MATLAB Answers


How to efficiently update an image if one matrix element is changed

Asked by Zoltán Csáti on 18 Oct 2015
Latest activity Commented on by Image Analyst
on 19 Oct 2015
I have a matrix in which I change only one element in every iteration of a loop. I want to display that matrix as an image using image or imagesc. What I am doing now is to update the CData property of the image handle so as to refresh the image of the matrix. With several thousands of iterations, it turned out that the bottleneck is the image/imagesc function according to the Profiler. So I thought of changing the color of the tile corresponding to the changed matrix entry. How can I do that? There would be no problem with the colormap since the matrix only contains zeros and ones.


And how could I achieve that?
If you are using R2014a or earlier, EraseMode 'xor' can be used to potentially speed up drawing. As of R2014b that is no longer an option.
Ewwww...that's a bummer, indeed! I'd also not thought of HG2 and latest releases in performance...I understand there is, so far at least, a big dropoff there.

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1 Answer

Answer by Image Analyst
on 18 Oct 2015
 Accepted Answer

Don't use image. Use set(). See this demo:
grayImage = imread('moon.tif');
hImage = imshow(grayImage);
promptMessage = sprintf('Do you want to assign new pixels,\nor Quit?');
titleBarCaption = 'Continue?';
buttonText = questdlg(promptMessage, titleBarCaption, 'Continue', 'Quit', 'Continue');
if strcmpi(buttonText, 'Quit')
for k = 1 : 50 : numel(grayImage);
grayImage(k) = 255; % Make this pixel bright white.
set(hImage, 'CData', grayImage);
I think it should be fast enough for you.


This is what I did as you can see it in the attached code in a few comments above. The performance drop is probably due to HG2. However, your code sample made my think of other useful things, so I accept it.
What attached code? Performance drop from what? On my system, my code was faster than a greased cheetah on steroids. So fast I couldn't even see it happen in real time - I just saw the final result.

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