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Sorting of points in 2D

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Vijay Anand
Vijay Anand on 17 Oct 2020
Commented: Stephen23 on 19 Oct 2020
Dear All,
I face issue in arranging the airfoil coordinates coming from a CAD program. The coordinates are arranged from minumum x value to maximum x value. But I require to arrange the points starting from Trailing Edge ( Max X coordinate) and then proceed on top side all the way to Leading Edge ( Minimum X coordinate) and then again proceed in the bottom side and terminate in the Trailing Edge.
The input.txt ( required input file to be sorted)
Output.txt (the coordinates sorted manually).
I have several airfoil coordinates of similar nature, so a matlab function could help me out doing the job manually everytime.
Any help is really appreciated.
Thanks in Advance,
K Vijay Anand
Vijay Anand
Vijay Anand on 17 Oct 2020
Edited: Vijay Anand on 17 Oct 2020
Dear KSSV,
Thanks a lot for your quick responce. I have gone through your algorithm and works perfect for convex polygons !!!
In my case, it is failing near the trailing edge. There should be a smart way to retify this. Kindly go through the attached code and the figures.
%% Program to sort 2D Airfoil Coordinates
clear all; close all; clc
%% Read Input File
Table1 = readtable('Input.txt','PreserveVariableNames',true);
x = Table1.x' ; y = Table1.y';
% Plot the input airfoil
h1 = figure(1); plot(x,y,'.-');
xlabel('X-Coordinate'); ylabel('Y-Coordinate'); grid on; title('Input Coordinates');
Input Airfoil
%% Sorting using Centroid Algorithm
P = [x; y]; % coordinates / points
c = mean(P,2); % mean/ central point
d = P-c ; % vectors connecting the central point and the given points
th = atan2(d(2,:),d(1,:)); % angle above x axis
[th, idx] = sort(th); % sorting the angles
P = P(:,idx); % sorting the given points
P = [P P(:,1)]; % add the first at the end to close the polygon
x1 = P(1,:);
y1 = P(2,:);
% Plot the output airfoil
h2 = figure(2); plot(x1,y1,'.-');
xlabel('X-Coordinate'); ylabel('Y-Coordinate'); grid on; title('Sorted Coordinates - Centroid Algorithm');

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Accepted Answer

Stephen23 on 17 Oct 2020
Edited: Stephen23 on 19 Oct 2020
Here is some simple code that that works quite nicely for your aerofoil. The code assumes that the top and bottom are both functions of x (i.e. no implicit curves in some other variable) and that they are both sampled with approximately the same step size (like your data). A convolution is used to smooth the data to create a moving average over 3 data points. The y-data are compared against this moving average to decide if the data point belongs to the top or bottom of the aerofoil, and then those two sets of points are sorted separately.
Inp = readtable('Input.txt','Delimiter','\t');
tmp = conv(Inp.y([1,1:end,end]),ones(1,3)/3,'valid'); % smoothed data
idx = Inp.y >= tmp; % true/false == top/bottom
idx(end) = true; % trailing edge -> top
Out = [sortrows(Inp(idx,:),-1);sortrows(Inp(~idx,:),+1)];
Checking against your original "Output.txt" and plotting the data:
>> Chk = readtable('Output.txt','Delimiter','\t');
>> isequal(Chk,Out)
ans =
>> plot(Inp.x,Inp.y,'-*', Inp.x,tmp,'-+', Out.x,Out.y,'-x')
>> legend('original','moving average','sorted')
Image Analyst
Image Analyst on 17 Oct 2020
What about boundary() or envelope()? Might those work?
Stephen23 on 17 Oct 2020
Edited: Stephen23 on 17 Oct 2020
@Image Analyst: good suggestions. The function boundary (available for >=R2014b) is certainly easy to use and provides a simple solution but is slower (it relies on the alphashape class, which is not trivial code) and requires a bit of fiddling to find a suitable shrink factor:
>> idy = boundary(T.x,T.y,0.97);
>> V = T(idy,:);
>> plot(V.x,V.y,'-d')
Below 0.97 misses out a few data points along the bottom towards the trailing edge**, above 0.98 starts to make extra connections between the top and bottom... I guess this depends on the sample step size and curvature, so most likely there is no universal solution which does not require fine-tuning.
** now that I look at the above figure, it also seems to be missing a few. 0.98 is probably about right.

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More Answers (1)

Vijay Anand
Vijay Anand on 18 Oct 2020
Edited: Vijay Anand on 18 Oct 2020
Fantastic Approach to the problem !! You saved my day !
I have tried with different airfoil profiles. Some profiles require moving average with a windows size of 5 or 7 instead of 3.
So, i have made use of the following function movingmean to change the window size after visual inspection of the output.
Glen (2020). Moving Average Function (, MATLAB Central File Exchange. Retrieved October 18, 2020.
The final code :
T = readtable('Input.txt','Delimiter','\t');
vec = movingmean(T.y,5); % Moving Average with Window Size 5
idx = T.y >= vec; % true/false == top/bottom
idx(end) = true; % trailing edge -> top
U = [sortrows(T(idx,:),-1);sortrows(T(~idx,:),+1)];
Steven Lord
Steven Lord on 18 Oct 2020
Consider using the movmean function included in MATLAB.
Stephen23 on 19 Oct 2020
I did some more experimentation and got better results replacing the moving average with a polynomial fitted to the data. First I tried polyfit, but quickly realized that the end points need to be specified. Luckily this FEX submission
allows constraints to be specified, including points that the curve has to pass through:
pts = Inp{[1,end],:}; % polynomial must pass through the leading & trailing edges
pwr = mmpolyfit(Inp.x,Inp.y, 5, 'Point',pts) % fit order 5 polynomial
tmp = polyval(pwr, Inp.x); % replaces TMP in the original answer
And checking the fit:
plot(Inp.x,Inp.y,'-*', Inp.x,tmp,'-+')

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