How to calculate the length of image?

Question

Steven 2013-12-22

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此问题的直接链接

https://ww2.mathworks.cn/matlabcentral/answers/110418-how-to-calculate-the-length-of-image

编辑： Steven 2016-4-26

采纳的回答： Image Analyst

Hi

Regarding the image below:

I want to calculate the length of the large black region, i.e., the perimeter of this region plus the length of the small white objects within it. How may I do so?

I know there are some functions for calculating the perimeter, but how to include the small objects?

Also please note that this is a binary image, but I can also do the calculations on grayscale image, if needed.

Thanks so much.

Steven

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Answer 1

Image Analyst 2013-12-22

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链接

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https://ww2.mathworks.cn/matlabcentral/answers/110418-how-to-calculate-the-length-of-image#answer_119074

在 MATLAB Online 中打开

Something is wrong with that image. I saved it but when I read it in it looks totally different. Anyway, you need to get the perimeters of everything with regionprops and sum them

measurements = regionprops(binaryImage, 'Perimeter');
allPerimeters = [measurements.Perimeter];
theSum = sum(allPerimeters);

But make sure the big perimeter is the inside one and not the outer edge of the image. If it is, then invert the image and fill holes before calling regionprops

binaryImage = imfill(~binaryImage, 'holes');
measurements = regionprops(binaryImage, 'Perimeter');
theBigPerimeter = measurements.Perimeter;

I can't really test it because, like I said, it seems the image is messed up.

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Steven 2013-12-22

Thanks again.

I get (naturally!) three wide different values from three different codes.

How can I know which one is correct? (because yet I have no idea of the scales)

Thanks.

Steven

Image Analyst 2013-12-22

There are different algorithms. Summing bwperim just sums up the pixels. regionprops computes the perimeter by calculating the distance between each adjoining pair of pixels around the border of the region. So the first method would have two pixels at 45 degrees to each other as a length of 2 while regionprops would give a distance of sqrt(2). You can use whichever method you like better. Optically the regionprops method is probably more accurate.

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