Rotation of a contour plot

I have a contour plot (Image1). I need to rotate it through a certain angle. If I save it as an image and then rotate it using transformation(or rotation) matrix, the whole plot including the axes are rotated(Image2). If I use imrotate for the matrix I have of the contour, the resultant contour doesn't actually rotate by the same angle which I specify (37 deg in the example, Image3) and more over sometimes the width of the plot decreases.

Can someone please help me overcome the problem. I want rotation of the contour plot without reducing the width and rotating exactly by the specified angle. Is it the aliasing effect?

Any help will be much appreciated. Many Thanks

Answer 1

[Grzegorz Knor]

Look at this example:

a = 37;
[x,y] = meshgrid(linspace(-4,4,30));
z = exp(-x.^2/15-y.^2);
contour(x,y,z)
xlim([-5 5])
ylim([-5 5])
x = x*cosd(a) - y*sind(a);
y = y*cosd(a) + x*sind(a);
figure
contour(x,y,z)
xlim([-5 5])
ylim([-5 5])

Is that what you want?

[Mahi Nazir]

Thank you so much for your reply

If you take a look at the results from your example, The angle(taken at the center of the plot) is only 25 deg and not 37 (Image attached).... and moreover the width of the contour is reduced as well. This is what happens to my plot as well. Any clues why does this happen.

[Grzegorz Knor]

The angle is 25 degree, because units on axes are not equal. Type

axis equal

after plots.

[Grzegorz Knor]

Ok, I think that now I understand your problem ;) So once again see at example:

a = 37;
[x,y] = meshgrid(linspace(-4,4,30));
z = exp(-x.^2/15-y.^2);
contour(x,y,z)
xlim([-5 5])
ylim([-5 5])
axis equal
figure
[C,h] = contour(x,y,z);
rotate(get(h,'children'),[0 0 1],a) 
xlim([-5 5])
ylim([-5 5])
axis equal

Original plot:

After rotation:

[Mahi Nazir]

This is super! That's what I exactly wanted. Thank you so much :) Cheers

[Mattalabba]

Is it correct that this method does not work anymore in MATLAB 2015 and newer?

I recieve the error 'Index exceeds matrix dimensions. Error in rotate (line 31): ax = ancestor(h(1),'axes');'

When I change

rotate(get(h,'children'),[0 0 1],a) 

to

rotate(h,[0 0 1],a) 

I get no error, but also nothing changes on the contourplot.

Or am I misunderstanding something?

[Alexander]

In the first example the y-transformation should include the original x values (i.e. prior to assignment):

% field to be rotated
a = 80;
[x,y] = meshgrid(linspace(-4,4,30));
z = exp(-x.^2/15-y.^2);
[~,h] = contour(x,y,z);
xlim([-5 5])
ylim([-5 5])

% use original x values in y-transformation
x_orig = x;
x = x*cosd(a) - y*sind(a);
y = y*cosd(a) + x_orig*sind(a);


figure
contour(x,y,z)
xlim([-5 5])
ylim([-5 5])    

●

Otherwise, the contour plot will be shear-transformed.

% accepted answer
[x,y] = meshgrid(linspace(-4,4,30));
x = x*cosd(a) - y*sind(a);
y = y*cosd(a) + x*sind(a);


figure
contour(x,y,z)
xlim([-5 5])
ylim([-5 5])  

Furthermore, as @Mattalabba mentioned

rotate(get(h,'children'),[0 0 1],a)

has no effect on contour objects. Use

[x,y] = meshgrid(linspace(-4,4,30));
figure
g = hgtransform;
contour(x, y, z, 'Parent', g);
set(g, 'Matrix', makehgtform('zrotate', deg2rad(a)));

instead. See rotating contourf in r2014b.