App designer - display multiple ROI on the same image

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I want to display a crosshair and a circle right in the middle of my image. I wanted to do this by defining a function, as below.
I'm not sure how to proceed but whilst a similar approach works in the live editor, the app design function doesn't like this syntax. Is there any way to approach this please? I'm not familiar with the app design environment.
The image is always displayed in a UIFigure object. I've input the values [472, 323] as a position vector for testing. Ideally, they would point to the centre of the image.
Thanks in advance!
function DisplayImage(app)
app.Image.ImageSource = app.fullname;
app.crossHair = images.roi.Crosshair(gca, 'Position', [472, 323], 'Color', 'w'); = images.roi.Circle(gca, 'Centre', [472, 323], 'Radius', 100, 'Color', 'w');

Accepted Answer

Adam Danz
Adam Danz on 10 Mar 2021
I assume "doesn't like this syntax" means that the crosshairs and circle are appearing on a different figure.
The reason that would happen is because of your use of gca instead of the actual axis handle.
Assuming your axis handle is app.UIAxes,
function DisplayImage(app)
app.Image.ImageSource = app.fullname; % ??? Not sure what this is
app.crossHair = images.roi.Crosshair(app.UIAxes, 'Position', [472, 323], 'Color', 'w');
% ^^^^^^^^^^ = images.roi.Circle(app.UIAxes, 'Center', [472, 323], 'Radius', 100, 'Color', 'w');
% ^^^^^^^^^^^

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