Use keyboard input to assign value to a variable

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?

[Aaron Fleming]

Had a question similar to this, but wanted to update a variable depending on the key that was pressed in 'real time'. Because it doesn't seem to exist anywhere else here is the code that will allow you to assign a variable, s, with either space, leftarrow, or rightarrow, when you click one of those three. It's a small variation on Geoff's code below.

function catchKeyPress

    h = figure; 
    set(h,'KeyPressFcn',@KeyPressCb);

    function y = KeyPressCb(~,evnt)

        fprintf('key pressed: %s\n',evnt.Key);
        global s;
        if strcmp(evnt.Key,'rightarrow')==1
        s = evnt.Key;
        elseif strcmp(evnt.Key, 'leftarrow')==1
        s = evnt.Key;
        elseif strcmp(evnt.Key,'space')==1
        s = evnt.Key;    

        end

    end

    end

In order for this to work you need to call the function catchKeyPress from a separate script. Thanks Geoff for the script which let me figure this out. end

Answer 1

[Geoff Hayes]

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!