Question

I have a GUI with two axes. The first axes has a low-resolution image.

What I would like to do is select an area on the first axes using IMRECT and then display that area as a high-resolution image on the second axes, while continuously updating as I move the IMRECT rectangle around.

The only way I have been able to do this is with a “for loop” with a 0.1 pause in it that just runs for a minute or two while I select and change the ROI with IMRECT, very cumbersome.

My thought was to use a function that ran whenever the mouse moved within the first axes, with the ploting and getPosition commands in that function. However, I’m not sure how to write such a function (triggering on mouse movement within an axes).

Any help would be greatly appreciated!

Answer 1

In general, you should assign a callback to your imrect. For example:

x = imrect();
x.addNewPositionCallback( @(x)(disp('The rect has changed')))

The callback should get additional parameters, such as the image and the second axes by utilizing anonymous functions.

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I wrote a small code snippet that does what you want. You should add boundary checks, since I did not bother. It updates CData instead of running imshow when you move the rectangle, so it is quite smooth.

function Zoomer
    figure();

    highResImage = imread('peppers.png');
    lowResImage = imresize(highResImage,0.5);

    a1 = subplot(2,1,1);
    a2 = subplot(2,1,2);

    imshow(lowResImage,'Parent',a1);
    initialPosition = [10 10 100 100];
    lowResRect = imrect(a1,initialPosition);

    lowResRect.addNewPositionCallback( @(pos)Callback(pos,a2,highResImage));

    Callback( initialPosition , a2, highResImage);
end

function Callback(position,axesHandle, highResImage)
    position = position * 2;
    x1 = position(1);
    y1 = position(2);
    x2 = position(1) + position(3);
    y2 = position(2) + position(4);

    highResThumbnail = highResImage( round(y1:y2),round(x1:x2),:);

    if isempty( get(axesHandle,'Children')) 
        imshow(highResThumbnail,'Parent',axesHandle);   
    else
        imHandle = get(axesHandle,'Children');
        oldSize = size(get(imHandle,'CData'));
        if ~isequal(oldSize, size(highResThumbnail))
            imshow(highResThumbnail,'Parent',axesHandle);
        else
            set( imHandle,'CData', highResThumbnail);
        end     
    end
end