How to find the range of a contour along each dimension of a plane with least possible error?

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Sourin 2023-7-10

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https://ww2.mathworks.cn/matlabcentral/answers/1993888-how-to-find-the-range-of-a-contour-along-each-dimension-of-a-plane-with-least-possible-error

编辑： Sourin 2023-7-20

I have a matrix of data which I plotted as a contour using the contour function. The y-axis of the plot is taken as log of the variable, i.e. I used:

contour(x,log2(y),Data,[-99,1],'k');

The resulting plot has an irregular shape. By "irregular" I mean that it's not a nice smooth elliptical or rectangular shape, it's quite jagged at places and not evenly spread everywhere. I am posting one such plot below.

Here, I want to find out the left most point and the right most point of this contour to find the range along x-axis. Similarly, I want to find the bottom most point and the top most point of this contour to find the range along y-axis. Obviously for such shape, the x coordinates of the y_bottom and y_top will be different; y coordinates of the x_left and x_right will be different.

So, how do I find out the four points? One way is to do it by observation and select the points myself to note down the values, but there is a chance of error there. How to find it exactly with code?

Also I have another minor doubt: since I have used log2 of the y data, is there any way to minimize approximation error that I should be careful about?

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Answer 1

KSSV 2023-7-10

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https://ww2.mathworks.cn/matlabcentral/answers/1993888-how-to-find-the-range-of-a-contour-along-each-dimension-of-a-plane-with-least-possible-error#answer_1270233

在 MATLAB Online 中打开

C = contour(x,log2(y),Data,[-99,1],'k');
x = C(1,:) ;
y = C(2,:) ;
iwant = [min(x) min(y) ;
        max(x) min(y) ;
        max(x) max(y) ;
        min(x) max(y)] ;

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Sourin 2023-7-12

No, I was not talking about different levels within a same contour. Such contours represent peak of a given height in my data, I am trying to detect the peaks. I just need the outermost level and its maximum spread. In some cases I got side by side contours (attaching a pic below for reference) and I am now trying to detect each of their ranges.

I am working on this on my own using the trick mentioned above to write a code that will separately give me the x_left, x__right; y_top, y_bottom points for each contour. However, I ran into a snag while using the technique suggested in the accepted answer so I am writing about it below:

I was first checking for the cases of a single contour on the plane (like the one I posted in main question). Quite often the first value of x-axis info, i.e. C(1,:) is giving wrong values, and sometimes the first value of y-axis info, i.e. C(2,:) is coming out as a weird number. I'm posting a pic of an output:

Here, it somehow gave C(1,:) 's first value as 1 when my x-axis data started from 14.7 and spacing between two data points was 0.01 units. Similarly, all my y-axis data were negative values but the code returned a weird large value of 1379 in the C(2,:). Only these two locations are getting affected somehow. Everything in the middle are fine.

What is going wrong here? How can I fix this?

Walter Roberson 2023-7-12

If you use https://www.mathworks.com/matlabcentral/fileexchange/38863-extract-contour-data-from-contour-matrix-c then you would get out a struct array that has the per-contour information.

Sourin 2023-7-20

编辑：Sourin 2023-7-20

@Walter Roberson Would you write this as answer in a separate comment? This is working without any error. It works on a single contour or multiple ones as well. If you post as a separate answer, I will be able to accept it as answer to my question. So other users can see it easily later.

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How to find the range of a contour along each dimension of a plane with least possible error?

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How to find the range of a contour along each dimension of a plane with least possible error?

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