Contour plot

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Falk 2011-5-24

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回答： Yusuf Yilmaz 2021-11-10

Hi,

How can I make contour plots that do not 'interpolate'? For example, suppose I have the matrix a = [0 0 0 0; 0 1 1 0; 0 1 1 0; 0 0 0 0], which I would like to look like a square when I use contour(a,1) (the ,1 is used in order to have only one contour line). However, Matlab outputs an octagon-like shape, i.e. it kind of rounds the corners. I have seen that certain Matlab based software can do such contour plots, but I can't access the code and haven't been able to get it to work myself. Any help would be greatly appreciated!

Thanks,

Falk

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Answer 1

Sean de Wolski 2011-5-24

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Multiple calls to line. You'll have to write some type of wrapper function but it should be doable.

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Falk 2011-5-25

Thanks, great idea! Definitely more work than just using contour, but works fine!

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Answer 2

Arnaud Miege 2011-5-24

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Try this:

contour(a,[1 1])

HTH,

Arnaud

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Falk 2011-5-24

Thanks a lot, but it's not quite what I was looking for. I guess I didn't explain clear enough. If I use the normal contour and overlay it on my matrix...

figure(1); imagesc(a); colormap gray; hold on; contour(a,1,'color',[1 0 0]);

... then I get the kind of 'cut' corners. However, when I use your version...

figure(2); imagesc(a); colormap gray; hold on; contour(a,[1 1],'color',[1 0 0]);

... it doesn't fit the matrix but is smaller. Any ideas? I'm actually working with arbitrary shapes (the square was just an example), so it would be quite nice if the contour really traced each outer pixel...

Thanks,

Falk

Arnaud Miege 2011-5-24

It seems that the contour matrix is computed automatically and is read-only, so you can't edit it - at least I haven't found a way to do it. Maybe contour isn't the right way to go about it?

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Answer 3

Sean de Wolski 2011-5-24

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If instead of a contour plot you could deal with an edge, this might work:

Lrgb = label2rgb(bwperim(kron(a,ones(10)))); %expanding with 10x10 to create an edge
imshow(Lrgb) 
%IPT required

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Falk 2011-5-24

Thanks a lot to both of you! The bwperim is an interesting idea, but still isn't what I am looking for, as it gives me a non-connected outline (i.e. filled boundary pixels) that is as thick as each pixel. Ideally, I would like to have a connected line (as thin as the contour line) that traces the boundary (and can be overlaid on another image / plotted matrix). Quite specific wishes, I know... Any further ideas?

Thanks,

Falk

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Answer 4

Matt Tearle 2011-5-24

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Massive hack that works in this case (not sure how generalizable it will be to whatever your application is):

a(a==0) = NaN;
mesh(a)
set(gca,'View',[0,90],'XGrid','off','YGrid','off')

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Falk 2011-5-25

Thanks, that's a cool trick, never thought about that. Unfortunately, it gives the same result as Arnaud's code, i.e. the square is too small if I overlay the line. Or did I do something wrong maybe?

Matt Tearle 2011-5-25

Oh, no, my bad. I hadn't read the comments on Arnaud's answer, sorry. Looks like you'll have to write your own routine to do multiple line calls, as Sean suggests.

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