MATLAB: Use keyboard input to assign value to a variable

keyboard input

I have a series of images that I'd like to display one at a time and be able to change something like the brightness or contrast by using keyboard keys (say up or down arrow) and when I'm satisfied with that image, use left or right arrows to move between images. To that end, I'd like to have some sort of function that, upon pressing a keyboard key, outputs a value that depends on the key pressed. I could do something like this with ginput, where the button output is 1, 2, or 3 depending on if I left, right, or middle click, but that would limit me to only 3 different options, while I want at least 4, if not 6 or 8 different options. Any ideas?

Best Answer

I'm guessing that you will have some sort of GUI or even just a figure that has the displayed image. And so long as your GUI or figure has focus, then you want it to respond to the key presses and act accordingly.
You can do this by simply adding a Key Press callback to a figure. When the user presses a key, the callback will fire and you can handle the event based on the key.
A very simple example that will change the image upon pressing the left and right arrows is
 function catchKeyPress
    h = figure; 
    set(h,'KeyPressFcn',@KeyPressCb) ;
    I = imread('someImageA.jpg');
    imshow(I);
    function KeyPressCb(~,evnt)
        fprintf('key pressed: %s\n',evnt.Key);
        if strcmpi(evnt.Key,'leftarrow')
            I = imread('someImageB.jpg');
            imshow(I);
        elseif strcmpi(evnt.Key,'rightarrow')
            I = imread('someImageA.jpg');
            imshow(I);
        end  
    end
 end
The above will create a figure and assign a callback for the KeyPressFcn event. The body of the callback fires for every key pressed and, in this case, handles the left and right arrow keys.
Try it and see what happens!
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