figure(3)
imshow(I)
hold on;
plot(mycontour(:,2),mycontour(:,1),'g','LineWidth',1);