MATLAB: Setting axis properties is cropping the graphic (imagesc)

imagesc

Hello 🙂

I am using imagesc to get a scaled coloured map of a matrix. I also want to decide the possibles values for the x and y axis.

When I try to set them using the following code, I get a graphic that is cropped in half (half of it is white, the other is coloured). Here is the code :

imagesc(matrixA(:,:,1),[0 1]);
set(gca,'XLim',[0 1],'XTick',0:0.1:1)

It is hard for me to tell if the image is cropped or if it is fitted to half the space…

I did try to search on MathWorks answers and on the internet, but to no avail for now. What am I doing wrong? Have you ever had a similar problem? Reading the doc leaves me clueless as to what the problem can be.

Thank you and have a most wonderful day!

Best Answer

When you use imagesc() without specifying XData or YData (or x and y), then the default XData is to place the center of the pixels one data unit apart, so they would be at 1, 2, 3, ... number of columns. When you then set your XLim to [0 1] you are restricting the view on the image to show only what is visible in x range 0 to 1. That is going to be only half of the first column of pixels (with 1 as the center, the first pixel would occupy from 0.5 to 1.5 on the x axis.)

If you want the image to be placed within the range [0 1] then you should instead be using

imagesc(matrixA(:,:,1), [0 1], 'XData', [0 1], 'YData', [0 1]);

However, you might want to adjust that slightly to take into account that those are the centers you are specifying. For example you might want something like

   x = linspace(0, 1, size(YourImage,2)+2 );
   xlimit = [x(2), x(end-1)];

and then use xlimit as the XData for imagesc

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MATLAB: How to shift tick marks on imagesc() instead of centered

The tick marks are probably in the correct place for what you asked imagesc() to do.

img = uint8(1:10);
subplot(4,1,1); h1 = imagesc(img); title('no XData')
subplot(4,1,2); h2 = imagesc(img, 'XData', 0, 'YData', 0); title('XData 0'); box on
subplot(4,1,3); h3 = imagesc(img, 'XData', 1, 'YData', 1); title('XData 1'); box on
subplot(4,1,4); h4 = imagesc(img, 'XData', 1/2, 'YData', 1/2); title('XData 1'); box on

You can see that your case corresponds to having used imagesc() and specifying XData of 0.

Why does using XData of 0 give you a box centered at 0, instead of the 0 being to the left of the box? The reason for that is that when you specify XData and YData, the coordinates you give are the data coordinates for the center of the pixel. So XData of 0 means that 0 should be in the center of the first pixel. If you wanted 0 to be at the left of the first pixel, then you need to place the center of the first pixel at half a pixel offset from 0. That will be at data 1/2 if you only specify one element each for XData and YData, as the default is one pixel per data unit. If you specify the [lower, upper] form for XData you will need to calculate (upper-lower+1) / pixels_in_this_edge / 2

MATLAB: Specific values in colorbar

The unique values in you original question are [0 1 2 15]. Do you want just those to appear in the colorbar, or all possible integers between 0 and 15?

For the former:

data = [2 1 0 0 0 0 0 0 0 0;1 2 15 0 0 0 0 0 0 0];
[unq, ~, iunq] = unique(data);
iunq = reshape(iunq, size(data));
ncol = length(unq);
cmap = hot(ncol*2);
cmap = cmap(1:2:end,:);
imagesc(iunq);
colormap(cmap);
cb = colorbar;
set(gca, 'clim', [0.5 ncol+0.5]);
set(cb, 'ticks', 1:ncol, 'ticklabels', cellstr(num2str(unq)));

Walter's code covers the latter case. As a side note, I'd usually just suggest hot(4) to get the 4-color hot colormap, but interpolating hot to only 4 colors results in two nearly-identical shades of yellow; hence the doubling of colors and then using every other in my cmap generation above.

Best Answer

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MATLAB: How to shift tick marks on imagesc() instead of centered

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