assign a variable using a position changed function?

I am trying to plot a line segment of an image array graphically using horizontal and vertical lines as cursors using imline. To continuously update the line plot I would need to store the position of the line every time it changes position. However, using a position change function, assignin('base','pos',getposition(h) doesnt work and I cant seem to assign a variable within the addnewpositionCallback.

handles.Line = imline (handles.axes1, [50 50], [0 400]); 
api = iptgetapi(handles.Line);
fcn = @(pos) [min(pos(:,1)) pos(1,2); min(pos(:,1)) pos(2,2)];
api.setDragConstraintFcn(fcn);   
fcn = makeConstrainToRectFcn('imline',get(gca,'XLim'),get(gca,'YLim'));

setPositionConstraintFcn(handles.Line,fcn);
addNewPositionCallback(handles.Line,@(p) assignin('base','x',getPosition(handles.Line)));
 addNewPositionCallback(handles.Line,@(j) plot((cropdata{1,roi}(:,x(1),p)),'Parent','handles.axes2'));

The last line returns an error as x is apparently undefined even though I can see it updating in the workspace when I move the line.

Any help is appreiated.

Answer 1

[Stephen23]

Forget about what you can see in the base workspace. It does not help you, because the workspace where you try to use that values is NOT the base workspace. It is the workspace of that particular callback. Every function, including callback functions, have their own independent workspace, as explained in the MATLAB documentation:

https://www.mathworks.com/help/matlab/matlab_prog/base-and-function-workspaces.html

There really is no point in spamming data into the base workspace using assignin, evalin, etc, because then these are just more difficult to access (slower, buggier, etc). The values you want is obtained in one callback, so all you need to do is transfer that value to whatever other callback you have where you want to use it. How to move values between callbacks is explained in the MATLAB documentation:

https://www.mathworks.com/help/matlab/creating_guis/share-data-among-callbacks.html

I would recommend that you use guidata or nested functions (sadly these do not work with GUIDE).

[SL: fixed typo MATOLAB -> MATLAB.]

[ryan muddiman]

Thanks for the reply, however, after trying the guidata approach and the set/get appdata in the addnewpositionCallback I still cant assign a variable in the function.

[Stephen23]

As far as I can tell you need to add one callback. Make sure that you have stored any data that you require in handles, and then you can access it later in the callback (untested):

handles.Line = ...
handles.roi = ...
addNewPositionCallback(handles.Line,@myfun);
guidata(...,handles) % make sure that you store handles!
...
function myfun(p)
handles = guidata(gcf);
x = getPosition(handles.Line);
plot((cropdata{1,roi}(:,x(1),p)),'Parent',handles.axes2)
end

See also:

https://www.mathworks.com/matlabcentral/answers/43390-getting-more-information-from-addnewpositioncallback

https://www.mathworks.com/help/images/ref/imroi.addnewpositioncallback.html

[ryan muddiman]

Ok I got it to work thanks to:

function myfun(p)
handles = guidata(gcf);
x = getPosition(handles.Line);
plot((cropdata{1,roi}(:,x(1),p)),'Parent',handles.axes2)
end

Thanks Stephen