How to find the coordinates of edge from a figure and plot it on a separate/second figure?

Question

Zhou Ci 2023-11-26

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https://ww2.mathworks.cn/matlabcentral/answers/2052182-how-to-find-the-coordinates-of-edge-from-a-figure-and-plot-it-on-a-separate-second-figure

评论： Walter Roberson 2023-11-26

Hello,

I am trying to find out the coordinates of the edge from this figure but it doesn't work for me. These are actually 35 bins in the image. I don't want any ellipse or rectangle around it. I want a proper boundary around these bins because I want to display that boundary in another figure. Here's the image;

I have tried this;

I = imread('myimage.jpg');    %2598x3071x3 uint8
II = rgb2gray(I);
edgeimage = edge(II, 'Canny');
[x y] = find(edgeimage);

Any help would be appreciated.

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Answer 1

Walter Roberson 2023-11-26

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https://ww2.mathworks.cn/matlabcentral/answers/2052182-how-to-find-the-coordinates-of-edge-from-a-figure-and-plot-it-on-a-separate-second-figure#answer_1359942

在 MATLAB Online 中打开

I = imread('myimage.jpg');    %2598x3071x3 uint8
is_occupied = any(I ~= 255,3);
B = bwboundaries(is_occupied);        %https://www.mathworks.com/help/images/ref/bwboundaries.html#bu1wval-1-B

However... you are using a jpg. It is almost guaranteed that jpg will give you the wrong results for this purpose. jpg is trash on sharp boundaries. Do not use jpg for any scientific measurement -- not unless the measurement you are doing is of exactly how trash jpg is for scientific use.

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Zhou Ci 2023-11-26

So, should I save my figure as png? Or which format is best to save such kind of figure?

Walter Roberson 2023-11-26

png would definitely be better than jpg .

However... if you have the data available to save into an image file, then why not work directly with the data instead of with the image of the data ?

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