UIMAGE - UIMAGESC

Display image with non-linearly spaced axis.

Description

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

• MATLAB release: MATLAB 7.2 (R2006a)

[natan]

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

[Sven]

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]

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]

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

[Frederic Moisy]

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]

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]

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!

[Jon-Fredrik Hopperstad]

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!

[Gabriel Akira Schreiber]

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!