Finding Area of Polygon defined by roipoly

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abdulla alyammahi 2020-3-25

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https://ww2.mathworks.cn/matlabcentral/answers/512971-finding-area-of-polygon-defined-by-roipoly

编辑： Image Analyst 2020-12-26

Hello,

I would like to know how I can use the function polyarea to find the area of a region defined by the function roipoly.

I am new to image processing with matlab and basically what I am trying to do is read an image, select a border for something within the image using roipoly, finding the area within the border in pixels and finally converting the area in pixels to area in m^2. Please advise.

Thank you,

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Answer 1

Matt J 2020-3-25

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https://ww2.mathworks.cn/matlabcentral/answers/512971-finding-area-of-polygon-defined-by-roipoly#answer_421962

编辑：Matt J 2020-3-25

在 MATLAB Online 中打开

roipoly returns a logical mask BW of the region

BW = roipoly;

so, its area in whole pixels would simply be

area = sum(BW(:));

If you have the coordinates of the vertices, because you called roipoly using this syntax

[BW,xi2,yi2] = roipoly(___);

then fractional pixels can be included in the area calculation by doing

area = polyarea(xi2,yi2)

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abdulla alyammahi 2020-3-30

Thank you very much for your response, I tried it and it worked perfectly and solved my problem.

Matt J 2020-3-31

You're welcome, but please click "Accept" to indicate that it worked.

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Answer 2

Image Analyst 2020-3-31

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https://ww2.mathworks.cn/matlabcentral/answers/512971-finding-area-of-polygon-defined-by-roipoly#answer_423005

编辑：Image Analyst 2020-12-26

在 MATLAB Online 中打开

spatial_calibration_demo.m

To get the answer in real world units like m^2 you have to first get the area in pixels, using one of these 3 ways:

[mask, x, y] = roipoly(); % Get binary image and (x,y) coordinates of polygon
% Method 1 to get area in pixels:
areaInPixels = polyarea(x, y); % Could have fractional pixel amounts.
% Method 2
areaInPixels = bwarea(mask); % Could have fractional pixel amounts.
% Method 3
areaInPixels = nnz(mask); % Always an integer count of pixels.

Then multiply by the spatial calibration factor to get the area in square meters or millimeters:

areaInM2 = areaInPixels * metersPerPixel ^ 2;

If you don't know how to arrive at meters per pixel, you basically have to draw a distance on your image and tell it how many real world units it is. I have a demo attached where it does exactly that. Please run it.