Extract specific X & Y data from Contour curve

Hello MATLAB experts,

I have this below contour plot. How can I get the X and Y coordinate data for the specific contour line 1. Either in an array or table format would do the job.

Thanks for looking at it. I have attached the script for the reference.

Answer 1

[Star Strider]

Try this —

     %% Input Parameters for Earth
     Ctheta = 5;
     Cphi = 0.001;
     phimax = deg2rad(58);
     thetamax = deg2rad(45);
     alpha = (pi/2)-thetamax;
     H1 = hypergeom([2],[3/2,5/2],-thetamax^2);
     H2 = hypergeom([2],[1/2,5/2],-thetamax^2);
     g = 3.72;
     rho = 0.0142;   
     ge = 9.81;
     rhoe = 1.225;
     r2 = 0.554;
     r3 = 0.59;
     Cl = 1.1;
     Ct = Cl/sin(2*alpha);
     Cd = Cl*tan(alpha);
     
     %% Kolomenskiy Model for Earth Bee
     
     R = 13.2*10^(-3);
     c = 4.02*10^-3;
     S = 2.*R.*c;
     M = 175*10^-6;
     mw = 0.2251*R^3;
     W = M*g;
     
     %% Kolomenskiy model for Mars Bee
     
     nx = 75;
     ny = 75;
     Wm_var_out = [];
     Klm_out = [];
     Kpm_out = [];
     Lavg_out =[];
     AeroPower_out = [];
     PosPinert_out = [];
     Iyy_out = [];
     
     N = linspace(1,6,nx);
     f = linspace(155,1,ny);
     
     for i = 1:nx
     
     R1 = N(i).*R;
     c1 = 0.273.*R1;
     
     mw_var = 0.2251.*R1^3;
     Mm_var = M+mw_var;
     
     Wm_var = Mm_var.*g;
     Wm_var_out=[Wm_var_out Wm_var];
     
     for j=1:ny
     
         %Average Lift
         Klm = rho.*f(j).*(R1.^(2) *r2^(2)).*(2.*R1.*c1).*Ct;
         Klm_out = [Klm_out Klm];
     
         Lavg1 = Klm.*sin(2*alpha);
         Lavg2 = ((Cphi/(asin(Cphi)))^(2) * (2*pi^(2).*f(j).*phimax^(2))./(1+sqrt(1-Cphi^(2))));
     
         Lavg = Lavg1.*Lavg2;
     
         Lavg_out = [Lavg_out Lavg];
     
         %Average Aerodynamic Power
         Kpm = rho.*f(j).*(R1.^(3) *r2^(3)).*(2.*R1.*c1).*(2*Ct);
         Kpm_out = [Kpm_out Klm];
     
         % Variation of Flapping Angle
         T   = 1./f(j);
         t   =(0:0.00001:T/2);
     
         phi_t = (phimax/asin(Cphi)).*(asin(Cphi.*cos(2*pi.*f(j).*t)));
     
         % Flapping Velocity
         phidot_t = (phimax/asin(Cphi)).*((-2*pi.*f(j).*Cphi.*sin(2*pi.*f(j).*t))./sqrt(1-(Cphi^2.*(cos(2*pi.*f(j).*t)).^2)));
     
         % Flapping Acceleration
         phi2dot_t = ((4*pi^2 .*f(j).^2 *Cphi *phimax)./asin(Cphi)).*((((Cphi^2.*cos(2*pi.*f(j).*t).*sin(2*pi.*f(j).*t).^2)./(1-(Cphi^2.*cos(2*pi.*f(j).*t).^2)).^(3/2))-(cos(2*pi.*f(j).*t)./sqrt(1-(Cphi^2.*cos(2*pi.*f(j).*t).^2)))));
     
         % Pitching Angle Time variation
         theta_t = (thetamax/tanh(Ctheta)).*tanh(Ctheta.*sin(2*pi.*f(j).*t));
     
         % Average Integral Aerodynamic Power
         alpha_t = (pi/2)-theta_t;
     
         Kpm = rho.*f(j).*(R1.^3 * r3^3).*(2.*R1.*c1)*(2*Ct);
      
         Pa = phidot_t.^3.*(sin(alpha_t)).^2;
     
         AeroPower = Kpm.*trapz(t,abs(Pa));
         AeroPower_out = [AeroPower_out AeroPower];
     
         Pa_I = Kpm.*abs(Pa);
     
         % Inertial Power
         Iyy = 0.0426.*R1.^5;
         Iyy_out = [Iyy_out Iyy];
     
         Pinert = phidot_t.*Iyy.*phi2dot_t;
     
         B = find(Pinert>=0);
     
         PosPinert = trapz(t(B),Pinert(B))/(T/4);
     
         PosPinert_out = [PosPinert_out PosPinert];
     
     end
     end
     
     Ptotal = AeroPower_out+PosPinert_out;
     
     A = reshape(Lavg_out,ny,nx);
     C = reshape(Ptotal,ny,nx);
     
     
     SpecificLift = A./Wm_var_out;
     
     figure
     contourf(N,f,SpecificLift,'ShowText','on')
     hold on
     [M,C] = contourf(N,f,SpecificLift,[1 2 4 6 8 10 12 14 16 18 20],'ShowText','on');
     colormap jet
     colorbar
     title('SF vs Freq vs Specific Lift')
     xlabel('Scaling Factor')
     ylabel('Frequency')

Levels = C.LevelList

Levels = 1×11

1 2 4 6 8 10 12 14 16 18 20

for k = 1:numel(Levels)
    idx = find(M(1,:) == Levels(k));
    ValidV = rem(M(2,idx),1) == 0;
    StartIdx{k,:} = idx(ValidV);
    VLen{k,:} = M(2,StartIdx{k});
end

figure
for k1 = 1:numel(Levels)
% k1 = 4;                                                                 % Index For Levels 'k1'
hold on
    for k2 = 1:numel(StartIdx{k1})
        idxv = StartIdx{k1}(k2)+1 : StartIdx{k1}(k2)+VLen{k1}(k2);          % Index For Contour 'k1'
        xv = M(1,idxv);
        yv = M(2,idxv);
        plot(xv, yv)
    end
end
hold off
hl = legend(compose('%2d',Levels), 'Location','best', 'NumColumns',2);
title(hl, 'Specific Lift')

xlabel('M(1,:)')
ylabel('M(2,:)')

% title(sprintf('Contour Level %.1f', Levels(k1)))
% axis('equal')

.

[Chinmayraj Doddarajappa]

Thank you,

Your code is working.

Can you please brief me what the code is doing to get the X and Y coordinated.

[Star Strider]

As always, my pleasure!

The ‘xv’ and ‘yv’ values plot the curves for the ‘x’.and ‘y’ values respectively. Subscript them as:

xv{k1,k2} = M(1,idxv);

yv{k1,k2} = M(2,idxv);

where ‘k1’ are the indices of the levels (not the levels themselves), and ‘k2’ are the components of the contours (since in some contour plots, there may be several disconnected contours). For this contour plot, there should be only one (x,y) pair for each contour, so you can customise my code using only ‘k1’ for your contour plot.

Answer 2

[Matt J]

This has worked pretty well for me,

https://www.mathworks.com/matlabcentral/fileexchange/74010-getcontourlinecoordinates?s_tid=srchtitle

[Chinmayraj Doddarajappa]

Hi,

Thanks for the response. I did follow this method before posting it here, it didn't work.

[Matt J]

Worked fine for me:

>> M=contourf(N,f,SpecificLift,[1 2 4 6 8 10 12 14 16 18 20],'ShowText','on');

>> contourTable = getContourLineCoordinates(M);

>> contourTable=contourTable(contourTable.Level==1,:)

contourTable =

113×4 table

Level Group X Y

_____ _____ ______ ______

1 1 6 32

1 1 5.9722 32.216

1 1 5.9324 32.517

1 1 5.8649 33.048

1 1 5.7973 33.608

1 1 5.7297 34.199

1 1 5.7187 34.297

1 1 5.6622 34.792

1 1 5.5946 35.413

: : : :

1 1 2.5541 141.32

1 1 2.5427 142.51

1 1 2.5234 144.59

1 1 2.5049 146.68

1 1 2.4872 148.76

1 1 2.4865 148.84

1 1 2.4689 150.84

1 1 2.4514 152.92

1 1 2.4345 155

Display all 113 rows.

[Chinmayraj Doddarajappa]

Do I need to install any add ons for this function to work?

[Matt J]

Yes, that was what the link was for - the link you said you already downloaded from and tried.

[Chinmayraj Doddarajappa]

Thank you, It is working.