Find the diameter of the parts in an image processing?

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Torkan 2020-1-10

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评论： Guillaume 2020-1-10

Hello,

Does anybody have any idea how can I calculate the average diameter of the parts attached here? The white parts ar cylinders actually with diameters in Nano size.

Would you please help me to write a code such that I can catch the diameters?

Thanks

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Answer 1

Alex Mcaulley 2020-1-10

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One approach would be:

First, you need to segment the objects you want using any segmentation function, for example imbinarize, or a manual threshold.
Then, Using regionprops you can obtain the perimeter of each segmented object
Finally, to obtain the diameters is just:

diameter = perimeter/pi

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Guillaume 2020-1-10

Comment by Torkan mistakenly posted as an answer:

Thanks all

First I have a problem. I wrote this code:

A = imread('12.png');

imshow(A)

%radiusRange = [150 220];

%[centers,radii] = imfindcircles(A,radiusRange) ;

%viscircles(centersBright, radiiBright,'Color','b');

Rmin = 1;

Rmax = 2;

[centersBright, radiiBright] = imfindcircles(A,[Rmin Rmax],'ObjectPolarity','bright');

The MATLAB mostly gives empty centersbright and radiiBright. I don't know

which number should I define based upon my scale?

Warning: You just called IMFINDCIRCLES with a large radius range. Large radius ranges reduce algorithm accuracy and increase

computational time. For high accuracy, relatively small radius range should be used. A good rule of thumb is to choose the

radius range such that Rmax < 3*Rmin and (Rmax - Rmin) < 100. If you have a large radius range, say [20 100], consider breaking

it up into multiple sets and call IMFINDCIRCLES for each set separately, like this:

[CENTERS1, RADII1, METRIC1] = IMFINDCIRCLES(A, [20 60]);

[CENTERS2, RADII2, METRIC2] = IMFINDCIRCLES(A, [61 100]);

Warning: You just called IMFINDCIRCLES with very small radius value(s). Algorithm accuracy is limited for radius values less

than or equal to 5.

Matt J 2020-1-10

Torkan's answer converted to comment:

I used stats = regionprops('table',BW,'Centroid','MajorAxisLength','MinorAxisLength') and it gives nothing for major and minor axis. It seems that as you said the resolution makes problem.

Guillaume 2020-1-10

As I said, use imtool or imshow to look at the image.

resolution is a bit of an overloaded word. You have the optical resolution, i.e. physical unit/pixel (in your case

) which you can determine by measuring the length of the line in your image (not the most accurate method but of it's all you have...). Resolution is also used for the size of the image in pixels. If your images are only 170x315, then that's a very low resolution. In that image your white cylinders are only 2 or 3 pixels in diameter, so an error of meaurement of one pixel is a 50 % mesurement error. The only way to reduce that error is to have a sensor with more pixels or increase the optical magnification so that the cylinders take more pixel. An error of 1 pixel for a cylinder of diameter 20 pixels drops the measurement error to 5 %.

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