Area of contor from Matrix

I have a matrix with 2 coordinates x, y and the force F

about 2 million rows (2M X 3)

I want to plot the contor of this data while F > 0.1 N.

after that I want to calculate the area of the contor.

I can't use the function "contour" because I don't have mesh data and I cant use meshgrid because the matrix is very large

Can any one help me :)

Thanks

Answer 1

[Matt J]

Use scatteredInterpolant to resample the data on a grid,

S=scatteredInterpolant(x,y,F);

xg=linspace(min(x),max(x),500);
yg=linspace(min(y),max(y),500);
Fg=S({xg,yg});

and then use contour().

cm=contour(xg,yg,Fg',[0.1,0.1]);

meshgrid is irrelevant to the discussion. As demonstrated above, you don't need meshgrids of the x,y data to use contour(), but even if you were to do that, they wouldn't be that large.

[Matt J]

To compute the area, you could extract the contour boundary points with,

https://www.mathworks.com/matlabcentral/fileexchange/74010-getcontourlinecoordinates?s_tid=srchtitle_getContourLineCoordinates_1

out=getContourLineCoordinates(cm)

and then use polyarea.

[Mad Gano]

clear all; close all; clc;

File=load('.\TestFile.mat');
x=File.Matrix(:,1);
y=File.Matrix(:,2);
F=File.Matrix(:,3);
S=scatteredInterpolant(x,y,F);
xg=linspace(min(x),max(x),500);
yg=linspace(min(y),max(y),500);
Fg=S({xg,yg});
cm=contour(xg,yg,Fg',[0.01,0.01]);
out_Detail=getContourLineCoordinates(cm);
out=out_Detail{:,:};
area = polyarea(out(:,3),out(:,4));

Thank you very much.

Could you tell me how to get only the outer contour (see picture)

And can you tell me if I have calculated the area correctly?

[Adam Danz]

getContourLineCoordinates produces a table indicating the (x,y) coordinates and level for all contourlines along with a grouping variable that groups each contour line group. You can loop through the groups to compute the area for each contour line and then select the largest group.

File=load('TestFile.mat');
x=File.Matrix(:,1);
y=File.Matrix(:,2);
F=File.Matrix(:,3);
S=scatteredInterpolant(x,y,F);
xg=linspace(min(x),max(x),500);
yg=linspace(min(y),max(y),500);
Fg=S({xg,yg});
cm=contour(xg,yg,Fg',[0.01,0.01]);
T=getContourLineCoordinates(cm); % Get contour table

% loop through each contour group
groups = unique(T.Group);
areas = nan(size(groups)); 
for i = 1:numel(groups)
    idx = T.Group==i;
    areas(i) = polyarea(T.X(idx), T.Y(idx));
end

% Find the max area
[maxArea, maxidx] = max(areas)

Returns

maxArea =

9.371e-05

maxidx =

1

The largest area is 9.371e-05 which is in the first index which is group groups(i)=1.

To isolate info about that contour line from the table ,

TLargest = T(T.Group==1,:)

[Mad Gano]

Thanks :)

Answer 2

[Walter Roberson]

See https://www.mathworks.com/matlabcentral/fileexchange/38858-contour-plot-for-scattered-data?focused=5249779&tab=function and notice the code needs to build the triangulation first before calling the function