Cannot calculate gradient of contour plot
22 vues (au cours des 30 derniers jours)
Afficher commentaires plus anciens
Hi all,
I am currently attempting to draw a quiver plot on top of a contour plot, such that I can see the gradient of the contour function. However for some reason the gradient calculation does not take place correctly as all the quivers are far from being perpendicuar to the contours. This suprises me as the task itself is pretty simple and I have managed to accomplish it with mock data, see here:
clear ; clc
% Grid matrices
x = -2:0.25:2;
y = x;
[X,Y] = meshgrid(x);
% Function Matrix
F = X.*exp(-X.^2-Y.^2);
% Calculate Gradient
[FU, FV] = gradient(F, diff(x(1:2))) ;
% Plot
figure
hold on
contour(X, Y, F)
quiver(X, Y, FU, FV)
However, using my own data (you can find them attached its a simple 30x30 matrix), the same process does not seem to accomplish the task:
clear ; clc
% Grid matrices
x = linspace(-0.1, 0.1, 30) ;
y = linspace(8, 12, 30) ;
[X, Y] = meshgrid(x, y) ;
% Function Matrix
load('my_data.mat')
% Calculate Gradient
[FU, FV] = gradient(F, diff(x(1:2)), diff(y(1:2))) ;
% Plot
figure
hold on
contour(X, Y, F)
quiver(X, Y, FU, FV)
The weird thing is that the countour plot gets generated correctly, however something is off with the quiver. Even when I scale down the quiver vectors to say 0.2, they don't have the correct orientation relative to the contours.
Thanks for your help in advance
2 commentaires
Réponse acceptée
Bjorn Gustavsson
le 24 Sep 2020
If you modify your call to gradient to:
[FU, FV] = gradient(F, diff(y(1:2)), diff(x(1:2)));
You get a more reasonable set of quivers:
contour(X, Y, F)
hold on
quiver(X(1:2:end,1:2:end), Y(1:2:end,1:2:end), FU(1:2:end,1:2:end)/2, FV(1:2:end,1:2:end)/2,0)
The zero is to turn off the automatic scaling in quiver.
HTH
Plus de réponses (0)
Voir également
Catégories
En savoir plus sur Vector Fields dans Help Center et File Exchange
Community Treasure Hunt
Find the treasures in MATLAB Central and discover how the community can help you!
Start Hunting!