How to make a contourf plot follow a line?

Question

Margaux 2023-6-23

0
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此问题的直接链接

https://ww2.mathworks.cn/matlabcentral/answers/1987249-how-to-make-a-contourf-plot-follow-a-line

评论： Mathieu NOE 2023-6-26

Good morning! I plotted a contourf plot in a figure, until then evrything is working, but now, I awant to make that contour follow a line. I have been trying to use a for loop but I cannot make it work. Any idea on what I could try to achieve my goal?

Here is my foor loop:

for i = 1:200

c = contourf(X, Y, -) %% different variables from my code to make it easier

xticks([])

yticks([])

cmap = colormap("summer");

xline(0, "black", "LineWidth", 2)

view([180 90]); % changes the view so that the pattern is facing down

pause(2)

drawnow

end

Thank you

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Mathieu NOE 2023-6-23

编辑：Mathieu NOE 2023-6-23

hello

can you share your data ? and a picture / sketch of what you want to achieve

Margaux 2023-6-23

Hi! Sadly, I cannot share the data as it is way to big. But currently, the output looks like this (image). it is basically an ellipse and an xline is going right down the middle. What I want (image(1)), is basically that same ellipse, just following the lne and leaving a trail behind so it basically becomes a big line. I hope that makes sense.

Thank you

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Answer 1

Mathieu NOE 2023-6-23

2
链接

此回答的直接链接

https://ww2.mathworks.cn/matlabcentral/answers/1987249-how-to-make-a-contourf-plot-follow-a-line#answer_1261403

在 MATLAB Online 中打开

intersections.m

hello again

here a small demo code based on a first given elipse (bottom left on the picture)

you can use a spacing between them if needed (xr_spacing)

the intersections function is attached

% dummy ellipse
n = 100;
alpha = pi/n+(0:n)/n*2*pi;
a = 1;
b = 3;
x0 = 2.5;
y0 = 3;
x = x0 + a*cos(alpha);
y = y0 + b*sin(alpha);
% create the reference line 
xref = (0:0.1:10);
yref = y0*ones(size(xref));
figure(1);
plot(x,y,'*-b',xref,yref,'r--')
xlim([0 8])
ylim([0 8])
axis square
% for fun let's apply a rotation angle to this 
% rotation around the origin(0,0)
% Create rotation matrix
theta = 45; % to rotate 25 degrees counterclockwise
R = [cosd(theta) -sind(theta); sind(theta) cosd(theta)];
% Rotate your point(s)
for k =1:numel(x)
    rotpoint = R*([x(k) ;y(k)]);
    xr(k) = rotpoint(1);
    yr(k) = rotpoint(2);
end    
for k =1:numel(xref)
    rotpoint = R*([xref(k) ;yref(k)]);
    xrefr(k) = rotpoint(1);
    yrefr(k) = rotpoint(2);
end
% 
figure(2);
plot(xr,yr,'*-b',xrefr,yrefr,'r--')
xlim([-4 8])
ylim([0 12])
axis square
hold on
% now create replicates of the rotated elipse along the reference line
% intersections makes finding the intersection points very easy.
[xout,yout] = intersections(xr,yr,xrefr,yrefr,1);
dx = diff(xout);
dy = diff(yout);
% do n duplicates
xr_spacing = 0.25; % along the reference line (rotated)
xy_spacing = R*([xr_spacing ;0]); % gives the true x and y spacing in the absolute referential
for n = 1:3
    newx = xr + n*(dx+xy_spacing(1));
    newy = yr + n*(dy+xy_spacing(2));
    plot(newx,newy);
end

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Mathieu NOE 2023-6-23

在 MATLAB Online 中打开

forgot the top and bottom lines

here is it :

% dummy ellipse (a,b)
n = 100;
alpha = pi/n+(0:n)/n*2*pi;
a = 1;
b = 3;
x0 = 2.5;
y0 = 3;
x = x0 + a*cos(alpha);
y = y0 + b*sin(alpha);
% create the reference line 
xref = (0:0.1:10);
yref = y0*ones(size(xref));
% create top and bottom lines 
[val,id_top]= max(y);
x_top = (x(id_top):0.1:xref(end));
y_top = val*ones(size(x_top));
[val,id__bottom]= min(y);
x_bottom = (x(id__bottom):0.1:xref(end));
y_bottom = val*ones(size(x_bottom));
figure(1);
plot(x,y,'*-b',xref,yref,'r--',x_bottom,y_bottom,'g--',x_top,y_top,'k--')
xlim([-1 8])
ylim([-1 8])
axis square
% for fun let's apply a rotation angle to this 
% rotation around the origin(0,0)
% Create rotation matrix
theta = 45; % to rotate 25 degrees counterclockwise
R = [cosd(theta) -sind(theta); sind(theta) cosd(theta)];
% Rotate your point(s)
for k =1:numel(x)
    rotpoint = R*([x(k) ;y(k)]);
    xr(k) = rotpoint(1);
    yr(k) = rotpoint(2);
end    
for k =1:numel(xref)
    rotpoint = R*([xref(k) ;yref(k)]);
    xrefr(k) = rotpoint(1);
    yrefr(k) = rotpoint(2);
end
for k =1:numel(x_top)
    rotpoint = R*([x_top(k) ;y_top(k)]);
    x_topr(k) = rotpoint(1);
    y_topr(k) = rotpoint(2);
end
for k =1:numel(x_bottom)
    rotpoint = R*([x_bottom(k) ;y_bottom(k)]);
    x_bottomr(k) = rotpoint(1);
    y_bottomr(k) = rotpoint(2);
end
% 
figure(2);
plot(xr,yr,'*-b',xrefr,yrefr,'r--',x_bottomr,y_bottomr,'g--',x_topr,y_topr,'k--')
xlim([-4 8])
ylim([0 12])
axis square
hold on
% now create replicates of the rotated elipse along the reference line
% intersections makes finding the intersection points very easy.
[xout,yout] = intersections(xr,yr,xrefr,yrefr,1);
dx = diff(xout);
dy = diff(yout);
% do n duplicates
xr_spacing = 0.25; % along the reference line (rotated)
xy_spacing = R*([xr_spacing ;0]); % gives the true x and y spacing in the absolute referential
for n = 1:3
    newx = xr + n*(dx+xy_spacing(1));
    newy = yr + n*(dy+xy_spacing(2));
    plot(newx,newy);
end

Mathieu NOE 2023-6-23

在 MATLAB Online 中打开

combine the two codes and you get the result :

x = 0:0.1:2;
y = 0:0.1:5;
[X,Y] = meshgrid(x,y);
Z = X.*exp(-X.^2-0.25*(Y-2.5).^2);
figure(1);
[cm, h] = contourf(X,Y,Z);
[contourTable, contourArray] = getContourLineCoordinates(cm);
    % Show all lines that match the 7th level
    hold on
    levelIdx = contourTable.Level == h.LevelList(7);
    x = contourTable.X(levelIdx);
    y = contourTable.Y(levelIdx);
    
plot(x, y, 'r*-', 'MarkerSize', 10)
    % create the reference line 
xref = (0:0.1:10);
yref = mean(y)*ones(size(xref));
% create top and bottom lines 
[val,id_top]= max(y);
x_top = (x(id_top):0.1:xref(end));
y_top = val*ones(size(x_top));
[val,id__bottom]= min(y);
x_bottom = (x(id__bottom):0.1:xref(end));
y_bottom = val*ones(size(x_bottom));
figure(2);
plot(x,y,'*-b',xref,yref,'r--',x_bottom,y_bottom,'g--',x_top,y_top,'k--')
xlim([-1 8])
ylim([-1 8])
axis square
% for fun let's apply a rotation angle to this 
% rotation around the origin(0,0)
% Create rotation matrix
theta = 45; % to rotate 25 degrees counterclockwise
R = [cosd(theta) -sind(theta); sind(theta) cosd(theta)];
% Rotate your point(s)
for k =1:numel(x)
    rotpoint = R*([x(k) ;y(k)]);
    xr(k) = rotpoint(1);
    yr(k) = rotpoint(2);
end    
for k =1:numel(xref)
    rotpoint = R*([xref(k) ;yref(k)]);
    xrefr(k) = rotpoint(1);
    yrefr(k) = rotpoint(2);
end
for k =1:numel(x_top)
    rotpoint = R*([x_top(k) ;y_top(k)]);
    x_topr(k) = rotpoint(1);
    y_topr(k) = rotpoint(2);
end
for k =1:numel(x_bottom)
    rotpoint = R*([x_bottom(k) ;y_bottom(k)]);
    x_bottomr(k) = rotpoint(1);
    y_bottomr(k) = rotpoint(2);
end
% 
figure(3);
plot(xr,yr,'*-b',xrefr,yrefr,'r--',x_bottomr,y_bottomr,'g--',x_topr,y_topr,'k--')
xlim([-4 8])
ylim([0 12])
axis square
hold on
% now create replicates of the rotated elipse along the reference line
% intersections makes finding the intersection points very easy.
[xout,yout] = intersections(xr,yr,xrefr,yrefr,1);
dx = diff(xout);
dy = diff(yout);
% do n duplicates
xr_spacing = 0.25; % along the reference line (rotated)
xy_spacing = R*([xr_spacing ;0]); % gives the true x and y spacing in the absolute referential
for n = 1:3
    newx = xr + n*(dx+xy_spacing(1));
    newy = yr + n*(dy+xy_spacing(2));
    plot(newx,newy);
end

Mathieu NOE 2023-6-23

在 MATLAB Online 中打开

this is my first plot

and my last

from the same code :

x = 0:0.1:2;
y = 0:0.1:5;
[X,Y] = meshgrid(x,y);
Z = X.*exp(-X.^2-0.25*(Y-2.5).^2);
figure(1);
[cm, h] = contourf(X,Y,Z);
[contourTable, contourArray] = getContourLineCoordinates(cm);
    % Show all lines that match the 7th level
    hold on
    levelIdx = contourTable.Level == h.LevelList(7);
    x = contourTable.X(levelIdx);
    y = contourTable.Y(levelIdx);
    
plot(x, y, 'r*-', 'MarkerSize', 10)
    % create the reference line 
xref = (0:0.1:10);
yref = mean(y)*ones(size(xref));
% create top and bottom lines 
[val,id_top]= max(y);
x_top = (x(id_top):0.1:xref(end));
y_top = val*ones(size(x_top));
[val,id__bottom]= min(y);
x_bottom = (x(id__bottom):0.1:xref(end));
y_bottom = val*ones(size(x_bottom));
figure(2);
plot(x,y,'*-b',xref,yref,'r--',x_bottom,y_bottom,'g--',x_top,y_top,'k--')
xlim([-1 8])
ylim([-1 8])
axis square
% for fun let's apply a rotation angle to this 
% rotation around the origin(0,0)
% Create rotation matrix
theta = 45; % to rotate 25 degrees counterclockwise
R = [cosd(theta) -sind(theta); sind(theta) cosd(theta)];
% Rotate your point(s)
for k =1:numel(x)
    rotpoint = R*([x(k) ;y(k)]);
    xr(k) = rotpoint(1);
    yr(k) = rotpoint(2);
end    
for k =1:numel(xref)
    rotpoint = R*([xref(k) ;yref(k)]);
    xrefr(k) = rotpoint(1);
    yrefr(k) = rotpoint(2);
end
for k =1:numel(x_top)
    rotpoint = R*([x_top(k) ;y_top(k)]);
    x_topr(k) = rotpoint(1);
    y_topr(k) = rotpoint(2);
end
for k =1:numel(x_bottom)
    rotpoint = R*([x_bottom(k) ;y_bottom(k)]);
    x_bottomr(k) = rotpoint(1);
    y_bottomr(k) = rotpoint(2);
end
% 
figure(3);
plot(xr,yr,'*-b',xrefr,yrefr,'r--',x_bottomr,y_bottomr,'g--',x_topr,y_topr,'k--')
xlim([-4 8])
ylim([0 12])
axis square
hold on
% now create replicates of the rotated elipse along the reference line
% intersections makes finding the intersection points very easy.
[xout,yout] = intersections(xr,yr,xrefr,yrefr,1);
dx = diff(xout);
dy = diff(yout);
% do n duplicates
xr_spacing = 0.25; % along the reference line (rotated)
xy_spacing = R*([xr_spacing ;0]); % gives the true x and y spacing in the absolute referential
for n = 1:3
    newx = xr + n*(dx+xy_spacing(1));
    newy = yr + n*(dy+xy_spacing(2));
    plot(newx,newy);
end

Mathieu NOE 2023-6-26

在 MATLAB Online 中打开

ok

just fyi this is just another way to plot the elipse on top of your contourf plot

I added a pause of 1 second in the last for loop that plots the duplicate of the elipse with some x spacing (as you wish)

x = 0:0.1:5;
y = 0:0.1:5;
[X,Y] = meshgrid(x,y);
Z = X.*exp(-X.^2-0.25*(Y-2.5).^2);
figure(1);
[cm, h] = contourf(X,Y,Z);
[contourTable, contourArray] = getContourLineCoordinates(cm);
    % Show all lines that match the 7th level
    hold on
    levelIdx = contourTable.Level == h.LevelList(7);
    xc = contourTable.X(levelIdx);
    yc = contourTable.Y(levelIdx);
    
    % create the reference line 
xref = x;
yref = mean(y)*ones(size(xref));
% create top and bottom lines 
[val,id_top]= max(yc);
x_top = (xc(id_top):0.1:xref(end));
y_top = val*ones(size(x_top));
[val,id__bottom]= min(yc);
x_bottom = (xc(id__bottom):0.1:xref(end));
y_bottom = val*ones(size(x_bottom));
plot(xc,yc,'-r',xref,yref,'r--',x_bottom,y_bottom,'g--',x_top,y_top,'k--')
hold on
% now create replicates of the elipse along the reference line
[xout,yout] = intersections(xc,yc,xref,yref,1);
dx = diff(xout);
dy = diff(yout);
% do n duplicates
x_spacing = 0.25; % 
y_spacing = 0; %
for n = 1:3
    pause(1)
    newx = xc + n*(dx+x_spacing);
    newy = yc + n*(dy+y_spacing);
    plot(newx,newy);
    
end

Margaux 2023-6-26

Oh yeah of course!

Again, thank you so much for your help! I will try to implement it that way!! Thank you!!!

Mathieu NOE 2023-6-26

as always, my pleasure !

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How to make a contourf plot follow a line?

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