Detect a circular shape inside image in MATLAB

image-processing, matlab, shapes

Solution

Here's what I propose: 1. convert to gray image, enhancing the "difference from white"

gimg = min( img, [], 3 );

2. Threshold to remove white area

BW = im2bw( gimg, .4 );

3. Get area and centroid porperties of image regions

st = regionprops( ~BW, 'Area', 'Centroid', 'PixelIdxList' );

4. select only large enough regions

sel = [st.Area] > numel(BW)*0.025; % at least 2.5% of image size
st = st(sel);

5. compute region distance to center of image

cntr = .5 * [size(BW,2) size(BW,1)]; % X-Y coordinates and NOT Row/Col
d = sqrt( sum( bsxfun(@minus,vertcat( st.Centroid ), cntr ).^2, 2 ) );

6. pick the region closest to center

[mn idx] = min(d);

7. Create a mask

res = false(size(BW)); 
res( st(idx).PixelIdxList ) = true;

You might also consider using other region properties (e.g., `'Eccentricity'`) to better reject regions that are not circular.

Problem

Whats the fastest way to detect these circular shapes in a image? The radius is always between(80-100mm). Background is always white. And the circle will always be in center. I have tried Hough Transform but I couldn't really get it to work. I am new to this, and I get a feeling like Hough Transform is a overkill for this. Kindly suggest me the right approach to do this. UPDATE Here is what I got after applying hough transform. I have used the algorithm mentioned here. Following is the relevant code from the bigger algorithm ``` % applying Hough Below [accum, circen, cirrad] = ... CircularHough_Grd(gR, [89 93],... 17.4, 13, 1); % this executes in 0.72 sec % Lets see what we got imshow(gR); hold on; plot(circen(:,1), circen(:,2), 'r+'); for ii = 1 : size(circen, 1) rectangle('Position',[circen(ii,1) - cirrad(ii), circen(ii,2) - cirrad(ii), 2*cirrad(ii), 2*cirrad(ii)],... 'Curvature', [1,1], 'edgecolor', 'b', 'linewidth', 1.5); end hold off; ``` The meaningful circle is the one in the middle.

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