Edge detection in gradient images

Hello,

I am trying to detect a smooth boundary of an image (attached) to find contact angles of each component in contact (the whole image is not attached).

Firstly, I averaged 3*3 pixels to reduce noises and then, used 'imgradient()' to detect the boundary (subplot(2,2,3)).

To get a single boundary line, I used 'edge()' and tried different methods (Solbel, Canny, etc.), but I am getting two boundaries, which I didn't intend... Is there any way that I can get only the inner line (orange arrows are indicating) in the last picture (subplot(2,2,4))?

I googled and searched but I could not find clear solutions on this so far. I really appreciate your time and comments in advance.

Answer 1

[Image Analyst]

Just because you want the edge does not mean you should call edge(). Like you said, it can produce 2 edges. For that image you posted, you can simply threshold and call bwboundaries

mask = grayImage > 128; % or whatever number works, or imbinarize().
% Here is where we actually get the boundaries for each blob.
boundaries = bwboundaries(binaryImage);
% boundaries is a cell array - one cell for each blob.
% In each cell is an N-by-2 list of coordinates in a (row, column) format.  Note: NOT (x,y).
% Column 1 is rows, or y.    Column 2 is columns, or x.
numberOfBoundaries = size(boundaries, 1); % Count the boundaries so we can use it in our for loop
% Here is where we actually plot the boundaries of each blob in the overlay.
hold on; % Don't let boundaries blow away the displayed image.
for k = 1 : numberOfBoundaries
	thisBoundary = boundaries{k}; % Get boundary for this specific blob.
	x = thisBoundary(:,2); % Column 2 is the columns, which is x.
	y = thisBoundary(:,1); % Column 1 is the rows, which is y.
	plot(x, y, 'r-', 'LineWidth', 2); % Plot boundary in red.
end
hold off;
caption = sprintf('%d Outlines, from bwboundaries()', numberOfBoundaries); 
title(caption, 'FontSize', fontSize); 
axis('on', 'image'); % Make sure image is not artificially stretched because of screen's aspect ratio.

[autumn]

Thanks a lot. It's worked. One follow-up question here, do you know how to remove the outer line (marked)?

[Image Analyst]

Find the indexes where x == 1, or y == rows

[rows, columns, numberOfColorChannels] = size(grayImage);
badindexes = x == 1 || y == rows;
x = x(~badindexes); % Take everything except the bad ones.
y = y(~badindexes); % Take everything except the bad ones.

[autumn]

Thanks a lot ! Just one more question please. How can I append x and y in the loop, and get the final x and y matrix?

Currently, in case I run a image with 3 bourdaries, resulting x and y values are only for the final boundary.

Since 'boundaries' is cell, 3x1 here, I am not sure how to index each resulting x and y, and then append them all..

[autumn]

I googled more and added this in the loop.

arr = [x, y];
arr(:,:,k) = arr;

Answer 2

[Simon Chan]

Try Otsu's method

rawdata=imread('capture 1.png');
I = rgb2gray(rawdata);
level = graythresh(I);
BW = imbinarize(I,level);
J = edge(BW);
imshow(J);

[autumn]

Thank you for the answer!