How to crop the coin circular and neglect the rest area?

I have a coin image and I want to crop the circular section of the coin, I tried below code but this doesn't worked well.

I want to crop out the coin with the outer boundary with a perfect circle. Any help would be appreciated. Thank you

Code which I tried:

%B1 is in coin.mat coin image
B2 = B1/max(B1(:));
k=B1*200;
k(k>-6000) = 1;
k(k<-6000) = 0;
bw2 = imfill(k,'holes');
figure,imshow(bw2)
bw3 = imopen(bw2, ones(3,3));
figure,imshow(bw3)
bw3 = imopen(bw2, ones(5,5));

Answer 1

[Image Analyst]

That's what I do in my popular tutorial.

See my Image Segmentation Tutorial in my File Exchange:

Image Analyst's File Exchange

It's a generic, general purpose demo of how to threshold an image to find blobs (coins), and then measure things about the blobs, and extract certain blobs based on their areas or diameters.

Answer 2

[Cris LaPierre]

I would use the Image Segmenter app. After loading the image, you can define a circuilar ROI, apply a mask, and export the result back to MATLAB.

Just be aware that the result is always going to be a square, as the image is stored in a matrix. There is no circle version of it. You can use the mask to adjust the background color if you want.

You can learn more in the Using the Image Segmenter App video, which is part of the Image Processing for Engineering and Science Specialization on Coursera. It is a self paced course created by MathWorks, and you can enroll for free.

[Cris LaPierre]

"The issue is I don't know the radius of the coin"

This reply was removed, but I wanted to respond to it just to clear up any misconceptinos. There are two potential approaches to address this concern:

Use the Ruler tool in the Find Circles Segmentation Tool to get an idea of the range of diameters. This tool will find perfect circles in your image. However, it may not include everything you want in the circle.

After tweaking the settings a little, I was able to obtain the following result.

The other option is to just draw the ROI yourself using the Draw ROIs Segmentation Tool. One of the options is Draw Circle, which will let you draw circles of any size in your image. You do not need to know the radius.

[shubham kumar gupta]

I appreaciate the efforts!

This really helped me, but still this poses human error.

Thank you

[Image Analyst]

If extremely high precision is really a requirement then you'd do things to enable that. This includes

1. Using as high a resolution camera as you can.
2. Using a telecentric lens (instead of a regular lens).
3. Using a uniform background of contrasting color (like black velvet).
4. Using proper lighting.

Alternatively you can use a caliper micrometer.