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Vesnog
Vesnog

Reputation: 785

Matlab imagesc for data unevenly spaced in y

Is it possible in imagesc to specify the x-axis a column vector such that the ticks and data points(pixels) are placed on the corresponding points? As far as I understood from the manual you only specify the corners of the image and this is not a problem as long as your data is evenly spaced like 100:100:1000.

In my case the x-axis consists of 21 elements which are evenly spaced like 1200:50:1700, whereas the y-axis is the vertical concatenation of two evenly spaced column vectors 200:50:450 and 500:25:725. My aim is to have the data points(pixels) at the correct locations, but it seems impossible to do so. Is there a workaround?

Upvotes: 1

Views: 1883

Answers (3)

EBH
EBH

Reputation: 10450

One option is to make Y also evenly spaced, and than use repelem:

X = 1200:50:1700;
Y = [0 50 100 200:50:450 500:25:725 800];
% set the spacing factor:
spacing = round(diff(Y)/min(diff(Y)));

The spacing vector looks like:

spacing =
  Columns 1 through 14
     2     2     4     2     2     2     2     2     2     1     1     1     1     1
  Columns 15 through 19
     1     1     1     1     3

and defines the distance between elements as multiplications of the smallest distance between elements in the vector.
We then define our new 'Y' for the image, so it will be evenly spaced, with the smallest space between elements in the vector.

% Define the new Y:
Y_spaced = Y(1):min(diff(Y)):Y(end); % = 0:25:800
% some arbitrary data:
data = rand(numel(Y),numel(X));

We use the spacing vector as input for repelem to duplicate each row in data as much as needed:

% spacing the data:
data_spaced = repelem(data,spacing([1 1:end]),1,1);

And we can plot it using imagesc (either in the matrix ij orientation, or in cartesian xy orientation):

imagesc(X,Y_spaced,data_spaced)
axis xy

The result:

spaced image

Upvotes: 3

user2707001
user2707001

Reputation: 1711

Use contourf or interpolate the data to a grid that fits all data points using e.g. interp2 or plot using contourf

Initialize data

X=[1200:50:1700]
Y=[200:50:450 500:25:725]

V = peaks(max(size(X,2),size(Y,2)))(1:size(Y,2),1:size(X,2));

Plot using contourf

contourf(X,Y,V);
view([0 -90]);

Interpolate and plot

Xq=linspace(min(X),max(X),100);
Yq=linspace(min(Y),max(Y),100)';
Vq = interp2(X,Y,V,Xq,Yq,'nearest');
imagesc(Vq)

If you want to have your pixels exactly at the correct location, you need to use a grid that has points exactly at all the point of your Y vector of course, so you need to adjust the "100"s accordingly to your screen and data resolution.

The alternative is to use pcolor

pcolor(X,Y,V)
view([0 -90]);

(Be careful with all the up-down flipping between image data and real data)

Upvotes: 1

Andrei Davydov
Andrei Davydov

Reputation: 315

Try replace imagesc with surf command:

surf(1200:50:1700, [200:50:450 500:25:725], rand(16,11), 'EdgeColor','none');
view([0 90]);

Note: with this way you loose last column and last row compare to imagesc, but you can just duplicated these data before displaying.

Upvotes: 2

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