Matlab: Real-time plotting of ROI selected with IMRECT

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.

---

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

Answer 2

Simillar functionality as @Andrey's reply, with three differences:

• Setting axis limits instead of 'CData' (which may be faster?)
• The magnification factor is variable and depends on the rectangle size, due to the 'fit' 'IniitalMagnification'.
• Added ConstraintFcn

would be:

function imZ = Zoom(im, s)

f = figure;
a1 = subplot(1,2,1);
imshow(im,'InitialMagnification', 'fit');
a2 = subplot(1,2,2);
imshow(im,'InitialMagnification', 'fit');

Ipos = [0 0 s];
rect = imrect(a1,Ipos);
rect.setPositionConstraintFcn(@(p) Const(p,size(im)));
rect.addNewPositionCallback(@(p) CB(p,a2));
CB(Ipos,a2);

if nargout > 0
    uiwait(f);
    imZ = im(pm(2):pm(2)+pm(4),pm(1):pm(1)+pm(3),:);
end

    function p = Const(p,imS)
        p(1:2) = max(1,p(1:2));
        p(1:2) = min(imS([2 1])-p(3:4),p(1:2));
    end

    function CB(p,a)
        pm = round(p);
        axes(a);       
        axis([pm(1),pm(1)+pm(3),pm(2),pm(2)+pm(4)]);
    end
end

which could be called as:

Zoom(imread('peppers.png'),[100 100]);