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UIMAGE - UIMAGESC

version 1.1 (3.36 KB) by

Display image with non-linearly spaced axis.

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UIMAGE(X,Y,C) displays matrix C as an image, using the vectors X and Y to specify the X and Y coordinates. X and Y may be unevenly spaced vectors, but must be increasing. The size of C must be LENGTH(Y)*LENGTH(X). (Most probably you'll want to display C' instead of C).

Contrary to Matlab's original IMAGE function, here the vectors X and Y do not need to be linearly spaced. Whereas Matlab's IMAGE function linearly interpolates the X-axis between X(1) and X(end), ignoring all other values (idem for Y), here UIMAGE allows for X and/or Y to be unevenly spaced vectors, by locally stretching the matrix C (ie, by duplicating some elements of C) for larger X and/or Y intervals.

Use UIMAGESC to scale the data using the full colormap. The syntax for UIMAGESC(X,Y,C,...) is the same as IMAGESC(X,Y,C,...).

Typical uses:
- Plotting a spatio-temporal diagram (T,X), with unevenly spaced time intervals for T (eg, when some values are missing, or when using a non-constant sampling rate).
- Plotting a set of power spectra with frequency in log-scale.

Example:
c = randn(50,20); % Random 50x20 matrix
x = logspace(1,3,50); % log-spaced X-axis, between 10 and 1000
y = linspace(3,8,20); % lin-spaced Y-axis, between 3 and 8
uimagesc(x,y,c'); % displays the matrix

Comments and Ratings (25)

lior michaeli

THIS IS AMAZING!

Giacomo

Thank you very much

Teresa Madsen

It's generating an easily fixable warning in the latest version of Matlab:

Warning: NARGCHK will be removed in a future release. Use NARGINCHK or NARGOUTCHK instead.
> In uimage

The first line of code in the function is currently:
error(nargchk(3,inf,nargin));

...which can be replaced by:
narginchk(3,inf);

Aaron

Aaron (view profile)

Works great.

Francesco

I suggest the following improvement: in the uimage.m file, when both X an Y are uneven, it is better to replace

for i=1:nx
for j=1:ny
indi = find(x<=xe(i));
indj = find(y<=ye(j));
ce(j,i) = c(indj(end), indi(end));
end
end

with

xi = zeros(nx,1);
yi = zeros(ny,1);
for i=1:nx
indi=find(x<=xe(i));
xi(i)=indi(end);
end
for j=1:ny
indj=find(y<=ye(j));
yi(j)=indj(end);
end
for i=1:nx
for j=1:ny
ce(j,i) = c(yi(j), xi(i));
end
end

Kind regards.

Peter Pablo

Dear Matt,
thank you for your suggestion. In order to remove the necessity of functions 'row' and 'col', the line 'ce = ...' should read:

ce = interp2(x,y,c,xe(:),ye(:)','nearest');

I can confirm that the speed for plot creation is increased by a factor of almost 500!!

(Tested using MATLAB 2014a)

Matt Raum

Output is different from imagesc for nominally identical inputs.

sz=21;
noise=1e-6;
x=linspace(0,1,sz)+noise*randn(1,sz);
y=linspace(0,1,sz)+noise*randn(1,sz);
z=randn(sz);
figure;
imagesc(x,y,z);hline(y,'-k');vline(x,'-k');
figure;
uimagesc(x,y,z);hline(y,'-k');vline(x,'-k');

I was able to correct this and speed up the code significantly by using the following replacement for the general (~evenx && ~eveny) case:

nx = round(1 + (x(end) - x(1))/dx);
nx = min(nx, nmax);
xe = linspace(x(1), x(end), nx);

ny = round(1 + (y(end) - y(1))/dy);
ny = min(ny, nmax);
ye = linspace(y(1), y(end), ny);

ce = interp2(x,y,c,col(xe),row(ye),'nearest');

The functions 'col' and 'row' just turn one dimensional vectors into column and row vectors, respectively. A similar loop eliminating modification made to the other two cases will presumably speed them up as well.

Thanks Frederic!

Anders

Anders (view profile)

perfect!

Gustaf Lönn

Very nice script! When using large matrices, panning and zooming is *much* faster with this script than with pcolor.

Natan

Natan (view profile)

very nice work, should be part of Matlab in my view...

Sven

Sven (view profile)

Secondly, imagine this scenario:

You have a volume of images, and you want to scroll through those images, viewing each one. If they were monotonically spaced, you could just call h=image(X,Y,C(:,:,1)) on the first slice, then call set(h,'CData',C(:,:,i)) on the rest of the slices.

When they are NOT monotonically spaced, you now need to call h=uimage(X,Y,C(:,:,1)). The problem here is that the 'CData' has a new number of rows/columns, so for subsequent images you can't just update 'CData' like you could before.

Instead, consider if uimage also gave the *indices* it used to resample C monotonically. Now on the first image you could call:
[h,Xinds,Yinds]=uimage(X,Y,C(:,:,1))
and on subsequent calls you could now use:
set(h,'CData',C(Yinds,Xinds,i))

This would save re-calculating the resampling locations for each separate slice (since they are all the same), and it would also save deleting and re-creating image handles for each slice you want to show.

Frederic, I've just sent you an overhaul of your code that does just this (and includes the speed-up from my above comment).

Sven

Sven (view profile)

There are a few places where big speed gains can be made. Firstly, the interpolation step can be improved by removing a loop near line 88:

tic % Original version
for j=1:ny
indj = find(y<=ye(j));
ce(j,1:nx) = c(indj(end), 1:nx);
end
toc
tic % Faster version
T = bsxfun(@minus, ye(:), y(:)')>=0;
% Following two lines can be replaced with FIND_NDIM: indsJ = find_ndim(T,2,'last');
[~, indsJ] = max(flipdim(T,2),[],2);
indsJ = length(y)+1 - indsJ;
ce2 = c(indsJ,:);
toc
isequal(ce, ce2)

Elapsed time is 0.013454 seconds.
Elapsed time is 0.001384 seconds.

ans =

1

So here we get a 13x speedup (and the sped up version gives identical output to the original)

Oliver Woodford

pcolor creates patch objects, which can create problems when exporting using the painters renderer. This method allows images with non-linear axes to be rendered as images, which export much better.

Oliver Woodford

Rob Campbell

Rob Campbell (view profile)

It's still a good idea. It's just a redundant good idea ;)

Frederic Moisy

Frederic Moisy (view profile)

Oooooooh... I must admit that Bjorn Gustavsson is totally right. This submission actually mimick to behavior of pcolor. Thank you for your remark, and sorry for this false good idea...
F.

Bjorn Gustavsson

Just a question: What improved features does this provide compared to simply calling:

pcolor(x,y,c),shading flat,axis ij

Tim

Tim (view profile)

I just debugged an error similar to that of Janti.... I think the i,j indexing is wrong in line 122. See below..

% ce(j,i) = c(indi(end), indj(end)); % original version
ce(j,i) = c(indj(end), indi(end)); % "corrected" version

Janti

Janti (view profile)

If you need to have both axis in logspace you get an error.

please add the following in the for loop in uimage.m line:120

indi = find(x<=xe(i));
indi = min(indi,length(y));
indj = find(y<=ye(j));
indj = min(indj,length(x));
ce(j,i) = c(indi(end), indj(end));

Thanks for this m file!

Excellent work. This was exactly what I was looking for. I just did 15 query-replace for 'imagesc' in my code and it works beautifully. Thanks!

Oh soo cool! Just what I needed.Very nicely done.

Dhanya parameshwaran

Thanks Frederic. Clever trick , works perfectly!

Sven Holcombe

Excellent work. Piggybacks neatly on core matlab image() and imagesc() functions so I can simply replace these calls. If my input is in fact perfectly linear, then these calls are used directly without further processing. Thank you.

RALIHALIZARA Julliard

BEautifull idea!

Updates

1.1

Bug fixed when both X and Y axis are not linearly spaced (thank you Janti and Tim!!)

help text improved

MATLAB Release
MATLAB 7.2 (R2006a)

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